Patent Publication Number: US-8121509-B2

Title: Image forming apparatus and method for controlling the same in a stepwise manner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2007-308055 filed in Japan on Nov. 28, 2007 and Japanese priority document 2008-277211 filed in Japan on Oct. 28, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and its controlling method. 
     2. Description of the Related Art 
     As an image forming unit for an electrophotographic printer, for example, an image forming unit  1100  as shown in  FIG. 11  is known.  FIG. 11  is a schematic diagram of the image forming unit  1100  that includes a photosensitive element  1101 , a charging unit  1102 , an exposing unit  1103 , a developing unit  1104 , a sheet conveying unit  1105 , a transfer unit  1106 , a cleaning unit  1107 , and a neutralization unit  1108 . The photosensitive element  1101  rotates in a direction indicated by an arrow “a” in  FIG. 11 , during which the surface thereof is uniformly charged by the charging unit  1102 . The charged surface is irradiated with light corresponding to printing data by the exposing unit  1103 , so that a latent image is formed on the surface of the photosensitive element  1101 . Then, toner is adhered to the latent image by the developing unit  1104  to form a toner image on the surface of the photosensitive element  1101 . The toner image is transferred onto a sheet conveyed by the sheet conveying unit  1105  in a direction indicated by an arrow “b” in  FIG. 11 . The sheet carrying the toner image is conveyed to a fixing unit (not shown), in which the toner image is fixed to the sheet. Toner remaining on the surface of the photosensitive element  1101  is removed by the cleaning unit  1107 , and the neutralization unit  1108  neutralizes the surface of the photosensitive element  1101 . 
     There is a demand for an image forming apparatus that includes such an image forming unit to enable restarting of printing promptly when a printing instruction was received while each unit in the image forming unit is in a stopping process. For example, a technology is disclosed in Japanese Patent Application Laid-open No. H06-143683, in which when an image forming apparatus is instructed to perform a stopping process, the image forming apparatus stands by for a certain period of time in a state of stopping only a developing unit. When a printing instruction is received during the standby, printing can be started only by restarting the developing unit. When the printing instruction is not received during the standby, the stopping process is performed also on the units other than the developing unit such as a transfer unit and a cleaning unit after the certain period of time. 
     However, in the above technology, the units other than the developing unit are driven uselessly for the certain period of time, so that the lifetime of the units may be shortened. Moreover, even when the printing instruction is received at the moment when the stopping process of each unit of the image forming unit is started, the stopping process of the developing unit is inevitably performed. Therefore, the time required for restarting printing cannot be shortened anymore. 
     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 one aspect of the present invention, there is provided an image forming apparatus including an image forming unit that performs an image forming process; a control unit that controls a starting process and a stopping process of the image forming unit in a stepwise manner; a determination unit that, upon receiving an instruction for executing the image forming process during the stopping process, inquires of the control unit about a stage of the stopping process and determines whether the stopping process in the stage is interruptible; a shutdown unit that instructs the control unit to interrupt the stopping process during or after the stage, based on a result of determination by the determination unit; and a start-up unit that instructs the control unit to perform the starting process after interrupting the stopping process. 
     Furthermore, according to another aspect of the present invention, there is provided a control method for an image forming apparatus including an image forming unit that performs an image forming process. The control method includes controlling a starting process and a stopping process of the image forming unit in a stepwise manner; determining, upon receiving an instruction for executing the image forming process during the stopping process, including inquiring about a stage of the stopping process, and determining whether the stopping process in the stage is interruptible; shutting-down including instructing to interrupt the stopping process during or after the stage, based on a result of determination at the determining; and starting-up including instructing to perform the starting process after interrupting the stopping process. 
     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 schematic diagram of a unit configuration of a laser printer according to a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an image forming unit for an electrophotographic printer of the laser printer; 
         FIG. 3  is a timing chart of a printing operation of the image forming unit; 
         FIG. 4  is a block diagram of a laser-printer control unit of the laser printer; 
         FIG. 5  is a block diagram of control modules for controlling the image forming unit; 
         FIG. 6  is a flowchart of a printing process, a starting process, and a stopping process of the image forming unit by the control modules; 
         FIG. 7  is a schematic diagram illustrating a table stored in an engine ROM in the laser-printer control unit according to a modified example of the present invention; 
         FIG. 8  is a schematic diagram of an image forming unit according to a second embodiment of the present invention; 
         FIG. 9  is a timing chart of a printing operation of the image forming unit; 
         FIG. 10  is a block diagram of control modules for controlling the image forming unit; and 
         FIG. 11  is a schematic diagram of a conventional image forming unit for an electrophotographic printer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
     In the following explanation, the present invention is employed to a laser printer as an example; however, it is not limited thereto. The present invention can be employed to, for example, a multi-function peripheral (MFP) having a copier function, a facsimile function, a printer function, and the like or an electrophotographic copier. 
       FIG. 1  is a schematic diagram of a unit configuration of a laser printer  100  according to a first embodiment of the present invention. The laser printer  100  includes a body unit  110 , a duplex unit  120 , a mailbox unit  130 , a finisher unit  140 , and a sheet feeding unit  150 . 
     The body unit  110  includes an image forming unit (not shown) for printing an image on a recording medium (sheet) that includes a registration unit and a fixing unit, controls a printing process (image forming process) such as imaging, developing, and fixing, and feeds sheets on which images are to be printed. 
