Abstract:
The attenuation characteristic of the surface potential of a photosensitive drum  20  is periodically detected. A target value −Vgt for a grid bias voltage −Vg applied to a charging unit 24, which is necessary to maintain, at a set value −Vods, the charging potential −Vod of the developing portion of the photosensitive drum  20 , is derived from the detected attenuation characteristic. During image forming in which a developing unit  24  operates, the grid bias voltage −Vg of the charging unit  24  is set to the derived target value −Vgt.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image forming apparatus capable of color printing and monochrome printing, and an image forming method employed in the apparatus.  
         [0003]     2. Description of the Related Art  
         [0004]     In image forming apparatuses such as copy machines, printers, etc., image pattern data corresponding to a document image is analyzed, thereby generating image data Y indicating a yellow image, image data M indicating a magenta image, image data C indicating a cyan image, and image data K indicating a black image. Specifically, if the image pattern data corresponds to a monochrome image, only image data K is generated. If the image pattern data corresponds to a color image, image data items Y, M, C and K are generated.  
         [0005]     However, there is a case where even if the image pattern data corresponds to a monochrome image, image data items Y, M and C are generated as well as image data K, based on the analysis result. In this case, although it is sufficient if monochrome printing using only a black developer is performed, color printing using all developers is performed. As a result, a yellow developer, magenta developer and cyan developer are wasted.  
         [0006]     In light of the quality of printing as well, it is not preferable that color printing is performed instead of monochrome printing when monochrome printing should be performed.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     It is an object of an embodiment of the invention to provide an image forming apparatus and image forming method capable of accurately determining whether monochrome printing or color printing is needed, thereby avoiding waste of developers and enhancing the quality of printing.  
         [0008]     In accordance with an aspect of the invention, there is provided an image forming apparatus comprising:  
         [0009]     a generation section configured to analyze image pattern data corresponding to an image of a document to thereby generate image data Y indicating a yellow image, image data M indicating a magenta image, image data C indicating a cyan image, and image data K indicating a black image;  
         [0010]     a first control section configured to permit monochrome printing corresponding to the image data K when the generation section generates only the image data K;  
         [0011]     a determination section configured to determine whether the image data items Y, M and C each correspond to a white image, when the generation section generates the image data items Y, M, C and K;  
         [0012]     a second control section configured to delete the image data items Y, M and C included in the image data items Y, M, C and K generated by the generation section, and to permit monochrome printing corresponding to the remaining image data K, when a determination result of the determination section is an affirmative result; and  
         [0013]     a third control section configured to permit color printing corresponding to the image data items Y, M, C and K generated by the generation section, when the determination result of the determination section is a negative result.  
         [0014]     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be leaned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiment of the invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.  
         [0016]      FIG. 1  is a view illustrating the outward appearance of an embodiment;  
         [0017]      FIG. 2  is a view illustrating the structure of a control panel employed in the embodiment;  
         [0018]      FIG. 3  is a view illustrating the internal structure of the embodiment;  
         [0019]      FIG. 4  is a block diagram illustrating a control circuit employed in the embodiment;  
         [0020]      FIG. 5  is a block diagram illustrating an essential part of an image processing unit employed in the embodiment;  
         [0021]      FIG. 6  is a flowchart useful in explaining the function of the embodiment; and  
         [0022]      FIG. 7  is a flowchart illustrating the process following  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     [1] An embodiment of the invention will be described with reference to the accompanying drawings.  
         [0024]     As shown in  FIG. 1 , a document table (glass plate)  2  is provided on the upper portion of the main unit  1  of an image forming apparatus. An openable/closable automatic document feeder (ADF)  3  serving also as a document table cover is provided on the main unit  1 . A control panel  4  is provided on the upper surface of the main unit  1 . A plurality of cassettes  5  that contain various sizes of paper sheets are provided at the lower portion of the main unit  1 .  
         [0025]     A paper ejection port  6  is formed in the side surface of the main unit  1 , and a paper ejection unit  10  is mounted at the paper ejection port  6  side. The paper ejection unit  10  has trays  11  and  12  for receiving paper sheets from the paper ejection port  6 , and has a staple unit  13  for stapling paper sheet ejected from the paper ejection port  6 .  
         [0026]     As shown in  FIG. 2 , the control panel  4  mainly comprises a liquid crystal screen  4   a  of a touch panel type, a start key  4   b , etc. The liquid crystal display screen  4   a  displays two-color copy button  4   c  when necessary.  
         [0027]      FIG. 3  shows the internal structure of the main unit  1 .  
