Patent Publication Number: US-8983310-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2012-018197, filed on Jan. 31, 2012. The entire subject matter of the application is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     Aspects of the present invention relate to an image forming apparatus. 
     2. Related Art 
     A multicolor image forming apparatus provided with chargers the number of which is equal to the number of colors of developer (e.g., yellow, magenta, cyan and black) is known. One of such image forming apparatuses is configured to reduce the number of components and to downsize by sharing a high-voltage power supply which applies a high voltage to each charger. 
     SUMMARY 
     In order to suppress decrease of image quality, it is preferable that a current flowing through each charger is controlled to be a target value. However, when the high-voltage power supply is shared as described above, it becomes impossible to individually adjust voltage levels to be applied to the respective chargers. In such a case, a designer may try to control voltage levels to be applied to the chargers so that a current flowing through a selected one of the chargers becomes a target value. However, if a process cartridge including a control target charger is not attached to the image forming apparatus, the current does not change even when control of voltage levels to be applied to the charger is performed. As a result, a possibility arises that a failure of the high-voltage power supply occurs due to a fact that an excessively high voltage level is applied to the chargers. In view of the circumstances, the image forming apparatus is required to be able to judge whether the process cartridge is attached thereto. 
     Aspects of the present invention are advantageous in that they provide an image forming apparatus which is configured to share a voltage supply circuit and is capable of judging whether a process cartridge is attached to the image forming apparatus. 
     According to an aspect of the invention, there is provided an image forming apparatus, comprising: a charge voltage application circuit configured to be connected to a plurality of chargers in a process cartridge and to apply a voltage to the plurality of chargers; a current detection unit configured to detect a current flowing through each of the plurality of chargers; and a controller. The controller judges that the process cartridge is not attached to the image forming apparatus when the current smaller than a first threshold is detected by the current detection unit in a state where the charge voltage application circuit generates a predetermined voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a cross sectional view generally illustrating an internal configuration of a printer according to an embodiment. 
         FIG. 2  is a cross sectional view generally illustrating a internal configuration of the printer around a process cartridge for black. 
         FIG. 3  schematically illustrates a configuration of a charger. 
         FIG. 4  is a block diagram illustrating an electrical configuration of the printer. 
         FIG. 5  is a circuit diagram illustrating an electrical configuration of a high voltage power supply. 
         FIG. 6  is a graph illustrating transition of a grid current of each channel. 
         FIG. 7  is a graph illustrating transition of a grid current of each channel. 
         FIG. 8  is a graph illustrating transition of a grid current of each channel. 
         FIG. 9  is a flowchart illustrating a control flow for a high voltage power supply. 
         FIG. 10  is a flowchart illustrating a control flow for a high voltage power supply according to a second embodiment. 
         FIG. 11  is a flowchart illustrating a control flow for a high voltage power supply according to a third embodiment. 
         FIG. 12  is schematically illustrates another example of a configuration of a printer. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     Hereafter, a first embodiment according to the invention will be described with reference to  FIGS. 1 to 9 . 
     1. Overall Configuration of Printer 
     In the following explanations, when a component is separately explained for each color, a suffix B (black), Y (yellow), M (magenta) or C (cyan) is added to a reference symbol of each component. 
     As shown in  FIG. 1 , the printer  1  includes a paper supply unit  3 , an image formation unit  5 , a conveying mechanism  7 , a fixing unit  9 , a belt cleaning mechanism  20  and a high voltage power supply  100 . The paper supply unit  3  is provided at a lowermost position in the printer  1 , and includes a tray  17  storing sheet-like medium  15  (e.g., a sheet of paper or an OHP sheet), and a pick-up roller  19 . The sheet-like medium  15  stored in the tray  17  is picked up one by one by the pick-up roller  19 , and is conveyed to the conveying mechanism  7  via a conveying roller  11  and a registration roller  12 . 
     The conveying mechanism  7  is configured to convey the sheet-like medium  15 , and is provided on the upper side of the paper supply unit  3  in the printer  1 . The conveying mechanism  7  includes a drive roller  31 , a driven roller  32  and a belt  34 . The belt  34  is provided to extend between the drive roller  31  and the driven roller  32 . When the drive roller  31  rotates, a surface of the belt  34  facing photosensitive drums  41 B,  41 Y,  41 M and  41 C moves from the right side to the left side in  FIG. 1 . As a result, the sheet-like medium  15  supplied from the registration roller  12  is conveyed to a position under the image formation unit  5 . 
     The belt  34  is provided with four transfer rollers  33 B,  33 Y,  33 M and  33 C respectively corresponding to the four photosensitive drums  41 B,  41 Y,  41 M and  41 C. The transfer rollers  33  are arranged at positions respectively facing the photosensitive drums  41 B,  41 Y,  41 M and  41 C while sandwiching the belt  34  therebetween. 