     The duplex unit  120  reverses a sheet by a reversing (switchback) mechanism and feeds the reversed sheet for a duplex printing. 
     The mailbox unit  130  includes a plurality of discharge ports for discharging sheets and discharges printed sheets from any discharge port. In the present embodiment, the mailbox unit  130  is a 2-bin mailbox. 
     The finisher unit  140  performs post-processing such as a stacking process for sorting printed sheets for each job by shifting them and a stapling process. 
     The sheet feeding unit  150  includes a sheet feeding tray in which sheets are loaded and conveys sheets to the registration unit. 
     As an example of the image forming unit in the body unit  110 , an image forming unit that includes an image forming unit  200  for an electrophotographic process is explained.  FIG. 2  is a schematic diagram of the image forming unit  200 . The image forming unit  200  performs an image forming process, and includes a photosensitive element (image carrier)  201 , a charging unit  202 , an exposing unit  203 , a developing unit  204 , a sheet conveying unit  205 , a transfer unit  206 , a cleaning unit (removing unit)  207 , a neutralization unit  208 , and a driving unit (not shown). The photosensitive element  201  is rotated by the driving unit in a direction indicated by an arrow “a” in  FIG. 2 , and the charging unit  202  charges the surface of the photosensitive element  201 . The charged surface of the photosensitive element  201  is exposed by the exposing unit  203 , whereby a latent image is formed thereon. The developing unit  204  causes charged toner to adhere to the latent image to form a toner image on the surface of the photosensitive element  201 . The sheet conveying unit  205  conveys a sheet in a direction indicated by an arrow “b” in  FIG. 2 , and the transfer unit  206  transfers the toner image onto the sheet conveyed by the sheet conveying unit  205 . The cleaning unit  207  removes residual toner remaining on the surface of the photosensitive element  201 , and the neutralization unit  208  neutralizes the surface of the photosensitive element  201  after the transfer process. 
     The procedure and the content of the printing operation of the image forming unit  200  when the image forming unit  200  performs the printing process are explained with reference to  FIG. 3 .  FIG. 3  is a timing chart of the printing operation of the image forming unit  200 . When printing is started, the image forming unit  200  starts the printing operation shown in  FIG. 3  to cause the image forming unit  200  to be in a printable state. The printing operation is classified into a start-up (starting process), a printing process, and a shutdown (stopping process). 
     The starting process of the image forming unit  200  is explained with reference to  FIG. 3 . 
     First, a laser-printer control unit causes a polygon mirror for exposure (not shown) to rotate stably (A 1 ). Then, the laser-printer control unit drives the photosensitive element  201  to rotate stably by the driving unit (A 2 ). Then, the laser-printer control unit drives the neutralization unit  208  (A 3 ). Then, the laser-printer control unit applies a preparation bias for transferring a toner image onto a sheet to the transfer unit  206  (A 4 ). Then, the laser-printer control unit causes the charging unit  202  to apply a primary charging bias to the surface of the photosensitive element  201  to charge the surface (A 5 ). When the charged surface of the photosensitive element  201  reaches the developing unit  204 , the laser-printer control unit causes the developing unit  204  to apply a primary developing bias to toner to charge the toner (A 6 ). Then, the laser-printer control unit causes the charging unit  202  to apply a secondary charging bias to the surface of the photosensitive element  201 , for example, 50 milliseconds after applying the primary charging bias for stabilizing the voltage (A 7 ). Then, in the similar manner, the laser-printer control unit causes the developing unit  204  to apply a secondary developing bias to the toner, for example, 50 milliseconds after applying the primary developing bias for stabilizing the voltage (A 8 ). When the entire surface of the photosensitive element  201  is charged with the secondary charging bias applied by the charging unit  202 , i.e., when the photosensitive element  201  rotates one revolution after the start of the charging with the secondary charging bias, the image forming unit  200  is ready for printing (A 9 ). 
     In the present embodiment, the starting process is classified into five stages: the operation A 1 , the operation A 2 , the operation A 3 , the operation A 4 , and the operations A 5  to A 9 , and is performed in a stepwise manner in accordance with the stages. 
     The printing process of the image forming unit  200  is explained. 
     For printing after starting up the image forming unit  200 , the laser-printer control unit causes the exposing unit  203  to expose the charged surface of the photosensitive element  201  thereby forming a latent image thereon, and causes toner to adhere to the latent image by the developing unit  204  thereby forming a toner image. The toner image on the photosensitive element  201  is transferred by the transfer unit  206  onto a sheet conveyed by the sheet conveying unit  205 . At this time, toner which is not transferred onto the sheet and remains on the photosensitive element  201  is removed from the photosensitive element  201  by the cleaning unit  207 . Thereafter, the laser-printer control unit causes the neutralization unit  208  to neutralize the surface of the photosensitive element  201 , so that the potential of the surface becomes zero to be ready for the next latent image formation. The neutralization of the surface of the photosensitive element  201  is performed by the neutralization unit  208  for the size of the sheet. The laser-printer control unit performs the above process for the required number of sheets. Upon completing the printing process, the laser-printer control unit shuts down the image forming unit  200 . 
     The stopping process of the image forming unit  200  is explained with reference to  FIG. 3 . 
