Patent Publication Number: US-7583919-B2

Title: Color image forming apparatus capable of effectively matching registration between elementary color images

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus capable of effectively matching registration between elementary color images. 
   2. Discussion of the Background 
   A background full-color image forming apparatus according to an electrophotographic method generally forms toner images of a plurality of elementary colors, such as yellow, magenta, cyan, and black, and sequentially superimposes them on an intermediate transfer member, thereby forming a full-color image on the intermediate transfer member. Then, the full-color image is transferred from the intermediate transfer member onto a recording medium. The apparatus thus performs a full-color image forming. In this operation, however, if the images of the respective elementary colors are not precisely superimposed with respect to the intermediate transfer member, the full-color image formed by the toner images of the elementary colors on the intermediate transfer member has a so-called color drift and is transferred with the color drift onto the recording medium. As a result, quality of an obtained image is deteriorated because of the color drift. Therefore, in order to form a high-quality image, a position control is necessary for aligning start positions of transferring images of the elementary colors on the intermediate transfer member. 
   There is one attempt to solve this problem. In this case, a full-color image forming apparatus has a structure in which a single photoconductor is employed, and an intermediate transfer belt and a plurality of development devices of different colors are placed to face this photoconductor. With this structure, each time a toner image of a color is formed on the photoconductor, the toner image is transferred to the intermediate transfer belt. Thus, a plurality of toner images are transferred to and superimposed into a full-color image on the intermediate transfer belt, and such a full-color image is transferred to a recoding medium. 
   This full-color image forming apparatus includes a first reference signal generating mechanism for generating a first reference signal indicating a reference position of the photoconductor, a mark forming mechanism for forming a reference mark according to the first reference signal, and a second reference signal generating mechanism for detecting the reference mark and generating a second reference signal for the intermediate transfer belt. To prevent the above-mentioned color drift in superimposing the color toner images, the full-color image forming apparatus starts transferring an image of a first color on the photoconductor according to the first reference signal, and further transfers images of the second and subsequent colors on the photoconductor based on the second reference signal. This attempt, however, cannot be applied to a full-color image forming apparatus having a structure in which a plurality of photoconductors are employed. 
   There is another attempt to solve the problem. In this attempt, a full-color image forming apparatus includes a plurality of image forming-mechanisms and an intermediate transfer member placed to face the plurality of image forming mechanisms. Each of the plurality of image forming mechanisms includes an image carrying member, a writing mechanism, at least two development mechanisms for developing an electrostatic latent image formed on the image carrying member by the writing mechanism, and a switching mechanism for alternatively selecting to drive one of the at least two development mechanisms. 
   The intermediate transfer member includes a non-image region in which a plurality of marks are formed at equal intervals in a rotation direction of the intermediate transfer member. The full-color image forming apparatus further includes a detection mechanism for detecting the plurality of marks, and a PLL (phase-locked loop) circuit to which a signal output from the detection mechanism is input. The PLL circuit outputs a reference signal for starting an image writing performed by the writing mechanism. With this configuration, the full-color image forming apparatus can prevent a jitter of the toner image in a sub-scanning direction and a color drift in the toner images superimposed on the intermediate transfer member. This attempt, however, is required to align the plurality of marks with extremely high accuracy. Further, this structure cannot cope with deterioration over time of the intermediate transfer member and changes of an environment in which the full-color image forming apparatus is used. 
   SUMMARY OF THE INVENTION 
   Under the above-described circumstances, the present invention aims to provide a color image forming apparatus which forms a full-color image by highly accurately superimposing images of a plurality of colors on an intermediate transfer member by using a plurality of image forming units placed along a moving surface of the intermediate transfer member, and which is capable of flexibly coping with a change over time and a change in speed of the intermediate transfer member having a simplified structure. 
   Accordingly, this patent specification describes a novel color image forming apparatus which includes a plurality of image forming units, an intermediate transfer member, a mark detecting mechanism, a counter, a count value memory unit, and a controller. The plurality of image forming units includes nth and (n+1)th image forming units numbered in an order of a predetermined image forming sequence and is configured to write latent images on rotating image carrying members at write positions and to develop the latent images. The intermediate transfer member is configured to rotate to pass respective transfer positions at which the developed images are transferred to the intermediate transfer member from the respective image carrying members of the plurality of image forming units. The intermediate transfer member is further configured to have a plurality of marks including nth and (n+1)th marks and placed along a rotation direction of the intermediate transfer member. An interval between the nth and (n+1)th marks is substantially equal to or smaller than a distance difference between a distance from a write position of the nth image forming unit to a primary transfer position of the (n+1)th image forming unit and a distance from a write position of the (n+1)th image forming unit to the primary transfer position of the (n+1)th image forming unit. The mark detecting mechanism is configured to detect the marks at a fixed position. The counter is configured to be reset to start counting upon a detection of each of the marks by the mark detecting mechanism. The count value memory unit is configured to store a count value C n  counted during a time period from a detection of the nth mark until a start of writing on the image carrying member in the nth image forming unit, and also a count value C (n+1)  counted during a time period from detection of the (n+1)th mark until a start of writing on the image carrying member in the (n+1)th image forming unit. The controller is configured to control the nth image forming unit to start writing on the image carrying member thereof when the mark detecting mechanism detects the nth mark and the counter reaches the count value C n , and to control the (n+1)th image forming unit to start writing on the image carrying member thereof when the mark detecting mechanism detects the (n+1)th mark and the counter reaches the count value C (n+1) . 