         [0028]     An indicator  23  is provided on one side of the document table  2 . The step between the indicator  23  and the document table  2  serves as a reference position for setting documents. Further, a window  24  for reading documents is provided near the indicator  23 .  
         [0029]     A carriage  25  is provided below the document table  2  and window  24 , ad an exposure lamp  26  is provided on the carriage  25 . The carriage  25  can move (reciprocate) along the lower surface of the document table  2 . When the carriage  25  reciprocates along the document table  2  and the exposure lamp  26  lights, a document placed on the document table  2  is exposed to the light.  
         [0030]     When the document is exposed, a reflection light image of the document is acquired, and is projected onto a CCD (Charge Coupled Device)  31  via reflection mirrors  27 ,  28  and  29  and a lens block  30 . The CCD  31  outputs an image pattern signal corresponding to the projected image. The image pattern signal output from the CCD  31  is digitally converted into image pattern data. The image pattern data is supplied to an image processing unit  75  described later.  
         [0031]     The automatic document feeder  3  has a tray  21  for placing documents thereon, and is arranged to feed one by one a plurality of documents D, set on the tray  21 , to the window  24  and passes therethrough, and to eject the documents D, passed therethrough, to the tray  22 . When the automatic document feeder  3  is operated, the exposure lamp  6  is lit at a position opposing the window  24 , whereby the light enters the window  24 . The light entering the window  24  is applied to each document D passing over the window  24 . As a result, a reflection light image is acquired from each document D, and is then projected onto the CCD  31  via the reflection mirrors  27 ,  28  and  29  and the magnifying lens block  30 .  
         [0032]     On the other hand, an exposure unit  32  for emitting a laser beam B is provided at substantially the central portion of the main unit  1 . A rotary image carrier such as a photosensitive drum  40  is provided near the exposure unit  32 . Around the photosensitive drum  40 , there are provided a de-electrifying unit  41 , charging unit  42 , black developing unit  44 , color developing unit  45 , transfer roller  46  and cleaning unit  47  in the order mentioned. The laser beam B emitted from the exposure unit  32  is guided to the surface of the photosensitive drum  40  through the space between the charging unit  42  and block developing unit  44 .  
         [0033]     The de-electrifying unit  41  emits lamp light or emission diode light to the photosensitive drum  40  to eliminate a charge remaining on the surface of the photosensitive drum  40  (de-electrification). The charging unit  42  applies a high voltage to the photosensitive drum  40  to charge the surface of the photosensitive drum  40  with static electricity. When the thus-charged surface of the photosensitive drum  40  is exposed to the laser beam B of the exposure unit  32 , an electrostatic latent image is formed on the photosensitive drum  40 .  
         [0034]     The black developing unit  44  is brought into contact with the surface of the photosensitive drum  40  when a black image is formed, and out of contact with the photosensitive drum  40  when a yellow, magenta or cyan image is formed. Further, the black developing unit  44  has a developing roller  44   a  that rotates in accordance with the rotation of the photosensitive drum  40  when the unit is brought into contact with the surface of the photosensitive drum  40 . Using the developing roller  44   a , the black developer (black toner) pre-contained is applied to the surface of the photosensitive drum  40 . As a result, the electrostatic latent image on the photosensitive drum  40  is developed into a black visible image.  
         [0035]     The color developing unit  45  is brought into contact with the surface of the photosensitive drum  40  when a yellow, magenta or cyan image is formed, and out of contact with the photosensitive drum  40  when a black image is formed. Further, the color developing unit  45  has a yellow developing section  45   y , magenta developing unit  45   m  and cyan developing unit  45   c . When a yellow image is formed, the yellow developing section  45   y  is brought into contact with the surface of the photosensitive drum  40 . When a magenta image is formed, the magenta developing unit  45   m  is brought into contact with the surface of the photosensitive drum  40 . When a cyan image is formed, the cyan developing unit  45   c  is brought into contact with the surface of the photosensitive drum  40 . When the yellow developing section  45   y  is brought into contact with the surface of the photosensitive drum  40 , it applies the contained yellow developer (yellow toner) to the surface of the photosensitive drum  40 . As a result, the electrostatic latent image on the photosensitive drum  40  is developed into a yellow visible image. When the magenta developing section  45   m  is brought into contact with the surface of the photosensitive drum  40 , it applies the contained magenta developer (magenta toner) to the surface of the photosensitive drum  40 . As a result, the electrostatic latent image on the photosensitive drum  40  is developed into a magenta visible image. When the cyan developing section  45   c  is brought into contact with the surface of the photosensitive drum  40 , it applies the contained cyan developer (cyan toner) to the surface of the photosensitive drum  40 . As a result, the electrostatic latent image on the photosensitive drum  40  is developed into a cyan visible image.  