     First, the laser-printer control unit causes the transfer unit  206  to stop applying the preparation bias. The laser-printer control unit does not move to the next operation (B 2 ) until the application of the preparation bias is completely stopped (B 1 ). Then, the laser-printer control unit changes the output of the charging unit  202  to the primary charging bias (B 2 ). When the surface of the photosensitive element  201  charged with the primary charging bias reaches the developing unit  204 , the laser-printer control unit changes the output of the developing unit  204  to the primary developing bias (B 3 ). Then, the laser-printer control unit causes the charging unit  202  to stop applying the primary charging bias, for example, 50 milliseconds after changing the output of the charging unit  202  to the primary charging bias for stabilizing the voltage (B 4 ). Then, in the similar mariner, the laser-printer control unit causes the developing unit  204  to stop applying the primary developing bias, for example, 50 milliseconds after changing the output of the developing unit  204  to the primary developing bias for stabilizing the voltage. Consequently, the photosensitive element  201  and the toner both are not charged (B 5 ). Then, the laser-printer control unit causes the neutralization unit  208  to continue neutralizing the photosensitive element  201  until the surface of the photosensitive element  201  to which the charging unit  202  has stopped applying the primary charging bias reaches the neutralization unit  208 , and thereafter stops driving of the neutralization unit  208  (B 6 ). Then, the laser-printer control unit waits until the surface of the photosensitive element  201  to which the developing unit  204  has stopped applying the primary developing bias reaches the cleaning unit  207 , and thereafter moves to the next operation (B 8 ) (B 7 ). The laser-printer control unit stops driving of the photosensitive element  201 . The laser-printer control unit does not move to the next operation (B 9 ) until the driving of the photosensitive element  201  is completely stopped (B 8 ). Then, the laser-printer control unit stops driving of the polygon mirror. When the driving of the polygon mirror is completely stopped, the stopping process is completed (B 9 ). 
     In the present embodiment, the stopping process of the image forming unit  200  is classified into six stages: the operation B 1 , the operations B 2  to B 5 , the operation B 6 , the operation B 7 , the operation B 8 , and the operation B 9 , and is performed in a stepwise manner in accordance with the stages. 
     In the present embodiment, in the operations A 5  to A 9  and the operations B 2  to B 5 , each of the charging unit  202  and the developing unit  204  applies the bias in two steps and waits 50 milliseconds between the two steps because the rising characteristics of the both biases are different. 
     Specifically, if each of the charging unit  202  and the developing unit  204  applies a target voltage in one step, time difference occurs between the charging unit  202  and the developing unit  204  to reach the target voltage. Consequently, imbalance occurs between the charging bias and the developing bias, which may result in adhering developer containing toner to the surface of the photosensitive element  201 . On the other hand, in the present embodiment, each of the charging unit  202  and the developing unit  204  applies the target voltage in two steps, so that the time difference that may occur between the charging unit  202  and the developing unit  204  is reduced. The image forming unit repeats the above printing operation. 
       FIG. 4  is a block diagram of the laser-printer control unit of the laser printer  100 . 
     The laser printer  100  includes a controller  410  that performs interface control with a host machine  450  and controls editing image data and an engine  420  that controls the start-up, the printing process, and the shutdown of the image forming unit  200 . 
     The host machine  450  is a general-purpose computer and transmits/receives data to/from the laser printer  100  in accordance with various operations by a user. 
     The controller  410  includes a host interface (host I/F)  411 , a program read only memory (ROM)  412 , a font ROM  413 , a panel I/F  414 , a controller central processing unit (CPU)  415 , a random access memory (RAM)  416 , an option RAM  417 , and an engine I/F  418 . Furthermore, the controller  410  is connected to the host machine  450  via the host I/F  411 , to an operation panel  440  via the panel I/F  414 , and to a font cartridge  460  that stores various fonts. 
     The host I/F  411  performs interface control with the host machine  450 . The program ROM  412  stores computer programs for editing image data and controlling the controller  410 . The font ROM  413  stores a standard font used in printing. The panel I/F  414  performs interface control with the operation panel  440  to serve as an interface with a user at the operation panel  440 . 
     The controller CPU  415  controls the controller  410  by controlling data transmission/reception to/from the host machine  450  and the like through executing various computer programs stored in the program ROM  412 . Each of the RAMs  416  and  417  is a memory in which computer programs for processing and control, image data, and the like are loaded. The engine I/F  418  is an interface between the controller  410  and the engine  420 . 
     The engine  420  includes an engine CPU  421 , a controller I/F  422 , a flash ROM  423 , an input port  424 , an output port  425 , an engine ROM  426 , a RAM  427 , a sensor  428 , a motor  429 , a clutch  430 , and a power supply  431 . 
     The sensor  428  detects setting conditions for the printing process, abnormality in the image forming unit  200 , and the like. The motor  429  includes a main motor for rotating the polygon mirror of the exposing unit  203  and the photosensitive element  201  and a conveying motor for rotating sheet feeding rollers of the sheet conveying unit  205 . The clutch  430  drives and stops driving of the sheet feeding rollers and the like. The power supply  431  supplies power necessary for applying bias by the charging unit  202 , the developing unit  204 , and the transfer unit  206 , driving the neutralization unit  208 , and the like, to the charging unit  202 , the developing unit  204 , the transfer unit  206 , the neutralization unit  208 , and the like. For example, the power supply  431  is a commercial power. 
     The flash ROM  423  stores therein various computer programs and data. The input port  424  performs an input process of inputting the setting conditions for the printing process, the state of the image forming unit  200 , and the like detected by the sensor  428 . The output port  425  performs an output processing for realizing the starting process, the printing process, and the stopping process of the image forming unit  200 . The engine ROM  426  stores therein computer programs for controlling engines. The RAM  427  has a function of a buffer register and is used as a working memory. 