     The image formation unit  5  includes four process cartridges  40 B,  40 Y,  40 M and  40 C and four exposure units  49 B,  49 Y,  49 M and  49 C. The process cartridges  40 B,  40 Y,  40 M and  40 C are arranged in a line along a conveying direction of the sheet-like medium  15  (i.e., the left and right direction in  FIG. 1 ). 
     The process cartridges  40  have the same configuration. Each process cartridge  40  includes the photosensitive drum ( 41 B,  41 Y,  41 M or  41 C) which is a photosensitive body for a corresponding color, a toner case  43  storing toner which is a developer for a corresponding color, a developer roller  45  and the charger ( 50 B,  50 Y,  50 M or  50 C). 
     Each photosensitive drum ( 41 B,  41 Y,  41 M or  41 C) is configured to have a photosensitive layer having a positive electrostatic property on a substrate made of, for example, aluminum. In this configuration, the substrate made of aluminum is connected to the ground of the printer  1 . 
     The development roller  45  is provided to face a supply roller  46  under the toner case  43 . The development roller  45  serves to positively charge the toner by friction caused by rotation when the toner passes through a position between the development roller  45  and the supply roller  46 , and to supply the toner to the photosensitive drum ( 41 B,  41 Y,  41 M or  41 C) as a thin uniform layer. 
     Each of the chargers  50 B,  50 Y,  50 M and  50 C is a scorotron charger, and includes a shield case  51 , a wire  53  and a grid electrode  55  made of metal. The shield case  51  has a shape of a long square tube extending in a rotation axis direction of the photosensitive drum  41 . In the shield case  51 , a side facing the photosensitive drum  441  is opened as a discharge opening  52 . 
     The wire  53  is made of, for example, a tungsten wire. The wire  53  is provided to extend in the rotation axis direction (the left and right direction in  FIG. 3 ) in the shield case  51 , and is applied a high voltage of 5 kV to 8 kV from a charge voltage application circuit  200  which is described later. Through application of the high voltage, the wire  53  causes corona discharge in the shield case  51 . Ions caused by the corona discharge flow, as a discharge current, from the discharge opening  52  to the photosensitive drum  41  side, and thereby the surface of the photosensitive drum  41  is charged positively and uniformly. 
     To the discharge opening  52  of the shield case  51 , the plate-like grind electrode  55  having slits and holes is attached. By applying a voltage to the grid electrode  55  and controlling the applied voltage, it becomes possible to control the charge voltage of the photosensitive drum  41 . 
     A wire cleaner  57  is provided for each of the chargers  50 B,  50 Y,  50 M and  50 C. The wire cleaner  57  is configured to be able to freely slide along the wire  53 . By moving the wire cleaner  57  to reciprocate along the wire  53  through operation by an operator, dust on the wire  53  can be removed. The exposure unit ( 50 B,  50 Y,  50 M and  50 C) includes a plurality of light emitting devices (e.g., LEDs or laser sources) arranged in a line along the rotation axis direction of the photosensitive drum ( 41 B,  41 Y,  41 M or  41 C). By emitting light in accordance with image data externally input, the exposure unit  49  serves to form an electrostatic latent image on the surface of the photosensitive drum ( 41 B,  41 Y,  41 M or  41 C). 
     A sequence of image formation process by the laser printer  1  described above is simply explained as follows. When the printer  1  receives print data D from a host apparatus (see  FIG. 4 ), a print process is started. The surface of each of the photosensitive drums  41 B,  41 Y,  41 M and  41 C is charged positively and uniformly by the charger ( 50 B,  50 Y,  50 M or  50 C). Then, the laser light is emitted to the photosensitive drums  41 B,  41 Y,  41 M and  41 C from the respective exposure units  49 . As a result, electrostatic latent images corresponding to the print data are respectively formed on the photosensitive drums  41 B,  41 Y,  41 M and  41 C. That is, on the surface of the photosensitive drum ( 41 B,  41 Y,  41 M or  41 C) charged positively and uniformly, a potential of a portion irradiated with the laser light decreases. 
     By rotation of the development roller  45 , the toner which is held on the development roller  45  and changed positively is supplied to the electrostatic latent image formed on the photosensitive drum  41  ( 41 B,  41 Y,  41 M or  41 C). As a result, the electrostatic latent image on the photosensitive drum  41  ( 41 B,  41 Y,  41 M or  41 C) is visualized, and a toner image by reversal development is formed on the surface of the photosensitive drum  41  ( 41 B,  41 Y,  41 M or  41 C). 
     Concurrently with the above described process for forming the toner image, a process in which the sheet-like medium  15  is conveyed is performed. That is, by rotation of the pick-up roller  19 , the sheet-like medium  15  is sent out from the tray  17  to a paper conveying path Y. The sheet-like medium  15  sent out to the paper conveying path Y is conveyed to a transfer position (where the photosensitive drum  41  and the transfer roller  33  contact with each other). 