   According to the above image forming apparatus of the present invention, optical writing of each color starts at predetermined counts after detection of a mark. Therefore, an interval between marks is not required to be highly accurately equalized with an interval between transfer positions of a plurality of image carrying members. Accordingly, the image forming apparatuses of the present invention can accurately superimpose toner images and flexibly cope with a change in speed and a change over time of an intermediate transfer member, while simplifying the structure of the intermediate transfer member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic illustration of a color image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a block diagram of a control system of the color image forming apparatus of  FIG. 1 ; 
       FIG. 3  is a time chart indicating operation timings of the color image forming apparatus of  FIG. 1 ; 
       FIG. 4  is a flowchart of a primary transfer operation for transferring a toner image of an A-color; 
       FIG. 5  is a flowchart of a primary transfer operation for transferring a toner image of a B-color; 
       FIG. 6  is a flowchart of a primary transfer operation for transferring a toner image of a C-color; 
       FIG. 7  is a flowchart of a primary transfer operation for transferring a toner image of a D-color; 
       FIG. 8  is a schematic illustration of a color image forming apparatus according to another embodiment of the present invention; 
       FIG. 9  is a time chart indicating operation timings of the color image forming apparatus of  FIG. 8 ; 
       FIG. 10  is a schematic illustration of a color image forming apparatus according to another embodiment of the present invention; 
       FIG. 11  is a block diagram of a control system of the color image forming apparatus of  FIG. 10 ; 
       FIG. 12  is a block diagram of a control system of a color image forming apparatus according to another embodiment of the present invention; and 
       FIG. 13  is a time chart indicating operation timings of a color image forming apparatus according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , a color image forming apparatus  1  according to an embodiment of the present invention is explained. 
   As illustrated in  FIG. 1 , the color image forming apparatus  1  includes a sheet-feeding unit  2 , a secondary transfer unit  3 , a first image forming unit  4 , a second image forming unit  5 , a fixing unit  6 , a sheet-discharging unit  7 , an electric device  8 , and a control unit  9 . 
   The sheet-feeding unit  2  includes a sheet-feeding tray  10 , a pair of feed rollers  11 , and a pair of registration rollers  12 . The sheet-feeding tray  10  stores a stack of transfer sheets. The pair of feed rollers  11  send out the transfer sheets one by one from the sheet-feeding tray  10  along a sheet passage indicated by dotted lines. The pair of registration rollers  12  convey the transfer sheets sent out of the sheet-feeding tray  10  to the secondary transfer unit  3  at an adjusted timing. 
   The secondary transfer unit  3  includes an intermediate transfer belt  20 , a first drive roller  21 , a second drive roller  22 , a mark detecting unit  23 , a speed detecting unit  24 , a secondary transfer roller  25 , and a belt cleaning unit  26 . 
   The intermediate transfer belt  20  is an endless belt supported by the first drive roller  21  and the second drive roller  22 , and is sequentially surrounded by the first image forming unit  4 , the second image forming unit  5 , the secondary transfer roller  25 , and the belt cleaning unit  26 . The first drive roller  21  and the second drive roller  22  drive the intermediate transfer belt  20  to rotate in a direction indicated by an arrow. The intermediate transfer belt  20  includes an outer circumferential surface edge, perpendicular to a belt rotation direction and outside an area of toner image forming, on which marks M 1  and M 2  are formed at an interval in the belt rotation direction. The marks M 1  and M 2  are line-shaped and are configured to reflect light more strongly than the other area of the intermediate transfer belt  20 . 
   The mark detecting unit  23  is fixed to a stationary member located above a traveling route of the marks M 1  and M 2 . The mark detecting unit  23  uses a light-emitting element such as an LED (light-emitting diode) for emitting light to the intermediate transfer belt  20 , and a light-receiving element such as a photosensor for receiving light reflected from the intermediate transfer belt  20 . Thereby, the mark detecting unit  23  detects a passage of the marks M 1  and M 2  based on a change in intensity of the reflected light. The speed detecting unit  24  measures a moving speed of the intermediate transfer belt  20 . 