     The engine CPU  421  controls the starting process, the printing process, and the stopping process by executing control programs stored in the engine ROM  426 . The controller I/F  422  performs interface control with the controller  410  via the engine I/F  418 . 
     A storage medium for storing the control programs is not limited to the engine ROM  426 , and a semiconductor storage unit, an optical and/or magnetic storage unit, and the like can also be used. It is possible to use the storage medium for storing such control programs in an external system different from the above, and execute the control programs in a CPU in the external system, whereby an advantage substantially the same as that of the present embodiment can be obtained. 
     Control modules of the image forming unit  200  are explained with reference to  FIG. 5 . The control modules are realized by the engine CPU  421  executing the control programs stored in the engine ROM  426 .  FIG. 5  is a block diagram of the control modules for controlling the image forming unit  200 . The control modules include a photosensitive-element (driving unit) control unit  501 , an exposing-unit control unit  502 , a transfer-unit control unit  503 , a neutralization-unit control unit  504 , a charging-unit control unit  505 , and a developing-unit control unit  506  for performing the printing process by the units in the image forming unit  200  such as the photosensitive element  201  and the exposing unit  203  and the stepwise stating process and stopping process of the units in the image forming unit  200 . Moreover, the control modules include an image-forming-unit (IFU) start-up unit  508  and an IFU shutdown unit  509  for instructing the above modules to sequentially perform the starting process and the stopping process. In the explanation below that is common to all of the photosensitive-element control unit  501 , the exposing-unit control unit  502 , the transfer-unit control unit  503 , the neutralization-unit control unit  504 , the charging-unit control unit  505 , and the developing-unit control unit  506 , they are referred to as the control units  501  to  506 . 
     The control modules further include an IFU cleaning process unit  510 , an IFU stopping-process-interruption determination unit  511 , an IFU printing unit  512 , an IFU emergency stopping unit  513 , an IFU control unit  514 , an IFU stopping-process-interruption control unit  515 , a sheet-conveying control unit  516 , a fixing control unit  517 , and a printing control unit  518 . 
     The IFU cleaning process unit  510  causes the cleaning unit  207  to remove residual toner on the photosensitive element  201 . The IFU stopping-process-interruption determination unit  511  determines whether the stopping process being performed can be interrupted. The IFU printing unit  512  instructs the control units  501  to  506  to perform the printing process. The IFU emergency stopping unit  513  urgently stops the stopping process. The IFU control unit  514  controls the image forming unit  200 . The IFU stopping-process-interruption control unit  515  instructs the IFU printing unit  512 , the IFU start-up unit  508 , and the IFU shutdown unit  509  to perform the printing process, the starting process, and the stopping process, in accordance with a printing process instruction. The sheet-conveying control unit  516  controls conveying of sheets by the sheet feeding unit  150 . The fixing control unit  517  controls a fixing unit (not shown) of the finisher unit  140 . The printing control unit  518  sends an operation instruction to the IFU control unit  514 , the sheet-conveying control unit  516 , and the fixing control unit  517 . 
     When the image forming unit  200  receives the printing process instruction while performing the stopping process, the image forming unit  200  needs to shift to the printing process as soon as possible. Therefore, the IFU stopping-process-interruption determination unit  511  interrupts the stopping process and controls to perform the starting process again. 
     The printing process, the starting process, and the stopping process of the image forming unit  200  by the control modules are explained in detail with reference to  FIG. 6 .  FIG. 6  is a flowchart of the printing process, the starting process, and the stopping process by the control modules. 
     When the laser printer  100  is turned on, the IFU stopping-process-interruption control unit  515  stands by until receiving the printing process instruction from the controller  410  via the controller I/F  422  (Step S 601 ). When the IFU stopping-process-interruption control unit  515  receives the printing process instruction from the controller  410  (YES at Step S 602 ), the IFU stopping-process-interruption control unit  515  instructs the IFU start-up unit  508  to perform the starting process, and the IFU start-up unit  508  instructs each of the control units  501  to  506  to perform the starting process in a stepwise manner (A 1  to A 9  shown in  FIG. 3 ) in response to the instruction (Step S 603 ). 
     When the starting process is completed, the IFU stopping-process-interruption control unit  515  instructs the IFU printing unit  512  to perform the printing process, and the IFU printing unit  512  instructs each of the control units  501  to  506  to perform the printing process in response to the instruction (Step S 604 ). 
     When the printing process is completed, the IFU stopping-process-interruption control unit  515  instructs the IFU shutdown unit  509  to perform the stopping process, and the IFU shutdown unit  509  instructs each of the control units  501  to  506  to perform the stopping process in a stepwise manner (B 1  to B 9  shown in  FIG. 3 ) in response to the instruction (Step S 605 ). In the stopping process, the IFU stopping-process-interruption control unit  515  checks whether the stopping process is completed (Step S 606 ). If the stopping process is completed (YES at Step S 606 ), the system control returns to Step S 601  and the IFU stopping-process-interruption control unit  515  stands by until receiving the printing process instruction from the controller  410  again. 