     When the sheet-like medium  15  passes the transfer position, the toner images of the respective colors held on the photosensitive drums  41  are sequentially transferred to the sheet-like medium  15  to be overlapped with each other. Thus, a color toner image (a developer image) is formed on the sheet-like medium  15 . Then, when the sheet-like medium  15  passes the fixing unit  9  provided on the rear side of the belt  34 , the transferred toner image (developer image) is heat-fixed and is discharged on a discharge tray  60 . 
     2. Configuration of High Voltage Power Supply 
     As shown in  FIG. 5 , the high voltage power supply  100  includes the charge voltage application circuit  200 , a PWM signal smoothing circuit  210 , a charge voltage detection circuit  240 , constant voltage circuits  250 B,  250 Y,  250 M and  250 C, grid current detection units  260 B,  260 Y,  260 M and  260 C, and a control unit  110 . 
     The PWM signal smoothing circuit  210  is an integration circuit including a resistance and a capacitor. The PWM signal smoothing circuit  210  smoothes a PWM signal S 1  outputted from a PWM port P 0  of the control unit  110 , and outputs the smoothed signal to a base of a transistor Tr 1  provided in the charge voltage application circuit  200 . 
     The charge voltage application circuit  200  serves to generate a high voltage of approximately 6 kV to 8 kV from the input voltage of DC24V, and to apply the high voltage to each charger  50 . In this embodiment, the charge voltage application circuit  200  employs a self-excitation flyback converter (RCC). The charge voltage application circuit  200  includes a transformer  201 , a smoothing circuit  203  provided on the secondary side of the transformer  201 , the transistor Tr 1  provided on the primary side of the transformer  201 , and a feedback coil  205 . 
     The transistor Tr 1  serves to perform switching of the transformer  201 , and an emitter thereof is connected to the ground, and a collector thereof is connected to a primary side winding of the transformer  201 . To the base of the transistor Tr 1 , the PWM signal smoothing circuit  201  is connected via the feedback coil  205 . 
     To an output line Lo of the charge voltage application circuit  200 , the wires  53  of the chargers  50 B,  50 Y,  50 M and  50 C are connected in common. With this configuration, the output voltage Vo of the charge voltage application circuit  200  is applied to the wires  53  of the chargers  50 B,  50 Y,  50 M and  50 C. 
     The charge voltage detection circuit  240  detects the output voltage Vo of the charge voltage application circuit  200 , and includes an auxiliary winding  241  provided on the primary side of the transformer  201 , and an integration circuit  243  having a resistance and a capacitor. The charge voltage detection circuit  240  is connected to an A-D port A 0  of the control unit  110  so that data of the output voltage Vo of the charge voltage application circuit  200  is input to the control unit  110 . 
     As shown in  FIG. 5 , in this embodiment, connection lines L 1  to L 4  are provided respectively for the chargers  50 B,  50 Y,  50 M and  50 C, and the grid electrodes  55  of the chargers  50 B,  50 Y,  50 M and  50 C are connected to the ground via the lines L 1  to L 4 . On each of the lines L 1  to L 4 , the constant voltage circuit  250  and the grid current detection unit  260  are provided. 
     Each of the constant voltage circuits  250 B,  250 Y,  250 M and  250 C includes three zener diodes, and serves to keep the voltage of the grid electrode  55  of each of the chargers  50 B,  50 Y,  50 M and  50 C at the three-fold value of the breakdown voltage of a single zener diode (e.g., 250V×3). 
     The grid current detection circuits  260 B,  260 Y,  260 M and  260 Y respectively include resistances R 1  to R 4  which are sequentially connected to the constant voltage circuits  250 B,  250 Y,  250 M and  250 C, respectively. A connection point between the resistance (R 1 , R 2 , R 3 , R 4 ) and the constant voltage circuit ( 250 B,  250 Y,  250 M,  250 C) is connected via a signal line to the A-D port (A 1 , A 2 , A 3 , A 4 ) provided on the control unit  110 . With this configuration, the voltage in proportional to the current flowing through the connection line (L 1 , L 2 , L 3 , L 4 ) is input to the A-D port (A 1 , A 2 , A 3 , A 4 ). Therefore, by reading the level of the input voltage of the A-D port (A 1 , A 2 , A 3 , A 4 ), it is possible to detect the grid current Ig of the charger ( 50 B,  50 Y,  50 NM,  50 C). 
     The control unit  110  has the function of controlling the grid current Ig flowing through the grid electrode  55  of the charger  50 , and the function of making a judgment on whether the process cartridge is attached when the high voltage power supply  100  is started up. The control unit  110  includes a PWM port P 0  and five A-D ports A 0  to A 4 . 
     Control of the grid current Ig is conducted by outputting the PWM signal S 1  from the PWM port P 0  and adjusting the output voltage Vo of the charge voltage application circuit  200 . 