   The secondary transfer roller  25  is placed to face the intermediate transfer belt  20  at a downstream position of the second image forming unit  5  in the belt rotation direction. The secondary transfer roller  25  performs a secondary transfer operation. In the secondary transfer operation, a plurality of elementary color toner images, i.e., a full-color toner image, formed on the intermediate transfer belt  20  by the first image forming unit  4  and the second image forming unit  5  are transferred at one time onto the transfer sheet conveyed by the pair of registration rollers  12 . After the secondary transfer operation, the belt cleaning unit  26  contacts the intermediate transfer belt  20  and removes therefrom residual toner. 
   The first image forming unit  4  includes a photoconductor drum  30 , a charging unit  31 , a writing unit  32 , a development unit  33 , a primary transfer unit  34 , and a drum cleaning unit  35 . The photoconductor drum  30  is arranged at a center and is surrounded by the other components mentioned above. 
   The photoconductor drum  30  serves as an image carrying member and is rotated with a surface moving speed being equal to a surface moving speed of the intermediate transfer belt  20  on which a toner image is formed. The charging unit  31  uniformly charges the photoconductor drum  30  in the dark. The writing unit  32  uses an optical system including an LED (light-emitting diode) combined with a convergent optical transmission member. The optical system writes an electrostatic latent image on the charged photoconductor drum  30  according to a write signal, as the photoconductor drum  30  rotates. 
   The development unit  33  includes an A-color development device  36  and a C-color development device  37 . Switching is performed between development of the A color by the A-color development device  36  and development of the C color by the C-color development device  37  to develop the electrostatic latent images formed on the photoconductor drum  30  into visible toner images of the A color and the C color. In this example, the A color and the C color are cyan and magenta, respectively. 
   The primary transfer unit  34  performs a primary transfer operation of transferring the toner image of the A color or the C color formed on the photoconductor drum  30  to the intermediate transfer belt  20 . The photoconductor drum  30  is usually placed at a slight distance from the intermediate transfer belt  20 . In the primary transfer operation from the photoconductor drum  30  to the intermediate transfer belt  20 , the primary transfer unit  34  causes the intermediate transfer belt  20  to contact the photoconductor drum  30 . After the primary transfer operation, the drum cleaning unit  35  removes residual toner and developer agent from the photoconductor drum  30 . 
   The second image forming unit  5  includes a photoconductor drum  40 , a charging unit  41 , a writing unit  42 , a development unit  43 , a primary transfer unit  44 , and a drum cleaning unit  45 . The photoconductor drum  40  is arrange at a center and is surrounded by other components mentioned above. 
   The photoconductor drum  40  serves as an image carrying member and is rotated with a surface moving speed being equal to a surface moving speed of the intermediate transfer belt  20  on which a toner image is formed. The charging unit  41  uniformly charges the photoconductor drum  40  in the dark. The writing unit  42  uses an optical system including an LED (light-emitting diode) combined with a convergent optical transmission member. The optical system writes an electrostatic latent image on the charged photoconductor drum  40  according to a write signal, as the photoconductor  40  drum rotates. 
   The development unit  43  includes a B-color development device  46  and a D-color development device  47 . Switching is performed between development of the B color by the B-color development device  46  and development of the D color by the D-color development device  47  to develop the electrostatic latent images formed on the photoconductor drum  40  into visible toner images of the B color and the D color. In this example, the B color and the D color are yellow and black, respectively. 
   The transfer unit  44  performs a primary transfer operation of transferring the toner image of the B color or the D color formed on the photoconductor drum  40  to the intermediate transfer belt  20 . The photoconductor drum  40  is usually placed at a slight distance from the intermediate transfer belt  20 . In the primary transfer operation, the transfer unit  44  causes the intermediate transfer belt  20  to contact the photoconductor drum  40 . After the primary transfer operation, the drum cleaning unit  45  removes residual toner and developer agent from the photoconductor drum  40 . 
   The fixing unit  6  includes a heating roller  50 , a pressure roller  51 , a coating roller  52 , and a separation claw  53 . The heating roller  50  rotates while being heated. The pressure roller  51  rotates together with the heating roller  50 , while the pressure roller  51  holds the transfer sheet carrying thereon the toner image at a nip formed between the heating roller  50  and the pressure roller  51  and applies pressure to the transfer sheet. Thereby, the pressure roller  51  conveys the transfer sheet onto which the toner image has been transferred by the secondary transfer unit  3 , and fixes the toner image on the transfer sheet. The coating roller  52  contacts the heating roller  50  to coat a surface of the heating roller  50  with an offset preventing liquid, when necessary. The separation claw  53  contacts the heating roller  50  at a downstream position of the nip in a sheet transfer direction to separate the transfer sheet which has passed the nip from the heating roller  50 . 
   The sheet-discharging unit  7  includes a pair of discharging rollers  54  and an output tray  55 . The transfer sheet conveyed from the fixing unit  6  is sent onto the output tray  55  by the pair of discharging rollers  54 . The electric device  8  includes an exhaust fan  56  and an electric component  57 . The exhaust fan  56  exhausts air from the interior of the color image forming apparatus  1 . Thereby, an area near the electric component  57  located under the output tray  55  is prevented from being heated by heat of the fixing unit  6 . 