         [0036]     A rotary transfer belt  50  is held between the transfer roller  46  and photosensitive drum  40 . The transfer roller  46  transfers, to the transfer belt  50 , the visible image on the photosensitive drum  40 . The transfer belt  50  is tensioned between rollers  51 ,  52 ,  53  and  54 . A transfer roller  55  is provided at a position opposing the roller  53  with the transfer belt  50  interposed therebetween.  
         [0037]     The cleaning unit  47  removes, for example, a developer remaining on the surface of the photosensitive drum  40 .  
         [0038]     Paper sheets P are picked up one by one from one of the cassettes  5  provided at the lower portion of the main unit  1 . A pick-up roller  61  for picking up paper sheets is provided for each cassette. Each picked paper sheet P is separated from each cassette  5  by the corresponding separating roller  62 , and sent to a resist roller  63 . The resist roller  63  sends each paper sheet P to a space between the transfer belt  50  and transfer roller  55  at the timing made in light of the rotation of the transfer belt  50 . The transfer roller  55  transfers a visible image on the transfer belt  50  to each paper sheet P fed by the resist roller  63 .  
         [0039]     The paper sheet P, to which the visible image on the transfer belt  50  is transferred, is sent to a fixing unit  64 . The fixing unit  64  fixes, by heat, the image transferred to the paper sheet P. The image-fixed paper sheet P is sent to the paper ejection unit  10 , provided outside the main unit  1 , through the paper ejection port  8 .  
         [0040]      FIG. 4  shows a control circuit contained in the main unit  1 .  
         [0041]     A control panel controller  71 , scanning controller  72 , image processing unit  74 , page memory  75 , hard disk unit  76  and print section  80  are connected to a main controller  70 .  
         [0042]     The main controller  70  wholly controls the control panel controller  71 , scanning controller  72 , image processing unit  74 , page memory  75 , hard disk unit  76  and print section  80 . The scanning controller  72  controls a canning unit  73  for optically reading an image of each document D. The scanning unit  73  comprises the document table  2 , automatic document feeder  3 , window  24 , carriage  25 , exposure lamp  26 , reflection mirrors  27 ,  28  and  29 , magnification lens block  30 , CCD  31 , etc., and outputs image pattern data corresponding to the image of each document D.  
         [0043]     The page memory  74  temporarily stores, in units of pages, image pattern data output from the scanning unit  73 . The image processing unit  75  reads image pattern data from the page memory  74  and processes it, thereby outputting image data items Y, M, C and K. The image data items Y, M, C and K are supplied to the hard disk unit  76  and print section  80 . The hard disk unit  76  stores, when necessary, the image data items Y, M, C and K output from the image processing unit  75 .  
         [0044]     As shown in  FIG. 5 , the image processing unit  75  comprises a generation section  91 , compression section  92  and white determination section  93 . The generation section  91  analyzes image pattern data read form the page memory  74 , thereby generating image data Y indicating a yellow image, image data M indicating a magenta image, image data C indicating a cyan image, and image data K indicating a black image. The compression section  92  performs compression processing on the image data items Y, M, C and K generated by the generation section  91 . The compressed image data items Y, M, C and K are sent to the hard disk unit  76  and print section  80 . The white determination section  93  belongs to the compression section  92 , and determines whether each of the image data items Y, M, C and K generated by the generation section  91  corresponds to a white image. The determination results are sent to the print section  80 .  
         [0045]     The print section  80  comprises a print controller  81 , ROM  82 , RAM  83 , sheet conveying unit  84 , print engine  85 , paper ejection unit  10 , fixing unit  64 , etc.  
         [0046]     The print controller  81  controls the sheet conveying unit  84 , print engine  85 , paper ejection unit  10  and fixing unit  64 . The ROM  82  stores a control program used by the print controller  81 . The RAM  83  stores data. The sheet conveying unit  84  comprises a conveyance mechanism for paper sheets P, a driving circuit for the mechanism, etc.  
         [0047]     The print engine  85  comprises the exposure unit  32 , photosensitive drum  40 , de-electrifying unit  41 , charging unit  42 , black developing unit  44 , color developing unit  45 , transfer roller  46 , cleaning unit  47 , transfer belt  50 , transfer roller  55 , developer detecting section  86 , etc. The print engine  85  performs monochrome printing in which a black image corresponding to image data K supplied from the image processing unit  75  or hard disk unit  76  is formed on the photosensitive drum  40  using a black developer, and then transferred to a paper sheet using the transfer belt  50 . The print engine also performs color printing in which yellow, magenta, cyan and black images corresponding to image data items Y, M, C and K are formed on the photosensitive drum  40  using yellow, magenta, cyan and black developers, and then transferred to a paper sheet.  