     The control unit  110  may be configured such that a CPU is embedded therein or may be formed as an ASIC (Application Specific Integrated Circuit). The control unit  110  includes a built-in non-volatile memory in which programs for control of the high voltage power supply  100  and various types of data for making a judgment on whether the process cartridge  40  is attached. The various types of data include data (a) to (c) indicated below. 
     (a) Data of a target value of the grid current Ig (250 μA) 
     (b) Data of a first threshold of the grid current Ig (80 μA) 
     (c) Data of a second threshold of the grid current Ig (160 μA) 
     The grid current Ig is approximately proportional to the discharge current flowing from the charger  50  to the photosensitive drum  41 , and serves as an indicator for measuring the level of the discharge current flowing through the photosensitive drum  41 . That is, if the grid current Ig is equal to 250 μA which is the target value, the discharge current flowing through the photosensitive drum  41  is a reference level which is a proper level concerning the charge amount of the photosensitive drum  41  to keep the image quality. 
     In the following explanations, channels mean the chargers  50 B,  50 Y,  50 M and  50 C. Specifically, the chargers  50 B,  50 Y,  50 M and  50 C are referred to as CH 1 , CH 2 , CH 3  and CH 4 , respectively. 
     3. Judgment on Whether Process Cartridge  40  is Attached 
     The printer according to the embodiment has the function of judging whether the process cartridge  40  is attached. More specifically, in the case where the process cartridge  40  is attached, the grid current of approximately 250 μA flows through the grid electrode  55  of each of the chargers  50 B,  50 Y,  50 M and  50 C when a predetermined high voltage is applied to the wire  53  of each of the chargers  50 B,  50 Y,  50 M and  50 C (see  FIG. 6 ). 
     On the other hand, when the process cartridge  40  of one of the four channels CH 1  to CH 4  is not attached, the grid current Ig does not flows through the channel CH which in not attached. For example, when the process cartridge  40 Y of the channel CH 2  is not attached, the level of the grid current Ig of the channel CH 2  becomes almost zero (see  FIG. 7 ). 
     Therefore, by measuring the grid current Ig of each channel when the predetermined high voltage is generated by the charge voltage application circuit  200  and thereby judging whether the grid current Ig flows for each of the channels, it is possible to make a judgment on whether the process cartridge  40  is attached. 
     The predetermined high voltage generated by the charge voltage application circuit  200  may be the voltage (7 kV to 8 kV) to be applied during image formation or may be the voltage (e.g., 5 kV) which is slightly lower than the voltage to be applied during image formation but is able to generate a detectable grid current Ig. 
     In the case where the charge voltage application circuit  200  is controlled to generate the high voltage larger than or equal to 5 kV, when the process cartridge  40  is attached to the printer  1 , at least the grid current Ig larger than equal to 80 μA flows through the channel of the attached process cartridge  40 . Therefore, in this embodiment, 80 μA is set as a threshold (the first threshold), and when the grid current Ig is smaller than 80 μA, it is judged that “the process cartridge  40  is not attached” excepting the following cases. 
     When the wire  53  is short-circuited, i.e., when the wire  53  is broken and contacts with the grid electrode  55 , the voltage of the wire  53  of the charger is brought to the voltage level equal to the grid electrode  55 , i.e., 800V to 900V. Similarly, the output voltage Vo of the charge voltage application circuit  200  is brought to 800V to 900V. 
     In this case, the voltage of each of the chargers  50 B,  50 Y,  50 M and  50 C connected in common to the charge voltage application circuit  200  decreases to 800V to 900V. Therefore, for example, when the wire  53  is short-circuited in the charger  50 B, the chargers  50 Y,  50 M and  50 C which are not short-circuited are brought to the state of not discharging. Therefore, in this case, the grid current Ig becomes almost zero, i.e., becomes smaller than 80 μA as in the case where the process cartridge  10  is not attached. 
     For this reason, in the printer  1 , when the grid current Ig is smaller than 80 μA, the output voltage Vo of the charge voltage application circuit  200  is compared with 3 kV, i.e., a reference voltage. When the output voltage Vo of the charge voltage application circuit  200  is larger than or equal to 3 kV, it is judged that “the process cartridge  40  is not attached” (see S 150  which is described later). When the output voltage Vo of the charge voltage application circuit  200  is smaller than 3 kV, it is judged that “the process cartridge  40  is attached” (see step S 170  which is described later). Specifically, with respect to the channel CH of which grid current Ig is the maximum, it is judged that the wire  53  of the charger  50  is short-circuited. 
     Furthermore, in the printer  1 , the second threshold (160 μA) is also used as the threshold of the grid current Ig, in addition to the above described first threshold (80 μA). The second threshold of 160 μA is the threshold for judging a dirty state of the wire  53 . That is, when the wire  53  of the charger  50  becomes larger, the resistance of the wire  53  increases accordingly. Therefore, the level of the grid current Ig flowing through the charger  50  decreases from the target value of 250 μA. For example, when the channel CH 2  is dirty, the level of the grid current Ig of the channel CH 2  decreases to the level approximately equal to the half of the target value of 250 μA (see  FIG. 8 ). 