     On the other hand, if the IFU stopping-process-interruption control unit  515  receives the printing process instruction from the controller  410  (YES at Step S 607 ) while the stopping process is not completed (NO at Step S 606 ), the IFU stopping-process-interruption control unit  515  instructs the IFU shutdown unit  509  to interrupt the stopping process (Step S 608 ). The IFU shutdown unit  509  inquires of the IFU stopping-process-interruption determination unit  511  whether the currently-performed stopping process can be interrupted (Step S 609 ). 
     The IFU stopping-process-interruption determination unit  511  inquires of each of the control units  501  to  506  about the state of the stopping process (Step S 610 ). Moreover, the IFU stopping-process-interruption determination unit  511  detects whether there is an abnormality in the laser printer  100  by the sensor  428  (Step S 611 ). If there is an abnormality in the laser printer  100  (YES at Step S 611 ), the IFU emergency stopping unit  513  instructs the control units  501  to  506  to continue the stopping process (Step S 623 ), so that the control units  501  to  506  continue the stopping process regardless of the printing process instruction. If there is no abnormality in the laser printer  100  (NO at Step S 611 ), the IFU stopping-process-interruption determination unit  511  specifies the stage of the stopping process being performed based on the state of the stopping process received from the control units  501  to  506 , determines whether the stopping process can be interrupted in accordance with the specified stage, and notifies the IFU shutdown unit  509  of the result of the determination (Step S 612 ). 
     An operation to be performed in accordance with the result of the determination is explained for each stage specified by the IFU stopping-process-interruption determination unit  511 . 
     If the IFU stopping-process-interruption control unit  515  instructs to interrupt the stopping process, i.e., receives the printing process instruction, in the operation B 1  shown in  FIG. 3 , the IFU stopping-process-interruption determination unit  511  determines that the operation B 1  can be interrupted because no problem (abnormality) will occur due to the interruption of the stopping process of the transfer unit  206 . Thereafter, only the operation A 4  is performed. 
     If the IFU stopping-process-interruption control unit  515  instructs to interrupt the stopping process in the operations B 2  to B 5  shown in  FIG. 3 , the IFU stopping-process-interruption determination unit  511  determines that the stopping process of the charging unit  202  and the developing unit  204  cannot be interrupted because developer (toner) may adhere to the photosensitive element  201  due to the interruption of the stopping process of the charging unit  202  and the developing unit  204  resulting in abnormality of the laser printer  100 . Thereafter, when the operations B 2  to B 5  are completed, the operations A 4  to A 9  are performed. 
     If the IFU stopping-process-interruption control unit  515  instructs to interrupt the stopping process in the operation B 6  shown in  FIG. 3 , the IFU stopping-process-interruption determination unit  511  determines that the operation B 6  can be interrupted because no problem will occur due to the interruption of the stopping process of the neutralization unit  208 . Thereafter, only the operations A 4  to A 9  are performed. 
     If the IFU stopping-process-interruption control unit  515  instructs to interrupt the stopping process in the operation B 8  shown in  FIG. 3 , the IFU stopping-process-interruption determination unit  511  determines that the stopping process of the photosensitive element  201  cannot be interrupted because the rotation of the photosensitive element  201  may become unstable resulting in abnormality of the laser printer  100 . Thereafter, when the operation B 8  is completed, the operations A 2  to A 9  are performed. 
     If the IFU stopping-process-interruption control unit  515  instructs to interrupt the stopping process in the operation B 9  shown in  FIG. 3 , the IFU stopping-process-interruption determination unit  511  determines that the operation B 9  can be interrupted because no problem will occur due to the interruption of the stopping process of the polygon mirror. Thereafter, the operations A 1  to A 9  are performed. At this time, the polygon mirror rotates by its rotational inertia, so that polygon mirror comes to rotate stably in a shorter time than the case of driving the polygon mirror in a stopped state. 
     Accordingly, the printing process can be started earlier than the case of starting the laser printer  100  after the stopping process is completely finished. 
     The IFU shutdown unit  509  instructs each of the control units  501  to  506  to interrupt the stopping process during or after the stopping process of the stage specified by the IFU stopping-process-interruption determination unit  511  based on the result of the determination notified from the IFU stopping-process-interruption determination unit  511  (Step S 613 ). Each of the control units  501  to  506  interrupts the stopping process based on the instruction from the IFU shutdown unit  509  (Step S 614 ). 
     Upon completing interruption of the stopping process (Step S 615 ), the IFU shutdown unit  509  notifies the IFU control unit  514  of the completion of the interruption (Step S 616 ). Then, the IFU stopping-process-interruption control unit  515  instructs the IFU start-up unit  508  to restart the image forming unit  200  (Step S 617 ). 
     The IFU start-up unit  508  instructs each of the control units  501  to  506  to perform the starting process (the operations A 1  to A 9  in  FIG. 3 ) (Step S 618 ). In response to the instruction, each of the control units  501  to  506  performs the starting process of a corresponding control target such as the photosensitive element  201  and the charging unit  202  (Step S 619 ). At this time, if the control target is operating, each of the control units  501  to  506  need not restart the control target. Upon completing the starting process (including the case where the control target need not be restarted), the control units  501  to  506  notifies the IFU start-up unit  508  of the completion of the starting process (Step S 620 ). 
     Accordingly, when the control target such as the photosensitive element  201  and the charging unit  202  is operating, the image forming unit  200  can be started in a shorter time than the case of starting the image forming unit  200  in a completely stopped state. 