   The control unit  9  is mounted on a circuit substrate forming a part of the electric component  57  of the electric device  8 , and controls the entirety of image forming operations of the color image forming apparatus  1 . 
   The color image forming apparatus  1  include, as described above, two image forming unit, i.e., the first and second image forming units  4  and  5 . The color image forming apparatus according to the present invention, however, is not necessarily limited to such a structure but it may include more than two image forming units. When a plurality of image forming units are provided, they are preferably placed at regular intervals along the same moving surface of the intermediate transfer belt  20 . Also, the toner colors are not necessarily limited to the four colors, i.e., cyan, yellow, magenta, and black. 
   Furthermore, the color image forming apparatus  1  may use not only the drum-shaped photoconductors, as the image carrying members, but also endless belt-shaped photoconductors. Also, the image carrying members are not necessarily limited to the photoconductors but may be other media on which latent images can be formed without using light. In the color image forming apparatus  1  illustrated above, each of the writing units  32  and  42  includes an LED (light-emitting diode) combined with a convergent optical transmission member. Alternatively, the writing unit may include a laser light source. If the image carrying members are the media on which the latent images are formed without using light, the writing unit effecting an electric or magnetic change on the media may be used. 
   As illustrated in  FIG. 2 , the control unit  9  includes a ROM (read only memory)  60 , a CPU (central processing unit)  61 , a RAM (random access memory)  62 , an input-output unit  63 , an image memory  64 , an image processing unit  65 , a counter  66 , a count value memory unit  67 , and an operation unit  68 . The CPU  61  is connected to the ROM  60 , the RAM  62 , the input-output unit  63 , the image memory  64 , the image processing unit  65 , the counter  66 , the count value memory unit  67 , and the operation unit  68 . The CPU  61  is also connected to the sheet-feeding unit  2 , the secondary transfer unit  3 , the first and second image forming units  4  and  5 , the fixing unit  6 , and the sheet-discharging unit  7 . 
   The ROM  60  stores programs including operation programs and the like used by the CPU  61  for executing the image forming operations. The CPU  61  appropriately reads out the programs including the operation programs and controls the entirety of the image forming operations. The RAM  62  is appropriately used by the CPU  61  as a memory area. The input-output unit  63  is connected to an external apparatus, and inputs and outputs a variety of signals of image data and so forth. The image memory  64  stores the image data for forming an image. The image processing unit  65  performs a variety of image processing to the image data and creates data writing signals. 
   The counter  66  performs a counting operation and a count resetting operation according to a command sent by the CPU  61 . The counter  66  may perform the counting operation in synchronization with a main-scanning synchronization signal of the writing unit  32 , as the CPU  61  uses an interrupt processing. Specifically, upon reception of a mark detection interrupt, the CPU  61  executes the count resetting operation, and executes the counting operation upon every interruption of the main-scanning synchronization signal. Since the counting operation is performed in synchronization with the main-scanning synchronization signal, the write position can be highly accurately controlled by aligning a start position in each scanning movement. Further, a special timer is unnecessary, and thus the structure of the apparatus can be simplified. 
   The count value memory unit  67  stores count values C 1 , C 2 , C 3 , and C 4 . As illustrated in a time chart of  FIG. 3  which indicates operation timings, the count value C 1  corresponds to a value counted by the counter  66  during a time period from the first detection of the mark M 1  by the mark detecting unit  23  until the start of optical writing of the A color. The count value C 2  corresponds to a value counted by the counter  66  during a time period from the first detection of the mark M 2  until the start of optical writing of the B color. The count value C 3  corresponds to a value counted by the counter  66  during a time period from the second detection of the mark M 1  by the mark detecting unit  23  until the start of optical writing of the C color. The count value C 4  corresponds to a value counted by the counter  66  during a time period from the second detection of the mark M 2  by the mark detecting unit  23  until the start of optical writing of the D color. 
   The operation unit  68  receives an input of the count values C 1 , C 2 , C 3 , and C 4  through operations performed by a user, and stores the count values C 1 , C 2 , C 3 , and C 4  in the count value memory unit  67 . It is possible to apply an alternative way of inputting and storing the count values C 1 , C 2 , C 3 , and C 4  into the count value memory unit  67 . 
   The count values C 1  and C 2  are adjusted such that a toner image of the B color is superimposed on a toner image of the A color. The count value C 3  is adjusted such that a toner image of the C color is superimposed on a composite toner image of the A color and the B color. The count value C 4  is adjusted such that toner images of the A color, the B color, and the C color are superimposed. The count value C 1  may be set to zero so that optical writing of the A color starts immediately after detection of the mark M 1 . 