     For this reason, in the printer  1 , the grid current Ig of each charger is measured by each grid current detection unit  260  when the predetermined high voltage is generated by the charge voltage application circuit  200 . When the grid current Ig is 80 μA to 160 μA, it is judged that the wire  53  of the channel is dirty (see S 120  which is described later). 
     4. Control Flow 
     Hereafter, a control flow for the high voltage power supply  100  to be executed by the control unit  110  is explained with reference to  FIG. 9 . As shown in  FIG. 4 , when print data D is outputted from a host apparatus, such as a host computer, the print data D is received by the printer  1  via an interface IF. Then, a print process start command is sent to the control unit  110  of the high voltage power supply  100  from a main control unit  80  which totally controls the printer  1 . 
     As a result, the control unit  110  executes the control flow of the high voltage power supply  100  shown in  FIG. 9 . The control flow of the high voltage power supply  100  is divided into control in a start-up stage for starting up the high voltage power supply  100  and control in a print stage executed after starting up of the high voltage power supply  100 . In the following, the control in the start-up stage is explained, and thereafter the control in the print stage is explained. 
     Control in Start-Up Stage 
     When the control flow of for the high voltage power supply  100  is started, the control unit  110  sets the target value of the grid current Ig to 250 μA. Then, the control unit  110  outputs the PWM signal S 1  through the PWM port P 0 . As a result, the charge voltage application circuit  200  is started up, and the voltage is generated. Then, the control unit  110  monitors the grid current Ig by calculating the grid current Ig of each channel CH from the input voltage of each of the A-D ports A 1  to A 4  (S 20 ). 
     Next, the control unit  110  judges whether a predetermined time T has elapsed from the start of the control flow. The predetermined time T is a time for starting up the high voltage power supply  100 , and is approximately 100 mSEC to 200 mSEC. During a time period from the start of the control flow to the time at which the predetermined time T has elapsed, i.e., when the elapsed time is smaller than the predetermined time T, the judgment result in step S 30  is YES. 
     When the judgment result in step S 30  is YES, the process proceeds to step S 40 . In step S 40 , a process for selecting the channel CH having the maximum current is executed by the control unit  110 . Specifically, the grid currents Ig of the channels are compared with each other, and the grid current Ig having the maximum value is selected. In the following, explanation is made assuming that the channel CH 1  is selected. 
     When the channel CH having the maximum current is selected in step S 40 , the process proceeds to step S 50 . In step S 50 , the grid current Ig of the selected channel is subjected to constant current control. Since the channel CH 1  is selected, the output voltage Vo of the charge voltage application circuit  200  is adjusted so that the grid current Ig of the channel all is brought to the target value of 250 μA. 
     Then, the process proceeds to step S 60 , and it is judged whether the process for applying the charge voltage has finished. If the process for applying the charge voltage has not finished, the judgment result of S 60  becomes NO. When the judgment result of step S 60  is NO, the process returns to step S 20 , and step S 20  and steps following step S 30  are executed. 
     During the time period from start of output by the charge voltage application circuit  200 , steps S 20 , S 30  (judgment result; YES), S 40 , S 50  and S 60  (judgment result; NO) are repeated. 
     With this configuration, until the predetermined time T has elapsed, the grid current Ig of “CH 1 ” is subjected to the constant current control to be the target value of 250 μA. Therefore, as shown in  FIG. 6 , the grid current Ig of the channel CH 1  becomes stable approximately at the target value of 250 μA within the predetermined time T, and the grid currents Ig of the other channels CH 2  to CH 4  become stable at the target value of 250 μA or the level smaller than 250 μA. 
     In  FIG. 6 , of the four channels CH, only the channel CH 1  having the maximum grid current Ig and the channel CH 2  having the minimum grid current Ig are shown, and the grid currents Ig of the other channels CH 3  and CH 4  are omitted. 
     When the predetermined time T has elapsed from start of output by the charge voltage application circuit  200 , the judgment result in step S 30  by the control unit  110  becomes NO. When the judgment result in step S 30  is NO, the process proceeds to step S 70 . 
     In step S 70 , the control unit  110  judges whether the grid current Ig is larger than or equal to 160 μA. Specifically, the grid currents of the channels CH 1  to CH 4  are compared with the second threshold of 160 μA. When the grid currents of the channels CH 1  to CH 4  are larger than or equal to 160 μA, the judgment result becomes YES. On the other hand, when there is a channel having the grid current Ig smaller than 160 μA, the judgment result becomes NO. When the judgment result of step  370  is YES, the process proceeds to step S 80 . When the judgment result of step  370  is NO, the process proceeds to step S 100 . In the following, first, the explanation is given assuming that the judgment result of step S 70  is YES. Then, the explanation is given for the case where the judgment result in step S 70  is NO. 