         [0048]     The developer detecting section  86  detects whether the black developing unit  44  contains a black developer, and whether the color developing unit  45  contains a yellow developer, magenta developer and cyan developer.  
         [0049]     The print controller  81  comprises the following sections (1) to (13) as main functional sections:  
         [0050]     (1) First control section for permitting monochrome printing corresponding to image data K when the generation section  91  generates only image data K;  
         [0051]     (2) Second control section for deleting image data items Y, M and C and permitting monochrome printing corresponding to image data K, when the generation section  91  generates image data items Y, M, C and K and the determination results of the white determination section  93  are affirmative ones (i.e., when image data items Y, M and C correspond to white images);  
         [0052]     (3) Third control section for permitting color printing corresponding to image data items Y, M, C and K generated by the generation section  91 , when the determination results of the white determination section  93  are negative ones (i.e., when image data items Y, M and C do not correspond to white images);  
         [0053]     (4) First determination section for determining whether the print section  80  can perform printing using a remaining developer (developers), if the developer detection section  86  detects that the black developer, yellow developer, magenta developer and/or cyan developer does not remain;  
         [0054]     (5) Fourth control section for causing the print section  80  to continue printing, if the determination result of the first determination section is an affirmative one (printing is possible);  
         [0055]     (6) Fifth control section for causing the print section  80  to interrupt printing, if the determination result of the first determination section is a negative one (printing is impossible);  
         [0056]     (7) Reporting means for reporting a message that the black developer, yellow developer, magenta developer and/or cyan developer does not remain, by displaying the message on the liquid crystal display screen  4   a , if the developer detection section  86  detects it;  
         [0057]     (8) Second determination section for determining whether there is another print job, when the fifth control section interrupts printing;  
         [0058]     (9) Third determination section for determining whether said another printing job is possible using the remaining developer(s), if the determination result of the second determination section is an affirmative one (said another print job exists);  
         [0059]     (10) Sixth control section for causing the print section  80  to perform said another print job if the determination result of the third determination section is an affirmative one;  
         [0060]     (11) Fourth determination section for determining whether the fifth control section interrupts printing, if the developer detection section  86  detects that all the black developer, yellow developer, magenta developer and cyan developer remain;  
         [0061]     (12) Fifth determination section for determining whether the interrupted printing requires stapling, if the determination result of the fourth determination section is an affirmative one (printing is interrupted);  
         [0062]     (13) Seventh control section for restarting the interrupted printing from the beginning, if the determination result of the fifth determination section is an affirmative one (stapling is required); and  
         [0063]     (14) Eighth control section for resuming the interrupted printing from the interrupted portion, if the determination result of the fifth determination section is a negative one  
         [0064]     Referring now to the flowcharts of  FIGS. 6 and 7 , the operation will be described.  
         [0065]     When a document D is set on the document table  2 , and the start key  4   b  of the control panel  4  is turned on, the document D is optically read by the scanning unit  73 . Image pattern data corresponding to the read image is supplied form the scanning unit  73  to the generation section  91  of the image processing unit  75 .  
         [0066]     The generation section  91  analyzes the image pattern data, and generates image data Y indicating a yellow image, image data M indicating a magenta image, image data C indicating a cyan image, and image data K indicating a black image. Specifically, if the image pattern data corresponds to a monochrome image, only image data K is generated. If the image pattern data corresponds to a color image, image data items Y, M, C and K are generated.  
         [0067]     However, there is a case where although the image pattern data corresponds to a monochrome image, image data items Y, M and C are generated, as well as image data K, because of an erroneous analysis of the generation section  91 . In this case, although it is sufficient if monochrome printing using only a black developer is performed, color printing using all developers is performed. As a result, a yellow developer, magenta developer and cyan developer are wasted. In light of the quality of printing as well, it is not preferable that color printing is performed instead of monochrome printing when monochrome printing should be performed.  
         [0068]     To avoid the above, if the generation section  91  generates only image data K (NO at step  101 , YES at step  102 ), monochrome printing corresponding to image data K is permitted (step  103 ).  
         [0069]     If the generation section  91  generates image data items Y, M, C and K (YES at step  101 ), the white determination section  93  determines whether each of image data items Y, M and C corresponds to a white image (steps  104 ,  105 ,  106 ).  
