Patent Publication Number: US-8995850-B2

Title: Image forming apparatus with cartridge-replacement indicator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2012-201864 filed on Sep. 13, 2012, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Illustrative aspects of the present invention relate to an image forming apparatus having a control device configured to indicate a necessity to replace a developer container. 
     BACKGROUND 
     For an image forming apparatus, there has been proposed a control device which indicates a necessity to replace a cartridge in a case where a remaining amount of toner in the cartridge detected by an optical sensor is less than a defined value, or in a case where a number of revolutions of a developing roller detected by a revolution counter exceeds a defined value. 
     SUMMARY 
     Illustrative aspects of the present invention appropriately determine a timing for replacement of each cartridge in an image forming apparatus having a plurality of cartridges. 
     According to one illustrative aspect of the present invention, there is provided an image forming apparatus comprising: a first cartridge configured to contain developer of a first color; a second cartridge configured to contain developer of a second color; a processor; and memory. The memory store computer readable instructions that, when executed by the processor, causing the image forming apparatus to indicate a necessity to replace the first cartridge and the second cartridge. The processor is configured to cause the image forming apparatus to: indicate the necessity to replace the first cartridge by using at least information based on a first use amount, which corresponds to an amount of use of the developer of the first color contained in the first cartridge, and information based on a number of printed sheets; and indicate the necessity to replace the second cartridge by using at least information based on a second use amount, which corresponds to an amount of use of the developer of the second color contained in the second cartridge, without using the information based on the number of printed sheets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view schematically illustrating a color printer according to a first exemplary embodiment; 
         FIG. 2  is an explanatory view illustrating components such as various sensors and a control device; 
         FIG. 3  is a flow chart illustrating a method of determining a timing for replacement of a first developing cartridge; 
         FIG. 4  is a flow chart illustrating a method of determining a timing for replacement of a second developing cartridge; 
         FIG. 5  is an explanatory view illustrating an example of a change in a first light reception amount and a first total number of revolutions; 
         FIG. 6  is a flow chart illustrating a method of determining a timing for replacement according to a second exemplary embodiment; 
         FIG. 7  is a flow chart illustrating a process of setting a threshold value for a light reception amount; 
         FIG. 8  is an explanatory view illustrating an example of a change in a first light reception amount and a first total number of revolutions according to the second exemplary embodiment; 
         FIG. 9  is a flow chart illustrating a method of determining a timing for replacement according to a third exemplary embodiment; 
         FIG. 10  is a flow chart illustrating another process of setting a threshold value for a light reception amount; and 
         FIG. 11  is an explanatory view illustrating an example of a change in a first light reception amount and a first total number of revolutions according to the third exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;General Overview&gt; 
     In the above-described related-art method, since an influence of paper dust coming into a cartridge during printing control is not considered, a printing defect may result from the influence of paper dust before the remaining amount of toner or the number of revolutions of a developing roller satisfies the above-described condition. In order to solve this problem, for example, it may be considered to add a condition for the number of printed sheets as a condition for determining replacement of a cartridge. However, in a color printer having a plurality of cartridges, there exist a cartridge which is easy for the paper dust to enter, and a cartridge which is difficult for the paper dust to enter due to layout of the cartridges. Thus, it may be difficult to perform appropriate control even if a condition for the number of printed sheets is simply added. 
     Therefore, illustrative aspects of the present invention appropriately determine a timing for replacement of each cartridge in an image forming apparatus having a plurality of cartridges. 
     According to a first illustrative aspect of the present invention, there is provided an image forming apparatus comprising: a first cartridge configured to contain developer of a first color; a second cartridge configured to contain developer of a second color; a processor; and memory. The memory store computer readable instructions that, when executed by the processor, causing the image forming apparatus to indicate a necessity to replace the first cartridge and the second cartridge. The processor is configured to cause the image forming apparatus to: indicate the necessity to replace the first cartridge by using at least information based on a first use amount, which corresponds to an amount of use of the developer of the first color contained in the first cartridge, and information based on a number of printed sheets; and indicate the necessity to replace the second cartridge by using at least information based on a second use amount, which corresponds to an amount of use of the developer of the second color contained in the second cartridge, without using the information based on the number of printed sheets. 
     According to this configuration, for example, in a case where the first cartridge is configured to be easier for powder generated from sheets to enter, compared to the second cartridge, it is possible to appropriately determine a timing for replacement of each cartridge. 
     Consequently, information based on the number of printed sheets (information corresponding to an amount of sheet powder) is used to indicate the necessity to replace the first cartridge. Thus, for example, even before the condition for the amount of use is satisfied, when the condition for the number of printed sheets is satisfied, the necessity to replace the first cartridge is indicated. Therefore, before the first cartridge is influenced by sheet powder entering the first cartridge, it is possible to replace the first cartridge, and to improve image quality. 
     Further, the information based on the number of printed sheets is not used to determine the timing for replacement of the second cartridge. Therefore, it is possible to appropriately determine the timing for replacement of the second cartridge unlikely to be influenced by sheet powder, according to the amount of use of the developer. 