   An interval between the marks M 1  and M 2  is set to a predetermined value in relation to two distances. A first distance is between a distance from a write position of the writing unit  32  on the photoconductor drum  30  to a primary transfer position of the second image forming unit  5  relative to the intermediate transfer belt  20 . A second distance is from a write position of the writing unit  42  on the photoconductor drum  40  to a primary transfer position of the second image forming unit  5  relative to the intermediate transfer belt  20 . The predetermined value of the interval between the marks M 1  and M 2  is set as not to exceed a difference between the above-mentioned first and second distances. That is, the distance between the marks M 1  and M 2  is shorter than a value of the above-mentioned difference between the first and second distances. By being so arranged, superimposing timing of two toner images at the primary transfer position of the second image forming unit  5  may intentionally be shifted. More specifically, it is assumed that writing of latent images by the writing units  32  and  42  start immediately after detections of the marks M 1  and M 2 , respectively. Then, two time periods are considered. A first time period is from a time the writing unit  32  starts writing a first latent image until a time the first latent image is developed into a first visible toner image and is transferred to the primary transfer position of the second image forming unit  5 . A second time period is from a time the writing unit  42  starts writing a second latent image until a time the second latent image is developed into a second visible toner image and is transferred to the primary transfer position of the second image forming unit  5 . Since the distance between the marks M 1  and M 2  is, as described above, shorter than the difference between the first and second distances, the second visible toner image reaches the primary transfer position of the second image forming unit  5  earlier than the first visible toner image. In other words, the timing of creating the second visible toner image is needed to be adjusted to synchronize with the timing of the first visible toner image reaching the primary transfer position of the second image forming unit  5 . Therefore, it becomes possible to accurately synchronize the superimposing timing of the second visible toner image to the first visible toner image at the primary transfer position of the second image forming unit  5  by adjusting this difference in the distances by using the count values. 
   Referring to  FIG. 4 , an A-color image forming operation of the color image forming apparatus  1  is explained. As illustrated in  FIG. 4 , the CPU  61  drives the photoconductor drums  30  and  40  and the intermediate transfer belt  20  upon a receipt of a command to start printing (Step S 10 ). Then, the CPU  61  switches the development unit  33  to the A-color development device  36 , and switches the development unit  43  to the B-color development device  46  (Step S 1 ). The CPU  61  uses the speed detecting unit  24  to detect an event that the speed of the intermediate transfer belt  20  reaches a constant value (Step S 12 ). Thereafter, when the CPU  61  receives an input of a mark detection signal representing the first detection of the mark M 1  from the mark detecting unit  23  (Step S 13 ), it resets the counter  66  to start the counting operation (Step S 14 ). When the count value of the counter  66  reaches the count value C 1  stored in the count value memory unit  67  (Step S 15 ), the CPU  61  causes the writing unit  32  to start an optical writing of the A color on the photoconductor drum  30 , so that a formation of an electrostatic latent image of the A color is started (Step S 16 ). Then, the CPU  61  causes the A-color development device  36  to start forming a toner image of the A color (Step S 17 ). The toner image of the A color formed on the photoconductor drum  30  is transferred to the intermediate transfer belt  20  at a contact point between the photoconductor drum  30  and the intermediate transfer belt  20  (Step S 18 ). Upon a completion of the development operation by the A-color development device  36 , the CPU  61  switches the development unit  33  to the C-color development device  37  (Step S 19 ). Then, the A-color image forming operation ends. 
   Referring to  FIG. 5 , a B-color image forming operation of the color image forming apparatus  1  is explained. As illustrated in  FIG. 5 , the CPU  61  determines a receipt of an input of a mark detection signal representing a first detection of the mark M 2  after having received an input of the mark detection signal representing a first detection of the mark M 1  (Step S 20 ). Upon a determination of detecting the mark M 2  after the first detection of the mark M 1 , the CPU  61  resets the counter  66  to start the counting operation (Step S 21 ). The mark M 2  is detected during a formation of the toner image of the A color. When the count value of the counter  66  reaches the count value C 2  stored in the count value memory unit  67  (Step S 22 ), the CPU  61  causes the writing unit  42  to start optical writing of the B color on the photoconductor drum  40 , so that a formation of an electrostatic latent image of the B color is started (Step S 23 ). Then, the CPU  61  causes the B-color development device  46  to start forming a toner image of the B color (Step S 24 ). The toner image of the B color is formed on the photoconductor drum  40  in accordance with the count value C 2 . Thus, the toner image of the B color is transferred to the intermediate transfer belt  20  so as to be superimposed on the toner image of the A color. Accordingly, a composite toner image of the A color and the B color is formed on the intermediate transfer belt  20  (Step S 25 ). Upon a completion of the development operation by the B-color development device  46 , the CPU  61  switches the development unit  43  to the D-color development device  47  (Step S 26 ). Then, the B-color image forming operation ends. 
