Patent Publication Number: US-2009225342-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Provisional U.S. Application 61/035,212 filed on Mar. 10, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus for performing registration adjustment of an image and image quality adjustment in a color copy machine or a multi-function peripheral. 
     BACKGROUND 
     In a color image, image adjustment is performed to maintain the image quality. In the image adjustment, registration adjustment to adjust a positional relation of plural images, and image quality adjustment of the plural images are performed. There is an image forming apparatus in which for the image adjustment, the registration adjustment is performed after density adjustment of an image is performed. 
     In order to perform stable density adjustment, it is necessary to sufficiently agitate a developer and to stabilize the charge amount of the developer. In order to perform the stable density adjustment, it takes a time to sufficiently agitate the developer. When the registration adjustment is performed after the density adjustment is performed at the time of the image adjustment, since it takes a time to perform the density adjustment, there is a fear that the waiting time of a user until the image adjustment is ended becomes long. 
     It is desired to develop an image forming apparatus in which the waiting time of the user until the image adjustment is ended is shortened, and the image forming speed is increased. 
     SUMMARY 
     In an aspect of the invention, registration adjustment and image quality adjustment of a color image are performed without requiring a long waiting time, and the image forming speed is increased. 
     According to an aspect, an image forming apparatus includes a latent image forming member to form an electrostatic latent image on each of plural image carriers, plural developing members to develop the electrostatic latent images formed on the plural image carriers and to form plural toner images, a transfer member to transfer the plural toner images formed on the plural image carriers, and a control member that detects positions of the plural toner images transferred to a transfer medium, causes a registration mode of performing registration adjustment of the respective toner images and an image quality maintaining mode of performing image quality adjustment of the respective toner images to be selectively performed, and performs the registration mode before the image quality maintaining mode. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural view showing a copy machine of a first embodiment; 
         FIG. 2  is a schematic explanatory view showing sensors in the first embodiment; 
         FIG. 3  is a block diagram showing a control system in the first embodiment; 
         FIG. 4  is a table showing an environmental characteristic of a developing device stored in a memory in the first embodiment; 
         FIG. 5  is a graph showing an aging characteristic of the developing device stored in the memory in the first embodiment; 
         FIG. 6  is a graph showing an environmental characteristic of a photoconductive drum stored in the memory in the first embodiment; 
         FIG. 7  is a graph showing an aging characteristic of laser light amount stored in the memory in the first embodiment; 
         FIG. 8  is a table showing an environmental coefficient of development contrast stored in the memory in the first embodiment; 
         FIG. 9  is a flowchart showing image adjustment in the first embodiment; 
         FIG. 10  is a schematic explanatory view showing patterns printed on a transfer belt in the first embodiment; 
         FIG. 11  is an explanatory view for setting an adjustment value of image inclination from the patterns in the first embodiment; 
         FIG. 12  is an explanatory view sowing an inclination shift on a photoconductive drum in the first embodiment; 
         FIG. 13A  is an explanatory view showing a toner image with an inclination shift in the first embodiment; 
         FIG. 13B  is an explanatory view showing adjustment of a tilt mirror in the first embodiment; 
         FIG. 14  is an explanatory view for setting an adjustment value of a position shift in a sub-scanning direction from the patterns in the first embodiment; 
         FIG. 15  is an explanatory view showing a sub-scanning position shift on the photoconductive drum in the first embodiment; 
         FIG. 16  is an explanatory view showing a toner image with a sub-scanning position shift in the first embodiment; 
         FIG. 17  is an explanatory view for setting an adjustment value of a position shift in a main scanning direction from the patterns in the first embodiment of the invention; 
         FIG. 18  is an explanatory view showing a main scanning position shift on the photoconductive drum in the first embodiment; 
         FIG. 19  is an explanatory view showing a toner image with a main scanning position shift in the first embodiment; 
         FIG. 20  is an explanatory view for setting an adjustment value of a main scanning magnification shift from the patterns in the first embodiment of the invention; 
         FIG. 21  is an explanatory view showing a main scanning magnification shift on the photoconductive drum in the first embodiment; 
         FIG. 22  is an explanatory view showing a toner image with a main scanning magnification shift in the first embodiment; 
         FIG. 23  is an explanatory view showing patches of density detection in the first embodiment; 
         FIG. 24  is an explanatory view showing a relation between an image density and a detected value of a density sensor in the first embodiment; 
         FIG. 25  is an explanatory view showing a relation between the charge amount of a developer and image adjustment in the first embodiment; 
         FIG. 26  is a schematic structural view showing a main part of a color copy machine of a first modified example; 
         FIG. 27  is a schematic perspective view showing the main part of the color copy machine of the first modified example; 
         FIG. 28  is a schematic structural view showing a main part of a color copy machine of a second modified example; 
         FIG. 29  is a schematic structural view showing a main part of a color copy machine of a third modified example; and 
         FIG. 30  is a schematic structural view showing a main part of a color copy machine of a fourth modified example. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a first embodiment will be described.  FIG. 1  is a schematic structural view of a color copy machine  1  of train-of-four tandem system as an image forming apparatus of the first embodiment. The color copy machine  1  includes a scanner unit  6  to read an original document supplied by an auto document feeder  4 . The color copy machine  1  includes four sets of image forming stations  11 Y,  11 M,  11 C and  11 K of yellow (Y), magenta (M), cyan (C) and black (K) disposed in parallel along a transfer belt  10 . 