     According to a second illustrative aspect of the present invention, the processor is configured to cause the image forming apparatus to: in a case where any one condition of a plurality of conditions including at least a first condition that the first use amount exceeds a first threshold value and a second condition that the number of printed sheets exceeds a third threshold value is satisfied, indicate the necessity to replace the first cartridge; and in a case where the second use amount exceeds a second threshold value, indicate the necessity to replace the second cartridge. 
     According to a third illustrative aspect of the present invention, the plurality of conditions includes a third condition that a driving amount of a rotator for performing conveyance of the developer exceeds a fourth threshold value. According to a fourth illustrative aspect of the present invention, the third condition is that a driving amount of a rotator of the first cartridge for performing conveyance of the developer of the first color exceeds the fourth threshold value. 
     According to this configuration, the condition for the driving amount of the rotator is also used to determine the timing for replacement of each cartridge. Therefore, even in a case where the driving amount of the rotator increases and the developer is deteriorated, it is possible to urge replacement of the corresponding cartridge. 
     According to a fifth illustrative aspect of the present invention, the processor is configured to cause the image forming apparatus to: in a case where the first use amount exceeds a first threshold value, indicate the necessity to replace the first cartridge; in a case where the second use amount exceeds a second threshold value, indicate the necessity to replace the second cartridge; and in a case where the number of printed sheets exceeds a third threshold value, change the first threshold value to a sixth threshold value that is smaller than the first threshold value. 
     According to a sixth illustrative aspect of the present invention, the processor is configured to cause the image forming apparatus to: in a case where the first use amount exceeds a first threshold value, indicate the necessity to replace the first cartridge; in a case where the second use amount exceeds a second threshold value, indicate the necessity to replace the second cartridge; and in a case where the number of printed sheets exceeds a third threshold value, perform correction on the first use amount such that the corresponding amount of use increases than normal. 
     According to a seventh illustrative aspect of the present invention, the image forming apparatus further comprises an optical sensor configured to measure the first use amount and the second use amount. 
     According to an eighth illustrative aspect of the present invention, the number of printed sheets is counted by a page counter. 
     According to a ninth illustrative aspect of the present invention, the image forming apparatus further comprises a sheet sensor configured to detect passage of a sheet, wherein the page counter is configured to count the number of printed sheets on a basis of an output value from the sheet sensor. 
     According to this configuration, the timing for replacement of the first cartridge is determined on the basis of the number of sheets actually printed. Therefore, it is possible to efficiently suppress the influence of sheet powder. 
     According to a tenth illustrative aspect of the present invention, the rotator is a developing roller, and the driving amount is the number of revolutions of the developing roller. 
     According to the present invention, in an image forming apparatus having a plurality of cartridges, it is possible to appropriately determine a timing for replacement of each cartridge. 
     &lt;Exemplary Embodiments&gt; 
     Exemplary embodiments of the present invention will now be described with reference to the drawings. 
     [First Exemplary Embodiment] 
     Subsequently, a first exemplary embodiment of the present invention will be described. In the following description, first, the overall configuration of a color printer which is an example of an image forming apparatus will be described, and then the characterizing portions of the present invention will be described in detail. 
     In the following description, directions of a color printer refer to the directions as seen from a user facing to the color printer during its use. To be more specific, referring to  FIG. 1 , a left-side direction and a right-side direction of the drawing sheet are referred to as a “front side” and a “rear side” of the color printer, respectively. Also, a direction away from a viewer of  FIG. 1  is referred to as a “left side”, and a direction toward the viewer of  FIG. 1  as a “right side”. An upward and downward direction in  FIG. 1  is referred to as a “vertical direction” or an “upward and downward direction” as it is. 
     As shown in  FIG. 1 , a color printer  1  includes a sheet feeding unit  20  configured to feed a sheet P, an image forming unit  30  configured to form an image on the fed sheet P, and a sheet discharging unit  90  configured to discharge the sheet P, on which the image has been formed, inside a main apparatus body  10 . 
     The sheet feeding unit  20  includes a sheet feeding tray  21  and a sheet conveying device  22 . The sheet feeding tray  21  is configured to accommodate the sheet P. The sheet conveying device  22  is configured to convey the sheet P from the sheet feeding tray  21  to the image forming unit  30 . 
     The image forming unit  30  includes a scanner unit  40 , a process unit  50 , a transfer unit  70  and a fixing unit  80 . 
     The scanner unit  40  is provided at an upper portion of the inside of the main apparatus body  10 . The scanner unit  40  includes a laser beam emitting unit, a polygonal mirror, lenses, a reflective mirror, etc. (not shown). Further, in the scanner unit  40 , laser beams are irradiated onto the surfaces of photosensitive drums  51  which are examples of photosensitive elements (which will be described later), through paths shown by alternate long and two short dashes lines in  FIG. 1 , during high-speed scanning. 