   Referring to  FIG. 6 , a C-color image forming operation of the color image forming apparatus  1  is explained. As illustrated in  FIG. 6 , the CPU  61  determines a receipt of an input of a mark detection signal representing the second detection of the mark M 1  from the mark detecting unit  23  after the intermediate transfer belt  20  has been rotated one round (Step S 30 ). Upon a determination of such second detection of the mark M 1 , the CPU  61  resets the counter  66  to start the counting operation (Step S 31 ). When the count value of the counter  66  reaches the count value C 3  stored in the count value memory unit  67  (Step S 32 ), the CPU  61  causes the writing unit  32  to start optical writing of the C color on the photoconductor drum  30 , so that formation of an electrostatic latent image of the C color starts (Step S 33 ). Then, the CPU  61  causes the C-color development device  37  to start forming a toner image of the C color (Step S 34 ). The toner image of the C color is formed on the photoconductor drum  30  in accordance with the count value C 3 . Thus, the toner image of the C color is transferred to the intermediate transfer belt  20  so as to be superimposed on the toner images of the A color and the B color. Accordingly, a composite toner image of the A color, the B color, and the C color is formed on the intermediate transfer belt  20  (Step S 35 ). Then, the C-color image forming operation ends. 
   Referring to  FIG. 7 , a D-color image forming operation of the color image forming apparatus  1  is explained. As illustrated in  FIG. 7 , the CPU  61  determines a receipt of an input of a mark detection signal representing a detection of the mark M 2  after having received the second input of the mark detection signal representing the detection of the mark M 1  (Step S 40 ). Upon a determination of such detection of the mark M 2 , the CPU  61  resets the counter  66  to start the counting operation (Step S 41 ). The mark M 2  is detected during formation of the toner image of the C color. When the count value of the counter  66  reaches the count value C 4  stored in the count value memory unit  67  (Step S 42 ), the CPU  61  causes the writing unit  42  to start optical writing of the D color on the photoconductor drum  40 , so that formation of an electrostatic latent image of the D color starts (Step S 43 ). Then, the CPU  61  causes the D-color development device  47  to start forming a toner image of the D color (Step S 44 ). The toner image of the D color is formed on the photoconductor drum  40  in accordance with the count value C 4 . Thus, the toner image of the D color is transferred to the intermediate transfer belt  20  so as to be superimposed on the toner images of the A color, the B color, and the C color. Accordingly, a full-color toner image of the A color, the B color, the C color, and the D color is formed on the intermediate transfer belt  20  (Step S 45 ). Then, the D-color image forming operation ends. 
   The full-color toner image formed by superimposing the toner images on the intermediate transfer belt  20  is transferred at one time onto the transfer sheet conveyed by the secondary transfer roller  25  from the sheet-feeding unit  2 . The full-color toner image transferred to the transfer sheet is fixed thereon by the fixing unit  6 , and then the transfer sheet is discharged by the sheet-discharging unit  7 . 
   Thus, in the image forming apparatus  1 , optical writing of each color starts at predetermined counts after detection of the mark M 1  or M 2 . Therefore, the marks M 1  and M 2  are not necessarily needed to be placed with their interval highly accurately equalized with the distance between the primary transfer position of the photoconductor drum  30  and the primary transfer position of the photoconductor drum  40 . Accordingly, the toner images can be accurately superimposed while simplifying the structure of the intermediate transfer belt  20 . 
   Referring to  FIG. 8 , a color image forming apparatus  70  according to another embodiment of the present invention is explained. The color image forming apparatus  70  of  FIG. 8  is similar to the color image forming apparatus  1  of  FIG. 1 , except for an intermediate transfer belt  71 . As illustrated in  FIG. 8 , the intermediate transfer belt  71  is provided on an upper surface thereon with five marks M 11 , M 12 , M 13 , M 21 , and M 22 , for example. These marks M 11 , M 12 , M 13 , M 21 , and M 22  are formed such that four of these marks M 11 , M 12 , M 13 , M 21 , and M 22  are located within a distance equivalent to a distance between the two primary transfer positions for the photoconductor drums  30  and  40  relative to the intermediate transfer belt  20 . Also, the marks M 11 , M 12 , M 13 , M 21 , and M 22  are placed at regular intervals in a non-image region on the intermediate transfer belt  71 . 
   The mark M 11  is used as a reference in the optical writing of the A color and the C color, while the mark M 21  is used as a reference in the optical writing of the B color and the D color. A distance between the marks M 11  and M 21  is substantially equal to or shorter than the distance between the primary transfer positions between the photoconductor drums  30  and  40 . More specifically, this distance between the marks M 21  and M 11  is the closest distance to the distance between the primary transfer positions of the photoconductor drums  30  and  40  among the distances between the marks M 11 , M 12 , M 13 , M 21 , and M 22 . 