     The respective image forming stations  11 Y,  11 M,  11 C and  11 K include photoconductive drums  12 Y,  12 M,  12 C and  12 K as image carriers. A rotation shaft of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is parallel to a main scanning direction. The main scanning direction is orthogonal to a traveling direction of an arrow n direction of the transfer belt  10  (sub-scanning direction). The respective rotation shafts of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are disposed to be spaced from each other at equal intervals in the sub-scanning direction. 
     Charging chargers  13 Y,  13 M,  13 C and  13 K, developing devices  14 Y,  14 M,  14 C and  14 K as developing members, and photoconductive cleaners  16 Y,  16 M,  16 C and  16 K are respectively disposed around the photoconductive drums  12 Y,  12 M,  12 C and  12 K along the rotation direction of an arrow m direction. The developing devices  14 Y,  14 M,  14 C and  14 K respectively include developers of different colors of yellow (Y), magenta (M), cyan (C) and black (K). The developing devices  14 Y,  14 M,  14 C and  14 K develop electrostatic latent images on the photoconductive drums  12 Y,  12 M,  12 C and  12 K to form toner images of the respective colors on the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K. 
     A laser exposure device  17  as a latent image forming member irradiates respective laser exposure beams to the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The respective laser exposure beams are based on data of the respective color components of the image data. The laser exposure device  17  forms the respective electrostatic latent image on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. 
     The transfer belt  10  is supported by a drive roller  20  and a driven roller  21 , and is rotated in the arrow n direction. The toner images formed on the photoconductive drums  12 Y,  12 M,  12 C and  12 K are transferred to a sheet P as a recording medium conveyed in the arrow n direction by the transfer belt  10  at positions of transfer rollers  15 Y,  15 M,  15 C and  15 K. A color toner image is formed on the sheet P conveyed by the transfer belt  10 . The transfer belt  10  and the transfer rollers  15 Y,  15 M,  15 C and  15 K constitute a transfer member. 
     The sheet P is fed from a cassette mechanism  3  including first and second paper feed cassettes  3   a  and  3   b  to the transfer belt  10  through a conveying unit  7 . The conveying unit  7  includes pickup rollers  7   a  and  7   b  to pick up a sheet from the paper feed cassettes  3   a  and  3   b , separation conveying rollers  7   c  and  7   d , a conveying roller  7   e  and a registration roller  8 . The color toner image formed on the sheet P is fixed by a fixing device  22  and a color image is completed, and then, the sheet is ejected through a paper discharge roller  25   a  to a storage tray  25   b . After the transfer is ended, residual toners on the photoconductive drums  12 Y,  12 M,  12 C and  12 K are cleaned by the photoconductive cleaners  16 Y,  16 M,  16 C and  16 K, and next printing becomes possible. 
     As shown in  FIG. 2 , first and second pattern sensors  36  and  37  are provided downstream of the image forming station  11 K of black (K) of the transfer belt  10 . A density sensor  42  is provided at an intermediate position between the first and the second pattern sensors  36  and  37 . The first and the second pattern sensors  36  and  37  are used for registration. The density sensor  42  is used for image quality adjustment. The first and the second pattern sensors  36  and  37  perform position detection of registration patterns formed on the transfer belt  10  at the time of a registration mode. The first and the second pattern sensors  36  and  37  are disposed to be spaced from each other by a specified distance in the main scanning direction. A temperature and humidity sensor  38  for detecting an environment in a main body of the color copy machine  1  is provided above the density sensor  42 . 