     The process unit  50  is configured to be installable in and removable from the main apparatus body  10  through an opening  10 A which is formed by opening a front cover disposed on a front surface of the main apparatus body  10 . The process unit  50  includes a drawer  60 , four photosensitive drums  51  that are rotatably supported by the drawer  60 , four developing cartridges  52  that are removably installed as examples of cartridges in the drawer  60  such that the developing cartridges correspond to the photosensitive drums  51 , respectively. 
     Incidentally, in this specification and the drawings, in a case of specifying the photosensitive drums  51 , the developing cartridges  52  and the like corresponding to colors of toner (e.g., developer), symbols of K, Y, M and C corresponding to black, yellow, magenta and cyan are added to their reference symbols. Further, in a case of describing the photosensitive drums  51  and so on, regardless of the colors of toner, the above-described symbols of K, Y, M and C are omitted. 
     The photosensitive drums  51  are arranged along a front-rear direction (e.g., a predetermined direction) in a state where the process unit  50  is installed in the main apparatus body  10 . In the drawer  60 , other components such as four chargers  55  configured to expose the photosensitive drums  51  are appropriately provided. Further, each of the developing cartridges  52  includes a developing roller  53  configured to feed toner to a corresponding photosensitive drum  51 , a toner container  54  configured to accommodate the toner, and other components such as a known layer-thickness regulating blade and a feeding roller. 
     Inside each developing cartridge  52 , as shown in  FIG. 2 , an agitator  300  configured to stir toner T contained in the corresponding developing cartridge is provided. The agitator  300  includes a rotating spindle  310 , a sheet attachment portion  320  and a sheet member  330 . 
     The rotating spindle  310  is a shaft extending along the axial direction of a corresponding developing roller  53  (e.g., a left-right direction), and both ends of the rotating spindle  310  are rotatably supported on left and right side walls  521  of the corresponding developing cartridge  52 . The sheet attachment portion  320  is formed to extend radially outward from the rotating spindle  310 , and on the fore end of the sheet attachment portion  320 , the sheet member  330  is fixed by adhering or the like. 
     The sheet member  330  is configured to stir the toner T while its fore end is in sliding contact with the wall of the toner container  54 , according to the rotation of the agitator  300 . 
     Further, at the left and right side walls  521  of each developing cartridge  52 , a pair of transmission windows  522  configured to transmit light of a toner sensor  130  (which will be described later) is provided. 
     Referring to  FIG. 1  again, the transfer unit  70  is provided between the sheet feeding unit  20  and the process unit  50 . The transfer unit  70  includes a driving roller  71 , a driven roller  72 , a conveyance belt  73  and transfer rollers  74  which are examples of transfer members. 
     The driving roller  71  and the driven roller  72  are disposed in parallel with an interval in the front-rear direction. The conveyance belt  73 , which is composed of an endless belt, is stretched between the driving roller  71  and the driven roller  72 . The outer surface of the conveyance belt  73  comes into contact with the photosensitive drums  51 . Further, inside the conveyance belt  73 , four transfer unit  70  are provided to face the photosensitive drums  51  with the conveyance belt  73  interposed therebetween. During transferring, a transfer bias is applied to each transfer roller  74  by constant current control. 
     The fixing unit  80  is disposed behind the process unit  50  and the transfer unit  70 . The fixing unit  80  includes a heating roller  81  and a pressing roller  82 . The pressing roller  82  is disposed to face the heating roller  81  and is configured to press the heating roller  81 . 
     In the image forming unit  30  having the above-described configuration, first, the surfaces of the photosensitive drums  51  are uniformly charged by the chargers  55 , and then are exposed by the scanner unit  40 . As a result, the potentials of exposed portions decrease, whereby electrostatic latent images based on image data are formed on the photosensitive drums  51 . Subsequently, the toner contained in the developing cartridges  52  is fed to the electrostatic latent images formed on the photosensitive drums  51  by the developing rollers  53 , whereby toner images are carried on the photosensitive drums  51 . 
     Next, a sheet P fed on the conveyance belt  73  passes between the photosensitive drums  51  and the transfer rollers  74 , whereby the toner images formed on the photosensitive drums  51  are transferred onto the sheet P. Then, the sheet P passes between the heating roller  81  and the pressing roller  82 , whereby the toner image transferred on the sheet P is thermally fixed. 
     The sheet discharging unit  90  includes a plurality of conveying rollers  91  configured to convey the sheet P. The sheet P with the toner image transferred and thermally fixed thereon is conveyed by the conveying rollers  91 , and is discharged to the outside of the main apparatus body  10 . 
     A control device  200  is provided inside the main apparatus body  10 . The control device  200  is configured to control the color printer  1  having the above-described configuration. 
     (Image Forming Unit  30  and Control Device  200 ) 
     Four image forming sections  30 K,  30 Y,  30 M and  30 C are sequentially arranged in order of black, yellow, magenta and cyan from an upstream side to a downstream side of a conveyance direction of a sheet P. In other words, in the present exemplary embodiment, among a plurality of image forming sections  30 , an image forming section  30 K for black (a first color) which is disposed on the most upstream side corresponds to a first image forming section, and an image forming section  30 Y,  30 M and/or  30 C for a color other than black (a second color) such as yellow, magenta and cyan corresponds to a second image forming section. Incidentally, in the following description, if necessary, the term “first” is added to the heads of names of members and the like corresponding to black, and the term “second” is added to the heads of names of members and the like corresponding to yellow, magenta and cyan. 