     Control in Print Stage (The Case Where the Grid Current Ig is Larger Than or Equal to 160 μA at the Stage Where the Predetermined Time T has Elapsed) 
     When the judgment result of step S 70  is YES, the process proceeds to step S 80 . In step S 80 , the control unit  100  executes a process for selecting the channel having the minimum current. Specifically, the grid currents Ig of the channels CH 1  to CH 4  are compared with each other, and the channel CH having the minimum grid current Ig is selected. In the following, explanation is given assuming that the “CH 2 ” is selected. 
     When the channel CH having the minimum current is selected in step S 80 , the process proceeds to step S 50 . In step S 50 , the control unit  110  executes the constant current control for the grid current Ig of the selected channel. Since in this case the CH 2  is selected, the control unit  110  adjusts the output voltage Vo of the charge voltage application circuit  200  so that the grid current of the CH 2  becomes 250 μA. 
     Then, the process proceeds to step S 60 , and the process for applying the charge voltage is judged to be finished. When the process for applying the charge voltage is not finished, the judgment result in step S 60  is NO. When the judgment result in step S 60  is NO, the process returns to step S 20 , and step S 20  and steps following S 30  are executed. 
     When the grid current Ig is larger than or equal to 160 μA, the judgment result of step S 70  becomes YES. Therefore, after the predetermined time T has elapsed, S 20 , S 30  (judgment result: NO), S 70  (judgment result: YES), S 80 , S 50  and S 60  (judgment result: NO) are repeated. 
     Therefore, after the predetermined time T has elapsed, the control unit  110  executes the constant current control so that the grid current of the channel having the minimum current (the “CH 2 ” in this example) is kept at the target value of 250 μA. With this configuration, as shown in  FIG. 6 , the grid current Ig of the channel CH 2  selected as the control target is brought to the stable state at approximately 250 μA, and the grid currents of the other channels CH 1 , CH 3  and CH 4  are brought to a stable state at the target value of 250 μA or the level larger than 250 μA. 
     As described above, in the printer  1 , after the predetermined time T has elapsed, the channel having the minimum current is selected and is subjected to the constant current control at the target value of 250 μA. Therefore, the grid currents Ig of all the channels CH 1  to CH 4  become larger than or equal to the target value of 250 μA. Therefore, the discharge current larger than or equal to the reference level flows from each of the scorotron charger  50  to the photosensitive drum  41 . Accordingly, the charge amount of the photosensitive drum  41  of each channel is brought to the proper level for maintaining the image quality. 
     Then, when the photosensitive drum  41  is brought to the state of being appropriately charged after the predetermined time T has elapsed, a print process for printing the print data on the sheet-like medium is executed. When the print process is finished, the process for applying the charged voltage is finished, and the judgment result of step S 60  becomes YES. When the judgment result of S 60  is YES, the process proceeds to step S 90 , and the process for stopping the charge voltage application circuit  200  is executed by the control unit  110 . Thus, the control flow for the high voltage power supply  100  is finished. 
     The Case where the Grid Current Ig is Smaller than 160 μA when the Predetermined Time T has Elapsed 
     Next, when the channel whose grid current Ig is smaller than 160 μA is found at the time when the predetermined time has elapsed from start of output by the charge voltage application circuit  200  (S 70 : NO), the process proceeds to step S 100 . 
     In step S 100 , the control unit  110  executes judges whether the grid current Ig of the channel for which the judgment result in step S 70  was NO is larger than or equal to 80 μA. When the judgment result in step S 100  is YES, the process proceeds to step S 110 . 
     In step S 110 , the control unit  110  executes a process for stopping the charge voltage application circuit  200 . In step S 120 , the control unit  110  indicates a wire cleaning error. For example, in the case, a message indicating “please clean the wire” is displayed on a display (not shown) of the printer  1 . By displaying such an error message, it becomes possible to urge a user to clean the wire. 
     On the other hand, when the judgment result of step S 100  is NO, the process proceeds to step S 130 . In step S 130 , the control unit  110  judges whether the output voltage Vo of the charge voltage application circuit  200  (i.e., the charge voltage) is larger than or equal to 3 kV. Specifically, in this case the control unit  110  judges whether the output voltage Vo of the charge voltage application circuit  200  is larger than or equal to 3 kV based on the detection value of the charge voltage detection circuit  240 . 
     When the output voltage Vo of the charge voltage application circuit  200  is larger than or equal to 3 kV, the process proceeds to step S 140 . In step S 140 , the control unit  110  stops the charge voltage application circuit  200 . Then, the process proceeds to step S 150 . In step S 150 , the control unit  110  indicates a “no process cartridge error”. For example, the control unit  110  displays an error message, the example, indicating that “process cartridge is not attached” on a display (not shown) of the printer  1 . By displaying such an error message, it becomes possible to urge a user to attach the process cartridge  40 . 