   The count value memory unit  67  stores the count values C 1 , C 2 , C 3 , and C 4 . As illustrated in  FIG. 9 , the count value C 1  corresponds to a value counted by the counter  66  during a time period from the first detection of the mark M 11  by the mark detecting unit  23  until the start of optical writing of the A color. The count value C 2  corresponds to a value counted by the counter  66  during a time period from the first detection of the mark M 21  until the start of optical writing of the B color. The count value C 3  corresponds to a value counted by the counter  66  during a time period from the second detection of the mark M 11  by the mark detecting unit  23  until the start of optical writing of the C color. The count value C 4  corresponds to a value counted by the counter  66  during a time period from the second detection of the mark M 21  by the mark detecting unit  23  until the start of optical writing of the B color. 
   After the mark detecting unit  23  detects the mark M 11 , the CPU  61  counts the number of marks detected by the mark detecting unit  23  and selects the mark M 21 . Control by the CPU  61  at the time of interruption may be performed to select the mark M 21  such that marks detected by the mark detecting unit  23  are ignored during a time from detection of the mark  11  by the mark detecting unit  23  until immediately before detection of the mark  21 . 
   The image forming operation performed by the color image forming apparatus  70  of  FIG. 8  is similar to the image forming operation performed by the color image forming apparatus  1  of  FIG. 1 , except that the mark M 11  replaces the mark M 1  as the reference in the optical writing of the A color and the C color, and the mark M 21  replaces the mark M 2  as the reference in the optical writing of the B color and the D color. 
   As described above, the color image forming apparatus  70  is configured to start an optical writing of each color at a predetermined number of counts after a detection of the mark M 11  or M 21 . Therefore, the marks M 11  and M 21  are not necessarily needed to be placed with their interval highly accurately equalized with the distance between the primary transfer positions of the photoconductor drum  30  and the photoconductor drum  40 . Therefore, the toner images can be accurately superimposed while simplifying the structure of the intermediate transfer belt  20 . 
   Further, the color image forming apparatus  70  is configured to select the mark M 21  and uses it as a reference in the optical writing of the B color and the D color. The mark M 21  is placed with a distance from the mark M 11 . This distance between the mark M 21  to the mark M 11  is substantially equal to or shorter than the distance between the primary transfer positions of the photoconductor drums  30  and  40 . Furthermore, this distance of the mark M 21  to the mark M 11  is the closest distance to the distance between the primary transfer positions of the photoconductor drums  30  and  40  among the distances between the marks M 11 , M 12 , M 13 , M 21 , and M 22 . Accordingly, the color image forming apparatus  70  is capable of preventing misalignment of the superimposed toner images caused by a change in the speed of the intermediate transfer belt  20  and the like. 
   Referring to  FIGS. 10 and 11 , a color image forming apparatus  72  according to another embodiment of the present invention is explained. The color image forming apparatus  72  of  FIGS. 10 and 11  is similar to the color image forming apparatus  1  of  FIG. 1 , except for a temperature sensor  73 . 
   The temperature sensor  73  is placed near the intermediate transfer belt  20  for detecting the temperature of the intermediate transfer belt  20 . The temperature sensor  73  is preferably used to measure temperatures at the primary transfer positions at which the intermediate transfer belt  20  faces the photoconductor drum  30  and the photoconductor drum  40 . The temperature sensor  73  also measures a temperature at a position somewhere between the primary transfer positions of the photoconductor drum  30  and the photoconductor drum  40 . 
   The count value memory unit  67  stores a count value C n  (n=1, 2, 3, and 4) in accordance with the temperature detected by the temperature sensor  73 . If the temperature of the interior of the color image forming apparatus  72  is increased due to continuous use of the color image forming apparatus  72 , the length of the intermediate transfer belt  20  changes. Therefore, a relationship between a change in the temperature detected by the temperature sensor  73  and a change in the length of the intermediate transfer belt  20  is previously measured, and the count value C n  (n=1, 2, 3, and 4) in accordance with the temperature detected by the temperature sensor  73  is stored. 
   The image forming operation performed by the color image forming apparatus  72  of  FIGS. 10 and 11  is similar to the color image forming operation performed by the color image forming apparatus  1  of  FIG. 1 , except that, when the CPU  61  reads out the count value C n  (n=1, 2, 3, and 4) from the count value memory unit  67 , the CPU  61  selects the count value in accordance with the temperature detected by the temperature sensor  73 . 
   In this way, the color image forming apparatus  72  is configured to select the appropriate count value C n  in accordance with the change in the length of the intermediate transfer belt  20  caused by the change in the temperature of the intermediate transfer belt  20 . Accordingly, a high-quality image having no color drift can be formed by finely adjusting the leading end position of an image in consideration of expansion and contraction of the intermediate transfer belt  20 . 
   Referring to  FIG. 12 , a color image forming apparatus  74  according to another embodiment of the present invention is explained. The color image forming apparatus  74  of  FIG. 12  is similar to the color image forming apparatus  1  of  FIG. 1 , except for a use frequency memory unit  75  and a delay amount memory unit  76 . 