     The first developing cartridge  52 K is disposed on the most upstream side in the conveyance direction of the sheet P, such that the first developing cartridge  52 K is easier for paper dust generated from the sheet P to enter, as compared to the second developing cartridges  52 Y,  52 M and  52 C disposed on the downstream side. Specifically, paper dust having not been recovered by a belt cleaning roller (not shown) is brought to the positions of the photosensitive drums  51  by rotation of the conveyance belt  73 . Then, paper dust attached to the photosensitive drums  51  is attached to the developing rollers  53  and enters the developing cartridges  52 . Since the belt cleaning roller is disposed below the conveyance belt  73 , a large amount of the paper dust having not been recovered by the belt cleaning roller enters the developing cartridge  52 K disposed on the most upstream side. Then, the timing for replacement of each developing cartridge  52  is determined by the control device  200 . 
     The control device  200  includes, for example, a CPU (which is an abbreviation for central processing unit), a RAM (which is an abbreviation for random access memory), a ROM (which is an abbreviation for read only memory) and an input/output circuit. The control device  200  is configured to perform computing processes on the basis of inputs from various sensors, the contents of a print command, and programs, data and the like stored in the ROM, thereby performing a known warm-up operation, printing control, etc. Here, the warm-up operation means a printing preparation operation, specifically, control to rotate the agitators  300 , the developing rollers  53  and the like for a predetermined period before feeding of the sheet P from the sheet feeding tray  21  starts. 
     Further, in the present exemplary embodiment, especially, the control device  200  is configured to perform control to determine a timing for replacement of each developing cartridge  52  on the basis of inputs from a sheet sensor  110 , which serves as an example of a sheet sensor, and the toner sensors  130  (shown in  FIGS. 1 and 2 ), and the numbers of revolutions of the developing rollers  53  counted by a revolution counter  220  (which will be described later). Incidentally, the toner sensors  130  are provided on a one-to-one basis for the developing cartridges  52 ; however, in  FIG. 2 , for the sake of convenience, only one toner sensor  130  is representatively shown. 
     The sheet sensor  110  is configured to detect passage of a sheet P. The sheet sensor  110  is disposed on a conveyance path between the sheet conveying device  22  and the image forming unit  30  (the conveyance belt  73 ). A detection value detected by the sheet sensor  110  is output to the control device  200 . 
     Incidentally, the sheet sensor  110  has a known structure. To provide a brief explanation, the sheet sensor  110  is composed of a detection arm  111 , which is configured to swing by being in contact with the sheet P, and an optical sensor configured to detect swinging of the detection arm. 
     The toner sensors  130  are optical sensors. Each toner sensors  130  include a light emitting unit  131  and a light receiving unit  132 . The light emitting unit  131  and the light receiving unit  132  are disposed to face each other such that a pair of transmission windows  522  provided at the left and right side walls  521  of a corresponding developing cartridge  52  is interposed therebetween in the left-right direction. 
     As shown by a broken line in  FIG. 2 , light emitted from the light emitting unit  131  enters the corresponding developing cartridge  52  through one transmission window  522  and is received by the light receiving unit  132  through the other transmission window  522 . The light receiving unit  132  is an element whose output voltage value varies according to the intensity of received light. The light receiving unit  132  is configured to receive light and output a light reception signal (a light reception amount) to the control device  200  (specifically, a determining unit  230 ). More specifically, the toner T is stirred by the agitators  300 , and then the light emitting units  131  emit light. Then, the light receiving units  132  detect light reception amounts corresponding to the amounts of use of the toner T. 
     Here, a first light reception amount, which is detected by a toner sensor  130  for the first developing cartridge  52 K, corresponds to a first use amount showing the amount of use of toner stored in the first developing cartridge  52 K. Also, a second light reception amount detected by a toner sensor  130  for the second developing cartridge (e.g.,  52 Y,  52 M and  52 C) corresponds to a second use amount showing the amount of use of toner stored in the second developing cartridge ( 52 Y,  52 M and  52 C). 
     The control device  200  is configured to include a page counter  210 , the revolution counters  220  and the determining unit  230 . 
     The page counter  210  is configured to count the number of sheets P printed using the first developing cartridge  52 K. Specifically, the page counter  210  is configured to count the number of sheets P printed using the first developing cartridge  52 K (that is, the total number of printed sheets) on the basis of an output value from the sheet sensor  110 . 
     More specifically, the page counter  210  starts to count the total number of printed sheets from when a new first developing cartridge  52 K is installed in the main apparatus body  10 , and counts up the total number of printed sheets whenever printing control is performed, until the life of the corresponding first developing cartridge  52 K comes to an end (that is, replacement timing of the corresponding first developing cartridge comes). Further, when the lifetime of the first developing cartridge  52 K comes (when performing indication of a necessity to replace, which will be described later), the page counter  210  resets the total number of printed sheets to zero. The resetting of the total number of printed sheets may be performed when a new developing cartridge is installed. 