     On the other hand, when the output voltage Vo of the charge voltage application circuit  200  is smaller than 3 kV, the process proceeds to step S 160 . In step S 160 , the control unit  110  judges that the scorotron charger is short-circuited, and stops the charge voltage application circuit  200 . Then, the process proceeds to step S 170 . In step S 170 , the control unit  110  indicates a short-circuit error. For example, in this case, an error message indicating “the scorotron charger is short-circuited” is displayed on a display (not shown) of the printer  1 . By displaying such an error message, it becomes possible to urge a user to exchange the process cartridge (exchange the scorotron charger). 
     5. Advantages 
     As described above, the printer  1  according to the embodiment is able to judge whether the process cartridge is attached. Furthermore, the printer  1  is able to judge the short-circuit of the charger  50  and the dirty state of the wire  53 . Since the printer  1  makes these three types of judgments based on the grid current and the output voltage Vo of the charge voltage application circuit  200 , the number of components and the cost can be reduced in comparison with the case where a dedicated sensor is used to make these judgments. 
     Furthermore, in this embodiment, the judgment on whether the process cartridge is attached is executed at the timing when the process is switched from the start-up stage to the print stage. Since in such a case the judgment is made when the grid current Ig of each channel is brought to the stable state, it is possible to precisely execute the judgment on whether the process cartridge is attached. This also applies to the judgment regarding the short-circuited state of the charger and the dirty state of the wire. 
     The printer  1  according to the embodiment selects the channel CH having the maximum current is selected and executes the constant current control for the grid current in the start-up stage, and then selects the channel having the minimum current and executes the constant current control for the grid current Ig. 
     If control in the start-up stage is performed such that the printer according to the embodiment selects the channel CH having the minimum current and executes the constant-current control for the grid current Ig of the selected channel as in the case of the control in the print stage, when a channel for which it is judged that the process cartridge is not attached is found, the channel is selected as a control target. In this case, in order to increase the grid current Ig, the feedback control is performed so that output of the charge voltage application circuit  200  is increased, and as a result the output of the charge voltage application circuit  200  might exceed the upper limit. 
     In this regard, the printer  1  according to the embodiment is configured to select the channel CH having the maximum current and execute the constant-current control for the selected channel. Therefore, even when the channel whose process cartridge  40  is not attached is found, such a channel is not selected as a control target. Accordingly, it becomes possible to suppress increase of the output voltage Vo of the charge voltage application circuit  200  in the start-up stage. 
     Although, in the above described first embodiment, the judgment on the short-circuit and the dirty state of the wire is made in addition to the judgment on whether the process cartridge  40  is attached, the judgment on the short-circuit and the dirty state of the wire may be omitted (i.e., steps S 100 , S 110 , S 120 , S 130 , S 160  and S 170  may be omitted). In step S 70 , it is judged whether the grid current Ig is larger than or equal to the first threshold of 80 μA. When the grid current Ig is larger than or equal to 80 μA, the process proceeds to step S 80 , the channel having the minimum current is selected, and the constant-current control is performed for the selected channel CH (S 50 ). On the other hand, when the grid current Ig is smaller than 80 μA, the process proceeds to step S 140 , the output of the charge voltage application circuit  200  is stopped, and the error indicating that the process cartridge is not attached is displayed (S 150 ). 
     Second Embodiment 
     Hereafter, a second embodiment of the invention is explained with reference to  FIG. 10 . In the first embodiment, the channel CH having the maximum current is selected and the constant-current control (feedback control) of the grid current Ig is performed in the start-up stage. In the second embodiment, the charge voltage application circuit  200  is controlled while fixing an instruction value (i.e., feedforward control is performed). Specifically, the charge voltage application circuit  200  is controlled while fixing, at 50%, the PWM value of the PWM signal S 1  to be outputted from the PWM port P 0  of the control unit  110  (step S 13  in  FIG. 10 ). 
     By thus controlling the charge voltage application circuit  200  while fixing the instruction value, it becomes possible to suppress increase of the output voltage Vo of the charge voltage application circuit  200  even when the channel for which the process cartridge  40  is not attached exists, as in the case of the first embodiment. Regarding the method according to the second embodiment where the charge voltage application circuit  200  is controlled while fixing the instruction value (the feedforward control), the time period within which the output becomes stable is short in comparison with the feedback control. Therefore, it is possible to shorten the predetermined time T in comparison with the first embodiment. 
     Similarly to the first embodiment, the printer  1  according to the second embodiment judges whether the process cartridge  40  is attached by measuring the grid currents Ig of the channels CH 1  to CH 4  and comparing the first threshold with the grid currents Ig at the time of switching from the start-up stage to the print stage. 
     In the second embodiment, the control flow is configured by simplifying the control flow of the first embodiment, and the process (step S 130  in the first embodiment) for judging whether the charger  50  is short-circuited and the process after detection of the short-circuit (steps S 160  and S 170  in the first embodiment) are omitted. 