   The use frequency memory unit  75  stores the number of printed sheets. The delay amount memory unit  76  stores data representing a relationship between the number of printed sheets and the delay amount. The delay amount is an amount of displacement of a position at which a full-color toner image is formed on the intermediate transfer belt  20 . The position is displaced each time the number of printed sheets exceeds a predetermined value, and the amount of displacement is expressed in the count value counted by the counter  66 . 
   The image forming operation performed by the color image forming apparatus  74  is similar to the image forming operation of the color image forming apparatus  1 , except for the following two operations. That is, a first operation is that the number of printed sheets stored in the use frequency memory unit  75  is counted up in each image forming operation executed by the CPU  61 . A second operation is that the CPU  61  reads the delay amount from the delay amount memory unit  76  in accordance with the number of printed sheets stored in the use frequency memory unit  75  and subsequently adds the read delay amount to each of the count values C n  (n=1, 2, 3, and 4) read out from the count value memory unit  67 . 
   The CPU  61  counts the number of printed sheets, and changes the timing of writing each color by the same count value at each time the count value exceeds a predetermined count value. By so controlling, the CPU  61  delays the timing of starting the writing operation and arbitrarily changes the position of an image formed on the intermediate transfer belt  20 . 
   In this way, the color image forming apparatus  74  is configured to change the position of the full-color toner image formed on the intermediate transfer belt  20  as the number of printed sheets increases. The intermediate transfer belt  20  can thereby have a relatively longer lifetime with a suppression of deterioration over time. 
   Alternatively, another unit of use frequency may be used. For example, the use frequency memory unit  75  may store a use time, and the delay amount memory unit  76  may store the delay amount in accordance with the use time. 
   Referring to  FIG. 13 , another image forming operation performed by the color image forming apparatus  1  of  FIG. 1  is explained. In this image forming operation, the color image forming apparatus  1  stores the count values C n  (n=1, 2, 3, and 4) in the count value memory unit  67  in accordance with a toner image count, that is, which of the image formations is performed in a continuous image forming operation. The image forming operations of the present embodiment are similar to the image forming operations of the color image forming apparatus  1 , except that the CPU  61  executes the counting operation to find on which one of the transfer sheets a toner image is formed in the continuous image forming operation and selects the count value to be read out from the count value memory unit  67  in accordance with on which one of the transfer sheets the toner image is formed. 
   As illustrated in  FIG. 13 , writing of the A color starts and then writing of the B color starts on the first sheet. Then, writing of the C color starts during the writing of the B color subsequent to the writing of the A color. Thereafter, writing of the D color starts during the writing of the C color subsequent to the writing of the B color. Timings of writing the respective colors of the A color, the B color, the C color, and the D color on the second and following sheets are the same as the timings for the writing on the first sheet. Writing of the A color on a sheet starts during writing of the D color on a previous sheet, and writing of the B color on the sheet starts after writing of the D color on the previous sheet. 
   During writing of the colors on the first sheet, the second transfer roller  25  does not contact the intermediate transfer belt  20  at the start of writing each color on the first sheet, but contacts the intermediate transfer belt  20  during writing of the C color and the D color. During writing of the colors on the second and following sheets, the secondary transfer roller  25  contacts the intermediate transfer belt  20  at the start of writing the A color and the B color on the sheets, separates from the intermediate transfer belt  20  during writing of the A color and the B color, and contacts the intermediate transfer belt  20  during writing of the C color and the D color. 
   During writing of the colors on the first sheet, the belt cleaning unit  26  contacts the intermediate transfer belt  20  prior to writing of the A color, separates from the intermediate transfer belt  20  during writing of the B color, i.e., subsequent to writing of the A color and prior to writing of the C color, and contacts the intermediate transfer belt  20  during writing of the D color subsequent to writing of the C color. 
   A load on the intermediate transfer belt  20  changes, as the secondary transfer roller  25  and the belt cleaning unit  26  come in contact with or separate from the intermediate transfer member  20 . Therefore, the moving speed of the intermediate transfer belt  20  changes. The count value stored in the snout value memory unit  67  is determined based on an expected change in the speed of the intermediate transfer belt  20  in consideration of differences in the load on the intermediate transfer belt  20  according to on which one of transfer sheets a toner image being formed is located. 
   In this way, the color image forming apparatus  1  is configured to perform the image forming operation of  FIG. 13  in which the count value read out from the count value memory unit  67  is selected in accordance with the toner image on which one of the transfer sheets is formed. Accordingly, the color drift can be prevented from occurring in the toner images superimposed on the intermediate transfer belt  20 . 
   This invention may be conveniently implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the computer art. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The present invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
   This patent specification is based on Japanese patent application, No. JP2005-048183 filed on Feb. 24, 2005 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.