     Upon completing the whole starting process, the IFU start-up unit  508  notifies the IFU control unit  514  of the completion (Step S 621 ), and the IFU stopping-process-interruption control unit  515  instructs the IFU printing unit  512  to perform the printing process to restart printing by the printing control unit  518  (Step S 622 ). 
     Accordingly, when there is a print instruction during the stopping process, the stopping process is interrupted, so that the time for starting the printing can be shortened. 
     Moreover, when the stopping process in a specified stage cannot be interrupted, the image forming unit  200  is restarted after completing the stopping process in the specified stage, so that the time for starting the printing can be shortened without causing any trouble in the image forming unit  200 . 
     In the present embodiment, each of the control units  501  to  506  starts the stopping process upon receiving the stopping process instruction from the IFU shutdown unit  509 ; however, it is not limited thereto. For example, each of the control units  501  to  506  can start the stopping process after standing by for a certain period of time. In this case, when the control units  501  to  506  receive the print instruction while in the standby state, the printing process can be performed promptly. 
     If an abnormality occurs in the image forming unit  200 , the stopping process is continued even if the IFU stopping-process-interruption control unit  515  receives the printing process instruction, so that the image forming unit  200  can be appropriately controlled. 
     When the stopping process is interrupted after receiving the printing process instruction during the stopping process or standby for the stopping process, the stopping process is restarted after completing the printing process. Therefore, the image forming unit  200  can be controlled efficiently. 
     A modified example of the first embodiment is explained. In the modified example, the stopping process in each stage is associated with interruption determination information indicating whether it is possible to interrupt the stopping process in each stage that is determined based on an abnormality occurred in the image forming unit  200  by the interruption, which is stored in the engine ROM  426  as a table. It is determined whether the stopping process in a stage specified by the IFU stopping-process-interruption determination unit  511  can be interrupted in accordance with the interruption determination information associated with the specified stage in the table. Only the components different from those of the first embodiment are explained below. 
       FIG. 7  is a schematic diagram illustrating the table stored in the engine ROM  426  as an example, in which the stopping process in each stage, the interruption determination information, and starting process information are associated with each other. The starting process information indicates whether each unit of the image forming unit  200  needs the starting process when the stopping process is interrupted based on the interruption determination information. 
     The IFU stopping-process-interruption determination unit  511 , after specifying a stage of the stopping process, specifies the interruption determination information associated with the stopping process in the specified stage in  FIG. 7 . Then, the IFU stopping-process-interruption determination unit  511  determines whether the stopping process in the specified stage can be interrupted based on the specified interruption determination information. 
     For example, when the IFU stopping-process-interruption determination unit  511  receives the printing process instruction during the standby period before the stopping process is started, the IFU stopping-process-interruption determination unit  511  determines that the stopping process can be interrupted in accordance with “INTERRUPTIBLE” associated with “WAITING FOR SHUTDOWN” in  FIG. 7 . The IFU start-up unit  508  does not instruct the control units  501  to  506  to perform the starting process because “UNNECESSARY” associated with “WAITING FOR SHUTDOWN” in  FIG. 7  indicates that the starting process is not needed. 
     When the IFU stopping-process-interruption determination unit  511  receives the printing process instruction while the stopping process of the transfer unit  206  is performed, the IFU stopping-process-interruption determination unit  511  determines that the stopping process can be interrupted in accordance with “INTERRUPTIBLE” associated with “STOPPING PROCESS OF TRANSFER UNIT” in  FIG. 7 . Because “NECESSARY” for the transfer unit  206  that is associated with “STOPPING PROCESS OF TRANSFER UNIT” in  FIG. 7  indicates that the starting process is necessary for the transfer unit  206 , the IFU start-up unit  508  instructs the transfer-unit control unit  503  to perform the starting process. 
     When the IFU stopping-process-interruption determination unit  511  receives the printing process instruction while the stopping process of the charging unit  202  is performed, the IFU stopping-process-interruption determination unit  511  determines that the stopping process cannot be interrupted, i.e., the IFU stopping-process-interruption determination unit  511  needs to wait for completion of the stopping process of the charging unit  202 , in accordance with “WAITING FOR COMPLETION OF STOPPING PROCESS” associated with “STOPPING PROCESS OF CHARGING UNIT” in  FIG. 7 . Because “NECESSARY” for the charging unit  202  and the transfer unit  206  that is associated with “STOPPING PROCESS OF CHARGING UNIT” in  FIG. 7  indicates that the starting process is necessary for the charging unit  202  and the transfer unit  206 , the IFU start-up unit  508  instructs the transfer-unit control unit  503  and the charging-unit control unit  505  to perform the starting process. 
     According to the modified example, it is determined whether the stopping process in a specified stage can be interrupted by referring to the table shown in  FIG. 7 , and an advantage the same as that of the first embodiment can be obtained. 
     In a second embodiment, an image forming unit that includes an image forming unit  800  is provided to the body unit  110 . The image forming unit  800  is a tandem color electrophotographic printer that includes an intermediate transfer body. Only the components different from those of the first embodiment are explained below. 