     Incidentally, determination on whether a developing cartridge  52  is new may be performed, for example, using a known new-article detecting mechanism which is provided to the corresponding developing cartridge, or using a known contactless memory which is provided to the corresponding developing cartridge. 
     The total number of printed sheets counted by the page counter  210  is output to the determining unit  230 . 
     Each revolution counter  220  has a function of counting the total number of revolutions of a corresponding developing roller  53 , as an example of a driving amount. Specifically, similarly to the page counter  210 , each revolution counter  220  counts up the total number of revolutions of a corresponding developing cartridge  52  from when the corresponding developing cartridge  52  is new to when the life of the corresponding developing cartridge  52  comes to an end. Further, each revolution counter  220  resets the total number of revolutions to zero upon indicating a necessity to replace the corresponding developing cartridge  52  is performed. The resetting of the total number of revolutions may be performed when a new developing cartridge is installed. 
     Here, a first total number of revolutions of the first developing cartridge  52 K counted by a revolution counter  220  corresponds to a first driving amount. A second total number of revolutions of the second developing roller (e.g.,  53 Y,  53 M and  53 C) counted by the revolution counter  220  corresponds to second driving amounts. Further, each total number of revolutions counted by the revolution counter  220  is output to the determining unit  230 . 
     The determining unit  230  is configured to determine a timing for replacement of each developing cartridge  52  on the basis of the light reception amounts from the toner sensors  130 , and the total numbers of revolutions from the revolution counters  220 . In addition, the determining unit  230  is configured to further use the total number of printed sheets from the page counter  210 , as a determination condition, to determine a timing of replacement of the first developing cartridge  52 K. In other words, the determining unit  230  is configured to: use a total number of printed sheets corresponding to an amount of paper dust intruded, as a condition, to determine a timing for replacement of the first developing cartridge  52 K disposed at the most upstream position where it is easy for paper dust to enter; and not to use a condition for a total number of printed sheets to determine a timing for replacement of each of the second developing cartridges  52 Y,  52 M and  52 C disposed on the downstream side where it is difficult for paper dust to enter. 
     Further, in a case where the determining unit  230  determines that it is a timing for replacement of a developing cartridge  52  by use of the above-described condition, a notification unit  400  is configured to indicate a message to a user that it is necessary to replace the corresponding developing cartridge  52 . As the notification unit  400 , for example, a liquid crystal display configured to indicate the message by a text, a picture, and so on, a speaker configured to indicate the message by sound, a lamp configured to indicate the message by flicker of light or the like can be used. Further, a notification unit obtained by combining two or more of a liquid crystal display, a speaker, a lamp and so on can be used. 
     In other words, the control device  200  is configured to use information based on an amount of use of toner and information based on the number of printed sheets to indicate the necessity to replace the first developing cartridge  52 K. Further, the control device  200  is configured to use information based on an amount of use of toner to indicate the necessity to replace each second developing cartridges  52 Y,  52 M or  52 C, without using the information based on the number of printed sheets. 
     Specifically, the control device  200  uses a flow chart of  FIG. 3  to determine the timing for replacement of the first developing cartridge  52 K, and uses a flow chart of  FIG. 4  to determine the timing for replacement of each second developing cartridge  52  (for example,  52 Y). Incidentally, control based on the flow chart shown in  FIG. 4  is performed separately with respect to each of the three second developing cartridges  52 Y,  52 M and  52 C; however, in the following description, determining of the timing for replacement of the second developing cartridge  52 Y will be representatively described. 
     (Control on Determining Timing for Replacement of First Developing Cartridge  52 K) 
     As shown in  FIG. 3 , upon a print command is received (“START”), first, in STEP S 1 , the control device  200  performs the above-described known warm-up operation control. 
     After STEP S 1 , in STEP S 2 , the control device  200  acquires a first light reception amount corresponding to an amount of use of toner stored in the first developing cartridge  52 K, from a corresponding toner sensor  130 . After STEP S 2 , in STEP S 3 , the control device  200  acquires a first total number of revolutions of the first developing roller  53 K from a corresponding revolution counter  220 . Then, in STEP S 4 , the control device  200  acquires the total number of sheets P printed using the first developing cartridge  52 K from the page counter  210 . 
     After STEP S 4 , in STEP S 5 , the control device  200  determines whether the first light reception amount exceeds a first threshold value T 1  (which means, whether the first light reception amount is larger than the first threshold value T 1 ). In a case where it is determined that the first light reception amount exceeds the first threshold value T 1  (“Yes”), in STEP S 6 , the control device  200  indicates the necessity to replace the first developing cartridge  52 K. The first threshold value T 1  and a second threshold value T 2  (see  FIG. 4 ), which will be described later, are determined by experiments, simulations and so on, and may be the same as or different from each other. 