         [0070]     If the determination result of the white determination section  93  is an affirmative one (each of image data items Y, M and C corresponds to a white image) (YES at step  104 , YES at step  105 , YES at step  106 ), image data items Y, M and C included in image data items Y, M, C and K generated by the generation section  91  are deleted (step  107 ). Monochrome printing corresponding to the remaining data item K is permitted (step  103 ).  
         [0071]     In other words, if image patter data corresponding to monochrome printing is supplied to the generation section  91 , and nevertheless the generation section  91  generates image data items Y, M, C and K, image data items Y, M and C do not correspond to yellow, magenta and cyan images, respectively, but each correspond to a white image. In this case, it is determined that the analysis of the generation section  91  is erroneous, and monochrome printing is performed instead of color printing. Since monochrome printing is thus performed, yellow, magenta and cyan developers are prevented from being wasted, and the quality of resultant printing is enhanced.  
         [0072]     If the determination result of the white determination section  93  is a negative one (none of image data items Y, M and C corresponds to a white image) (NO at step  104 , NO at step  105 , NO at step  106 ), color printing corresponding to image data items Y, M, C and K generated by the generation section  91  is performed (step  108 ).  
         [0073]     If at least image data Y, M or C does not correspond to a white image, it is determined that the analysis of the generation section  91  is correct, and color printing is performed.  
         [0074]     As described above, regardless of whether the analysis of the generation section  91  is erroneous, it can be accurately determined whether monochrome or color printing is necessary. This can avoid waste of developers, and enhance the quality of printing.  
         [0075]     On the other hand, as shown in the flowchart of  FIG. 7 , the developer detection section  86  detects whether each of the black, yellow, magenta and cyan developers remains (step  109 ).  
         [0076]     If it is detected that the black, yellow, magenta or cyan developer does not remain, while a print job corresponding to several sheets of paper is being performed (NO at step  110 ), this is reported on the liquid crystal display screen  4   a  of the control panel  4  (step  111 ). Further, it is determined whether printing can be continued using the remaining developers (step  112 ).  
         [0077]     For example, even if it is detected during monochrome printing that no black developer remains, if the yellow, magenta and cyan developers remain, monochrome printing can be continued.  
         [0078]     Even if it is detected during color printing that no yellow developer remains, color printing except for yellow, green and red can be continued using the remaining black, magenta and cyan developers.  
         [0079]     Even if it is detected during color printing that no magenta developer remains, color printing except for magenta, red and blue can be continued using the remaining black, yellow and cyan developers.  
         [0080]     Even if it is detected during color printing that no cyan developer remains, color printing except for cyan, blue and green can be continued using the remaining black, yellow and magenta developers.  
         [0081]     Furthermore, if the two-color copy button  4   c  of the liquid crystal display screen  4   a  of the control panel  4  is in the ON state, and if, for example, a two-color copy mode of yellow and red is set, the yellow and magenta developers are necessary for yellow and red color printing. Accordingly, printing can be continued if the yellow and magenta developers remain, and if the black or cyan developer does not remain.  
         [0082]     If it is determined that printing can be continued using the remaining developers (YES at step  112 ), printing is continued (step  113 ).  
         [0083]     If it is determined that printing cannot be continued using the remaining developers (NO at step  112 ), printing by the print section  80  is interrupted (step  115 ). At this time, it is determined whether there is another print job (step  116 ).  
         [0084]     If there is another print job (YES at step  116 ), it is determined that said another print job is possible using the remaining developers (step  117 ). If possible (YES at step  117 ), said another print job is performed (step  118 ). The user who has set said another print job can immediately acquire a necessary printing result without waiting for replenishment of the exhausted developers.  
         [0085]     After the user replenishes the exhausted developers to complete all developers (YES at step  110 ), it is determined whether there is a print job interrupted (step  119 ). If there is no interrupted print job (NO at step  119 ), print jobs from now on are permitted (step  120 ).  
         [0086]     If there is a print job interrupted (YES at step  119 ), it is determined whether the interrupted print job requires stapling (step  121 ). If stapling is required (YES at step  121 ), the interrupted print job is restarted from the beginning (step  122 ). By restarting the interrupted print job, a required number of paper sheets can be reliably stapled.  
         [0087]     If no stapling is required, the interrupted print job is resumed from the interrupted portion (step  123 ).  
         [0088]     In the above embodiment, determination as to whether stapling is required is set as a determination condition. However, if a saddle stitch function is employed, determination as to whether saddle stitching is required may be set as a determination condition for resuming printing. If a function for designating one of the trays  11  and  12  as the ejection destination of each paper sheet P is employed, determination as to whether each of the trays  11  and  12  has room may be used as a determination condition for resuming printing.  
         [0089]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiment shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.