       FIG. 8  is a schematic diagram of the image forming unit  800 . The image forming unit  800  includes photosensitive elements  801 Y,  801 M,  801 C, and  801 K for four colors of yellow (Y), magenta (M), cyan (C), and black (K), developing units  802 Y,  802 M,  802 C, and  802 K, charging units  803 Y,  803 M,  803 C, and  803 K, cleaning units  804 Y,  804 M,  804 C, and  804 K, intermediate transfer rollers  805 Y,  805 M,  805 C, and  805 K, an exposing unit  806 , and an intermediate transfer belt  807 . The developing units  802 Y,  802 M,  802 C, and  802 K develop latent images formed on the photosensitive elements  801 Y,  801 M,  801 C, and  801 K into toner images with different colors, respectively. The intermediate transfer belt  807  is an endless belt that rotates in a direction indicated by an arrow A in  FIG. 8  and onto which the toner images are primary transferred in a superimposed manner. In the explanation below that is common to all of the photosensitive elements  801 Y,  801 M,  801 C, and  801 K, all of the developing units  802 Y,  802 M,  802 C, and  802 K, all of the charging units  803 Y,  803 M,  803 C, and  803 K, all of the cleaning units  804 Y,  804 M,  804 C, and  804 K, all of the intermediate transfer rollers  805 Y,  805 M,  805 C, and  805 K, they are referred to as photosensitive elements  801 , developing units  802 , charging units  803 , and cleaning units  804 , intermediate transfer rollers  805 , respectively. 
     The photosensitive elements  801  are arranged in a line above the intermediate transfer belt  807  along the direction indicated by the arrow A in  FIG. 8 . The developing unit  802 , the charging unit  803 , the cleaning unit  804 , and the intermediate transfer roller  805  are provided around the photosensitive element  801 . 
     The photosensitive element  801  is driven to rotate in a direction indicated by an arrow B in  FIG. 8 , during which the charging unit  803  applies a bias to the photosensitive element  801 , whereby the surface of the photosensitive element  801  is uniformly charged. Then, the exposing unit  806  radiates a laser beam to the uniformly charged surface of the photosensitive element  801  to form a latent image thereon. The developing unit  802  develops the latent image into a toner image of a corresponding color. 
     The intermediate transfer roller  805  is arranged to oppose the photosensitive element  801  with the intermediate transfer belt  807  therebetween, and the intermediate transfer belt  807  is driven to rotate in a state of being nipped between the intermediate transfer roller  805  and the photosensitive element  801 . The intermediate transfer belt  807  is supported by a driving roller  808  as a secondary transfer roller and a supporting roller  810 . The intermediate transfer belt  807  can be supported by more than two rollers; however, the height of the image forming unit  800  can be suppressed by supporting the intermediate transfer belt  807  by only two rollers, enabling to reduce the size of the image forming unit  800 . 
     The toner images of respective colors on the photosensitive elements  801  are sequentially transferred onto the intermediate transfer belt  807  by the intermediate transfer rollers  805  in a superimposed manner, whereby a full-color toner image is formed on the intermediate transfer belt  807 . Thereafter, the cleaning unit  804  cleans the surface of the corresponding photosensitive element  801  to remove residual toner on the surface. 
     A pair of registration rollers  812  as a sheet-conveying unit conveys a sheet to a portion between the driving roller  808  and a transfer roller  809  as a transfer unit at a predetermined timing. The driving roller  808  and the transfer roller  809  are arranged to oppose each other with the intermediate transfer belt  807  therebetween. When the sheet is conveyed between the driving roller  808  and the transfer roller  809 , the full-color toner image on the intermediate transfer belt  807  is collectively transferred onto the sheet by the action of the transfer roller  809 . Then, the sheet is conveyed to a fixing unit  813 , in which the full-color toner image is fixed to the sheet by heat and pressure. The sheet with the full-color toner image fixed thereto is discharged onto a discharge tray (not shown). Toner remaining on the intermediate transfer belt  807  after transferring the full-color toner image onto the sheet is removed by a cleaning unit  811 . The image forming unit  800  includes neutralization units (not shown) each of which is arranged between the cleaning unit  804  and the charging unit  803  to neutralize the surface of the photosensitive element  801 . 
     The procedure and the content of the printing operation of the image forming unit  800  when the image forming unit performs the printing process are explained with reference to  FIG. 9 .  FIG. 9  is a timing chart of the printing operation of the image forming unit  800 . When printing is started, the image forming unit starts the printing operation shown in  FIG. 9  to cause the image forming unit  800  to be in a printable state. The printing operation is classified into the starting process, the printing process, and the stopping process. 
     The starting process of the image forming unit  800  is explained with reference to  FIG. 9 . 
     First, a laser-printer control unit causes a polygon mirror for exposure (not shown) to rotate stably (A 1 ). Then, the laser-printer control unit drives the photosensitive element  801  and the intermediate transfer belt  807  to rotate stably by the driving unit and the driving roller  808  (A 2 ). Then, the laser-printer control unit drives the neutralization unit (A 3 ). Then, the laser-printer control unit applies a preparation bias for transferring an image to the intermediate transfer roller  805  and the transfer roller  809  (A 4 ). Then, the laser-printer control unit causes the charging unit  803  to apply a primary charging bias to the surface of the photosensitive element  801  to charge the surface (A 5 ). When the charged surface of the photosensitive element  801  reaches the developing unit  802 , the laser-printer control unit causes the developing unit  802  to apply a primary developing bias to toner to charge the toner (A 6 ). Then, the laser-printer control unit causes the charging unit  803  to apply a secondary charging bias to the surface of the photosensitive element  801 , for example, 50 milliseconds after applying the primary charging bias for stabilizing the voltage (A 7 ). Then, in the similar manner, the laser-printer control unit causes the developing unit  802  to apply a secondary developing bias to the toner, for example, 50 milliseconds after applying the primary developing bias for stabilizing the voltage (A 8 ). When the entire surface of the photosensitive element  801  is charged with the secondary charging bias applied by the charging unit  803 , i.e., when the photosensitive element  801  rotates one revolution after the start of the charging with the secondary charging bias, the image forming unit  800  is ready for printing (A 9 ). 