     After STEP S 6 , the control device  200  resets the individual counters  210  and  220  (the total number of revolutions and the total number of printed sheets corresponding to the first developing cartridge  52 K) in STEP S 7  and then terminates the present control (“END”). 
     In a case where it is determined in STEP S 5  that the first light reception amount does not exceed the first threshold value T 1  (which means, the first light reception amount is equal to or less than the first threshold value T 1 ) (“No”), in STEP S 8 , the control device  200  determines whether the first total number of revolutions exceeds a fourth threshold value T 4 . The fourth threshold value T 4  and a fifth threshold value T 5  (see  FIG. 4 ), which will be described later, are determined by experiments, simulations and so on, and may be the same as or different from each other. 
     If it is determined in STEP S 8  that the first total number of revolutions exceeds the fourth threshold value T 4  (“Yes”), the control device  200  proceeds to the process of STEP S 6 ; otherwise, the control device  200  proceeds to the process of STEP S 9 . In STEP S 9 , the control device  200  determines whether the total number of printed sheets exceeds a third threshold value T 3 . The third threshold value T 3  may be determined by experiments, simulations and so on. 
     If it is determined in STEP S 9  that the total number of printed sheets exceeds the third threshold value T 3  (“Yes”), the control device  200  proceeds to the process of STEP S 6 ; otherwise (“No”), the control device  200  proceeds to the process of STEP S 10 . In STEP S 10 , the control device  200  performs printing control corresponding to one page. Consequently, the control device  200  is configured to indicate the necessity to replace the first cartridge in a case where any one condition of the first condition of STEP S 5 , the second condition of STEP S 9  and the third condition of STEP S 8  is satisfied. 
     After STEP S 10 , in STEP S 11 , the control device  200  determines whether the printing control has terminated, that is, whether printing of all pages designated by the print command has terminated. If it is determined in STEP S 11  that the printing control has not terminated (“No”), the control device  200  returns to the process of STEP S 2 ; otherwise (“Yes”), the control device  200  terminates the present control (“END”). 
     (Control on Determining Timing for Replacement of Second Developing Cartridge  52 Y) 
     In a control of determine the timing for replacement of the second developing cartridge  52 Y, the condition for the total number of printed sheets is not considered. Therefore, the flow chart shown in  FIG. 4  is basically a flow chart obtained by deleting the processes of STEPS S 4  and S 9  of  FIG. 3 . In the following description, only differences from the flow chart of  FIG. 3  will be described, and identical steps to those of  FIG. 3  are denoted by the same reference symbols and detailed descriptions thereof will be omitted. 
     STEPS S 102  and S 103  are substantially identical to STEPS S 2  and S 3  of  FIG. 3  except that data which are acquired are a second light reception amount and a second total number of revolutions corresponding to the second developing cartridge  52 Y. In STEP S 105 , the control device  200  performs a process substantially identical to that of STEP S 5  of  FIG. 3 . Specifically, the control device  200  determines whether the second light reception amount exceeds the second threshold value T 2 . 
     STEP S 106  is substantially identical to STEP S 6  of  FIG. 3 , except that a target which is notified is the second developing cartridge  52 Y. In STEP S 107 , the control device  200  resets a corresponding revolution counter  220  (the total number of revolutions corresponding to the second developing cartridge  52 Y). 
     In STEP S 108 , the control device  200  performs a process substantially identical to that of STEP S 8  of  FIG. 3 . Specifically, the control device  200  determines whether the second total number of revolutions exceeds the fifth threshold value T 5 . 
     According to the above-described configuration, in the present exemplary embodiment, it is possible to obtain the following effects. 
     Not only the first light reception amount and the first total number of revolutions but also the condition for the total number of printed sheets are used to determine the timing for replacement of the first developing cartridge  52 K. Therefore, for example, as shown in  FIG. 5 , before the first light reception amount reaches the first threshold value T 1 , and even before the first total number of revolutions reaches the fourth threshold value T 4 , when the total number of printed sheets exceeds the third threshold value T 3 , the necessity to replace the first developing cartridge  52 K is indicated. For this reason, it is possible to replace the first developing cartridge  52 K before the first developing cartridge is influenced by paper dust entering the first developing cartridge  52 K, and to improve image quality. 
     Further, the condition for the total number of printed sheets is not used to determine the timing for replacement of each second developing cartridges  52 Y,  52 M or  52 C. Therefore, it is possible to appropriately determine the timings for replacement of each second developing cartridges  52 Y,  52 M or  52 C, which is hard to be influenced by paper dust, according to an amount of use of toner, a deterioration of toner, etc. 
     Not only the light reception amount but also the total number of revolutions of each developing roller  53  are used to determine a timing for replacement of a corresponding developing cartridge  52 . Therefore, even in a case where toner has been deteriorated due to an increase in the total number of revolutions, it is possible to urge replacement of the corresponding developing cartridge  52 . 
     The page counter  210  counts the total number of printed sheets on the basis of the output value from the sheet sensor  110 . therefore, it is possible to determine the timing for replacement of the first developing cartridge  52 K on the basis of the number of sheets P actually printed, and to efficiently suppress the influence of sheet powder. 