     Although, in the second embodiment, the judgment on the dirty state of the wire  53  is performed in addition to the judgment on whether the process cartridge  40  is attached, the judgment on the dirty state of the wire  53  may be omitted (steps  100  to S 120  are omitted). In this case, the control flow shown in  FIG. 10  may be altered as follows. In step S 70 , it is judged whether the grid current Ig is larger than or equal to the first threshold of 80 μA. When the grid current Ig is larger than or equal to 80 μA, the process proceeds to step S 80 , the channel CH having the minimum current is selected, and the constant-current control is performed for the selected channel (S 85 ). On the other hand, when the grid current Ig is smaller than 80 μA, the process proceeds to step S 140 , output of the charge voltage application circuit  200  is stopped, and then an error message indicating that the process cartridge  40  is not attached is displayed (S 150 ). 
     Third Embodiment 
     In the following, the third embodiment of the invention is described with reference to  FIG. 12 . In the first embodiment, the channel CH having the maximum current is selected and the constant-current control (feedback control) of the grid current Ig is performed in the start-up stage. The third embodiment is different from the first embodiment in that, in the start-up stage, the constant-current control is performed for the charge voltage application circuit  200  so that the output voltage Vo becomes 5 kV. Specifically the constant-current control is performed for the charge voltage application circuit  200  based on the detected value of the charge voltage detection circuit  240 . 
     By thus performing the constant-current control for the charge voltage application circuit  200  in the start-up stage, it is possible to suppress increase of the output voltage Vo of the charge voltage application circuit  200  even when the channel for which the process cartridge  40  is not attached is found, as in the case of the first embodiment. 
     Similarly to the first embodiment, the printer  1  according to the third embodiment makes a judgment on whether the process cartridge  40  is attached by measuring the grid currents Ig of the channels CH 1  to CH 4  and comparing the grid currents Ig with the first threshold. 
     In the third embodiment, the control flow is configured by simplifying the control flow of the first embodiment. In the third embodiment, the process for judging the short-circuit of the charger (S 130  in the first embodiment) and the process after detection of the short-circuit (steps S 160  and S 170  in the first embodiment) are omitted. 
     Although, in the third embodiment, the judgment on the dirty state of the wire  53  is performed in addition to the judgment on whether the process cartridge  40  is attached, the judgment on the dirty state of the wire  53  may be omitted (steps  100  to S 120  are omitted). In this case, the control flow shown in  FIG. 11  may be altered as follows. In step S 70 , it is judged whether the grid current Ig is larger than or equal to the first threshold of 80 μA. When the grid current Ig is larger than or equal to 80 μA, the process proceeds to step S 80 , the channel CH having the minimum current is selected, and the constant-current control is performed for the selected channel (S 85 ). On the other hand, when the grids current Ig is smaller than 80 μA, the process proceeds to step S 140 , output of the charge voltage application circuit  200  is stopped, and then an error message indicating that the process cartridge  40  is not attached is displayed (S 150 ). 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. 
     (1) In the above described first to third embodiments, the printer  1  is configured such that one charger  50  is associated with one photosensitive drum  41  (i.e., the photosensitive drums  41  are provided respectively for the colors). However, the present invention can be applied to a printer configured such that a plurality of chargers  310  and  320  are associated with one photosensitive drum  300  as shown in  FIG. 12  (where after toner images of respective colors are overlaid on the photosensitive drum  300 , the toner images are simultaneously transferred to the sheet-like medium). In  FIG. 12 , a component assigned a reference number  315  is a process cartridge (developing unit) associated with the charger  310 , and a component assigned a reference number  325  is a process cartridge associated with the charger  320 .
 
(2) In the first to third embodiments, the grid current detection units  260 B,  260 Y,  260 M and  260 C are provided respectively for the channels CH 1  to CH 45 . However, a common grid current detection unit may be shared between the channels. In this case, the grid currents of the channels may be detected in a time division manner.
 
(3) In the first to third embodiments, the process cartridge  40  is configured to include the toner case  43 , the development roller  45  and hr charger  50 . However, it should be understood that it is sufficient for the process cartridge to include at least the charger  50 . As the charger  50 , a corotron charger may be used in place of the scorotron charger.
 
(4) In the above described first to third embodiments, whether the process cartridge is attached and the dirty state of the wire are judged by comparing the grid current Ig of each channel with the first threshold and the second threshold. In the first to third embodiments, the target value of the grid current Ig is set to 250 μA, the first threshold is set to 80 μA, and the second threshold is set to 160 μA. However, it should be understood that these values are examples, and the numerical values may be determined by considering the electric property of the printer  1 .
 
     It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. Aspects of the invention may be implemented in computer software as programs storable on computer-readable media including but not limited to RAMs, ROMs, flash memory, EEPROMs, CD-media, DVD-media, temporary storage, hard disk drives, floppy drives, permanent storage, and the like.