     In the present embodiment, the starting process is classified into five stages: the operation A 1 , the operation A 2 , the operation A 3 , the operation A 4 , and the operations A 5  to A 9 , and is performed in a stepwise manner in accordance with the stages. 
     The printing process of the image forming unit  800  is explained. 
     For printing after starting up the image forming unit  800 , the laser-printer control unit causes the exposing unit  806  to expose the charged surfaces of the photosensitive elements  801  thereby forming a latent image on each of the photosensitive elements  801 , and causes toner to adhere to the latent image by each of the developing units  802  thereby forming a toner image. The toner images on the photosensitive elements  801  are transferred onto the intermediate transfer belt  807  by the intermediate transfer rollers  805  to form a full-color toner image on the intermediate transfer belt  807 . At this time, toner which is not transferred onto the intermediate transfer belt  807  and remains on the photosensitive elements  801  is removed from the photosensitive elements  801  by the cleaning units  804 . Thereafter, the full-color toner image on the intermediate transfer belt  807  is conveyed to the transfer roller  809  to be transferred onto a sheet conveyed by the registration rollers  812 . The laser-printer control unit causes the neutralization units to neutralize the surfaces of the photosensitive elements  801 , so that the potential of the surface of each of the photosensitive elements  801  becomes zero to be ready for the next latent image formation. The neutralization of the surface of each of the photosensitive elements  801  is performed by the neutralization unit for the size of the sheet. The laser-printer control unit performs the above process for the required number of sheets. Upon completing the printing process, the laser-printer control unit shuts down the image forming unit  800 . 
     The stopping process of the image forming unit  800  is explained with reference to  FIG. 9 . 
     First, the laser-printer control unit causes the intermediate transfer roller  805  and the transfer roller  809  to stop applying the preparation bias. The laser-printer control unit does not move to the next operation (B 2 ) until the application of the preparation bias is completely stopped (B 1 ). Then, the laser-printer control unit changes the output of the charging unit  803  to the primary charging bias (B 2 ). When the surface of the photosensitive element  801  charged with the primary charging bias reaches the developing unit  802 , the laser-printer control unit changes the output of the developing unit  802  to the primary developing bias (B 3 ). Then, the laser-printer control unit causes the charging unit  803  to stop applying the primary charging bias, for example, 50 milliseconds after changing the output of the charging unit  803  to the primary charging bias for stabilizing the voltage (B 4 ). Then, in the similar manner, the laser-printer control unit causes the developing unit  802  to stop applying the primary developing bias, for example, 50 milliseconds after changing the output of the developing unit  802  to the primary developing bias for stabilizing the voltage. Consequently, the photosensitive element  801  and the toner both are not charged (B 5 ). Then, the laser-printer control unit causes the neutralization unit to continue neutralizing the photosensitive element  801  until the surface of the photosensitive element  801  to which the charging unit  803  has stopped applying the primary charging bias reaches the neutralization unit, and thereafter stops driving of the neutralization unit (B 6 ). Then, the laser-printer control unit waits until the surface of the photosensitive element  801  to which the developing unit  802  has stopped applying the primary developing bias reaches the cleaning unit  804 , and thereafter moves to the next operation (B 8 ) (B 7 ). The laser-printer control unit stops driving of the photosensitive element  801  and the intermediate transfer belt  807 . The laser-printer control unit does not move to the next operation (B 9 ) until the driving of the photosensitive element  801  is completely stopped (B 8 ). Then, the laser-printer control unit stops driving of the polygon mirror. When the driving of the polygon mirror is completely stopped, the stopping process is completed (B 9 ). 
     In the present embodiment, the stopping process of the image forming unit  800  is classified into six stages: the operation B 1 , the operations B 2  to B 5 , the operation B 6 , the operation B 7 , the operation B 8 , and the operation B 9 , and is performed in a stepwise manner in accordance with the stages. 
     Control modules of the image forming unit  800  are explained with reference to  FIG. 10 . The control modules are realized by the engine CPU  421  executing the control programs stored in the engine ROM  426 .  FIG. 10  is a block diagram of the control modules for controlling the image forming unit  800 . The configuration of the control modules is the same as that in the first embodiment except that a transfer-roller control unit  1001  for controlling the intermediate transfer roller  805  and the transfer roller  809  is provided instead of the transfer-unit control unit  503 , and an intermediate-transfer-belt control unit  1002  for controlling driving of the intermediate transfer belt  807  by the driving roller  808  is added. 
     The flow of each of the printing process, the starting process, and the stopping process in the image forming unit  800  by the control modules is the same as that in the flowchart of  FIG. 6  in the first embodiment, so that explanation thereof is omitted. 
     According to the present embodiment, even when the image forming unit that includes the image forming unit  800  as a color electrophotographic printer is provided to the body unit  110 , an advantage the same as that of the first embodiment can be obtained. 
     According to one aspect of the present invention, even when an image forming process instruction was received in the stopping process, a stopping process can be interrupted and a starting process can be restarted. Thus, printing can be performed corresponding to the instruction promptly. 
     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.