     [Second Exemplary Embodiment] 
     Subsequently, a second exemplary embodiment of the present invention will be described in detail with reference to appropriate drawings. The present exemplary embodiment is obtained by changing a portion of the above-described control of the control device according to the first exemplary embodiment. Therefore, steps substantially identical to those of the first exemplary embodiment are denoted by the same reference symbols and detailed descriptions thereof will be omitted. 
     In the first exemplary embodiment, in a case where the total number of printed sheets exceeds the third threshold value T 3  (“Yes” in STEP S 9 ), immediately, the control device proceeds to STEP S 6  to indicate the necessity to replace the first developing cartridge  52 K. However, in the second exemplary embodiment, when the total number of printed sheets exceeds the third threshold value T 3 , the control device changes a threshold value for a light reception amount which is compared with the first light reception amount, from the above-described first threshold value T 1  to a sixth threshold value T 6  that is smaller than the first threshold value T 1 . As a result, in the case where the total number of printed sheets exceeds the third threshold value T 3 , the control device proceeds to STEP S 6  to indicate the necessity to replace the first developing cartridge  52 K. The sixth threshold value T 6  needs only to be a value (for example, zero) surely resulting in “Yes” in determination of STEP S 50 , which will be described later, and may be determined by experiments, simulations and so on. 
     Specifically, the control device is configured to perform control according to flow charts shown in  FIGS. 6 and 7 . Here, the flow chart shown in  FIG. 6  is obtained by eliminating STEPS S 4  and S 9  from the flow chart shown in  FIG. 3 , adding a new process of STEP S 200  between STEPS S 3  and S 5 , and replacing STEP S 5  of  FIG. 3  with a process of STEP S 50 . 
     In STEP S 200 , the control device sets a threshold value for a light reception amount. Specifically, as shown in  FIG. 7 , in STEP S 200 , first, in STEP S 201 , the control device acquires the total number of printed sheets from the page counter  210 . 
     After STEP S 201 , in STEP S 202 , the control device determines whether the total number of printed sheets exceeds the third threshold value T 3 . If it is determined in STEP S 202  that the total number of printed sheets does not exceed the third threshold value T 3  (“No”), the control device sets the threshold value for the light reception amount to the first threshold value T 1  in STEP S 203 , and then proceeds to STEP S 50  of  FIG. 6 . 
     Meanwhile, if it is determined in STEP S 202  that the total number of printed sheets exceeds the third threshold value T 3  (“Yes”), the control device changes the threshold value for the light reception amount from the first threshold value T 1  to the sixth threshold value T 6  that is smaller than the first threshold value T 1  in STEP S 204 , and then proceeds to STEP S 50  of  FIG. 6 . In STEP S 50 , the control device determines whether the first light reception amount exceeds the threshold value for the light reception amount set in STEP S 200 . 
     By configuring the control device as described above, as shown in  FIG. 8 , before the first light reception amount reaches the first threshold value T 1 , and even before the first total number of revolutions reaches the fourth threshold value T 4 , when the total number of printed sheets exceeds the third threshold value T 3 , the threshold value for the light reception amount is changed to the sixth threshold value T 6  smaller than the first threshold value T 1 . Therefore, it is possible to advance the timing for replacement of the first developing cartridge from a normal timing (a timing when the first light reception amount reaches the first threshold value T 1 ). Therefore, even in this case, it is possible to replace the first developing cartridge before the first developing cartridge is influenced by paper dust, and to improve image quality. 
     [Third Exemplary Embodiment] 
     Subsequently, a third exemplary embodiment of the present invention will be described in detail with reference to appropriate drawings. The present exemplary embodiment is obtained by changing a portion of the above-described control of the control device according to the first exemplary embodiment. Therefore, steps substantially identical to those of the first exemplary embodiment are denoted by the same reference symbols and detailed descriptions thereof will be omitted. 
     In the third exemplary embodiment, the control device performs correction to increase the first light reception amount when the total number of printed sheets exceeds the third threshold value T 3 . Consequently, in a case where the total number of printed sheets exceeds the third threshold value T 3 , the control device proceeds to STEP S 6  to indicate the necessity to replace the first developing cartridge  52 K. 
     Specifically, the control device is configured to perform control according to flow charts shown in  FIGS. 9 and 10 . The flow chart shown in  FIG. 9  is obtained by eliminating STEPS S 4  and S 9  from the flow chart shown in  FIG. 3  and adding a new process of STEP S 300  between STEPS S 3  and S 5 . 
     In STEP S 300 , the control device sets the first light reception amount. Specifically, as shown in  FIG. 10 , in STEP S 300 , first, the control device performs the same processes as those of STEPS S 201  and S 202  of the second exemplary embodiment. 
     If it is determined in STEP S 202  that the total number of printed sheets does not exceed the third threshold value T 3  (“No”), the control device proceeds to STEP S 5  of  FIG. 9 , without correcting the first light reception amount. Meanwhile, if it is determined in STEP S 202  that the total number of printed sheets exceeds the third threshold value T 3  (“Yes”), in STEP S 303 , the control device performs correction to increase the first light reception amount. 
     The correcting method may be any method, for example, a method of multiplying the first light reception amount by a predetermined coefficient, or a method of adding a predetermined correction value to the first light reception amount. Also, the value of the corrected light reception amount needs only to be a value (for example, a value larger than the first threshold value T 1 ) surely resulting in “Yes” in the determination of STEP S 5 , and the above-described coefficient or correction value may be determined by experiments, simulations and so on, such that the value of the corrected light reception amount becomes that value. 
     By configuring the control device as described above, as shown in  FIG. 11 , before the first light reception amount (uncorrected) reaches the first threshold value T 1 , and even before the first total number of revolutions reaches the fourth threshold value T 4 , when the total number of printed sheets exceeds the third threshold value T 3 , the first light reception amount is corrected to increase such that the first light reception amount instantaneously reaches the first threshold value T 1 . Therefore, it is possible to advance the timing for replacement of the first developing cartridge from the normal timing (a timing when the uncorrected first light reception amount reaches the first threshold value T 1 ). Therefore, even in this case, it is possible to replace the first developing cartridge before the first developing cartridge is influenced by paper dust and to improve image quality. 
     Incidentally, in the second and third exemplary embodiments, the light reception amount or the threshold value to be compared with the corresponding light reception amount is changed to advance the timing for replacement of the first developing cartridge from the normal timing. However, the present invention is not limited thereto. For example, the total number of revolutions of the first developing roller or the threshold value to be compared with the corresponding total number of revolutions is changed to advance the replacement timing. 
     Specifically, for example, in a case where the total number of printed sheets does not exceed the third threshold value T 3 , the threshold value for the number of revolutions may be set to the fourth threshold value T 3 , and in a case where the total number of printed sheets exceeds the third threshold value T 3 , the threshold value for the number of revolutions may be changed from the fourth threshold value T 4  to a threshold value smaller than the fourth threshold value T 3 . Further, for example, in a case where the total number of printed sheets does not exceed the third threshold value T 3 , the total number of revolutions may not be corrected, and in a case where the total number of printed sheets exceeds the third threshold value T 3 , correction may be performed on the total number of revolutions such that the total number of revolutions increases. 
     In the above-described exemplary embodiments, as photosensitive elements, the photosensitive drums  51  have been exemplified. However, the present invention is not limited thereto. For example, belt-type photosensitive elements may be used. 
     In the above-described exemplary embodiments, each developing roller  53  has been provided integrally with a corresponding developing cartridge  52  serving as a cartridge. However, the present invention is not limited thereto. Each developing roller may be provided to a developing unit which is a component separate from toner cartridges serving as cartridges. Also, a cartridge may be a process cartridge having a photosensitive drum  51  and a developing cartridge  52  integrated with each other. 
     In the above-described exemplary embodiments, the sheet P such as cardboard, postcard and thin paper have been exemplified as the sheet. However, the present invention is not limited thereto. For example, an OHP sheet may be used. 
     In the above-described exemplary embodiments, the transfer rollers  74  have been exemplified as the transfer member. However, the present invention is not limited thereto. The transfer member may be any other member such as a conductive brush or a conductive leaf spring configured to transfer bias can be applied. 
     In the above-described exemplary embodiments, as an amount of use of toner, a light reception amount detected by an optical sensor (a toner sensor  130 ) has been exemplified. However, the present invention is not limited thereto. For example, a value obtained by integrating the numbers of dots of images printed on sheets may be used as an amount of use. 
     In the above-described exemplary embodiments, the page counter  210  configured to the total number of printed sheets on the basis of an output value of the sheet sensor  110  has been exemplified. However, the present invention is not limited thereto. The page counter may be configured to integrate data on the numbers of printed sheets included in print commands, thereby computing the total number of printed sheets. 
     In the above-described exemplary embodiments, the first developing cartridge  52 K has been disposed on the most upstream side such that it is easy for paper dust to enter the first developing cartridge. However, the present invention is not limited thereto. For example, a first developer container may be configured to be easy for paper dust to enter due to the features such as color of toner contained therein. That is, since the present invention needs only to be applicable to the first developing cartridge easiest for paper dust to enter, for example, in a case where a first developer container which is easiest for paper dust to enter is specified from among a plurality of developer containers by experiments, simulations and so on, only with respect to a timing for replacement of the specified first developer container, a condition for the total number of printed sheets may be used. 
     In the above-described exemplary embodiments, as a driving amount of a rotator, the total number of revolutions of each developing roller  53  has been exemplified. However, the present invention is not limited thereto. For example, the total number of revolutions of a member (such as an agitator) connected to a driving gear of a developing roller through a gear, the total number of revolutions of a photosensitive drum, a driving time of a developing roller, and so on may be used. 
     In the above-described exemplary embodiments, the present invention has been applied to the color printer  1 . However, the present invention is not limited thereto. The present invention may be applied to other image forming apparatuses such as a copy machine and a multi-function apparatus.