       FIG. 3  is a block diagram showing a control system  100  as a control member mainly relating to image adjustment. In the image adjustment, registration adjustment of an image and image quality adjustment are performed. The first and the second pattern sensors  36  and  37 , the density sensor  42 , and the temperature and humidity sensor  38  are connected to the input side of a CPU  101  of the control system  100 , which controls the whole color copy machine  1 . Counters  40  to count the number of rotations of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K of the respective color components and the number of rotations of each of developing rollers of the developing devices  14 Y,  14 M,  14 C and  14 K of the respective color components are connected to the input side of the CPU  101 . Other sensors  41  necessary for the image formation are connected to the input side of the CPU  101 . 
     The CPU  101  is connected to a laser control unit  110  and a print control unit  120  through an input and output interface. The CPU  101  is connected to a scanner control unit  130  to control the auto document feeder  4  and the scanner unit  6 . 
     The CPU  101  includes a memory  102  to store various settings for controlling the laser control unit  110  and the print control unit  120 . The memory  102  stores, for example, a table of an environmental characteristic of each of the developing devices  14 Y,  14 M,  14 C and  14 K shown in  FIG. 4 . The table of the environmental characteristic of each of the developing devices  14 Y,  14 M,  14 C and  14 K represents a development contrast Vc with respect to the inside humidity of the color copy machine  1 . 
     The memory  102  stores, for example, a graph of an aging characteristic of each of the developing devices  14 Y,  14 M,  14 C and  14 K shown in  FIG. 5 .  FIG. 5  shows a contrast coefficient with respect to the drive time of each of the developing devices  14 Y,  14 M,  14 C and  14 K (that is, the number of rotations of the developing roller). The contrast coefficient is used for adjustment of a variation of the development contrast (life control). 
     The memory  102  stores, for example, a graph of an environmental characteristic of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K shown in  FIG. 6 . The graph of the environmental characteristic of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K represents a variation of a residual potential Ver of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K with respect to the inside temperature of the color copy machine  1 . 
     The memory  102  stores, for example, life control of a laser light amount with respect to the drive time of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K shown in  FIG. 7 .  FIG. 7  shows a laser coefficient to adjust the laser light amount with respect to the drive time of each of the photoconductive drums  12 Y,  12 M,  12 C and  12 K (that is, the number of rotations of the photoconductive drum). The laser coefficient is used to adjust the variation of the development contrast. 
     The memory  102  stores, for example, a development bias of each of the developing devices  14 Y,  14 M,  14 C and  14 K and a laser light amount of each of the color components of the laser exposure device  17 , which are prior density adjustment values at the time of the image formation performed just before. 
     The memory  102  stores, for example, an environmental coefficient based on a humidity change of a development bias of each of the developing devices  14 Y,  14 M,  14 C and  14 K shown in  FIG. 8 . 
     The CPU  101  includes an arithmetic unit  103  to calculate an adjustment value of the registration adjustment and an adjustment value of the image quality adjustment by using the laser control unit  110  and the print control unit  120 . In the registration adjustment and the image quality adjustment, the adjustment values are calculated by using the detection results of the registration patterns formed on the transfer belt  10 . The laser control section  110  adjusts the laser exposure device  17  based on the adjustment values obtained by the calculation. The print control unit  120  adjusts the developing devices  14 Y,  14 M,  14 C and  14 K based on the adjustment values. 
     The laser control unit  110  includes a laser driver  111  to adjust writing timings of laser oscillators  111 Y,  111 M,  111 C and  111 K of the respective color components or laser light amounts of the respective color components. The laser control unit  110  includes a mirror driver  112  to adjust angles of tilt mirrors  112 Y,  112 M,  112 C and  112 K of the respective color components. 
     The print control unit  120  controls the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the transfer belt  10 , the charging chargers  13 Y,  13 M,  13 C and  13 K, the developing devices  14 Y,  14 M,  14 C and  14 K, the photoconductive cleaners  16 Y,  16 M,  16 C and  16 K, and the conveying unit  7 . The print control unit  120  adjusts the developing biases of the developing devices  14 Y,  14 M,  14 C and  14 K. 
     The first and the second pattern sensors  36  and  37 , the laser control unit  110  and the print control unit  120  are used for the registration adjustment. The density sensor  42 , the laser control unit  110  and the print control unit  120  are used for the image quality adjustment. 
     For example, when power is turned ON, the color copy machine  1  starts the image adjustment shown in a flowchart of  FIG. 9 . In the image adjustment, the registration mode for performing the registration adjustment of plural toner images, and the image quality maintaining mode for performing the image quality adjustment of plural toner images are performed. 
     The color copy machine  1  forms a color image by superimposing toner images of four colors of Y, M, C and K. In the color image, the color balance is lost when for example, image density of merely one color of the four colors of Y, M, C and K is shifted. Fine image quality adjustment is required for all of the four colors of Y, M, C and K. In the color copy machine  1 , when the superimposing positions of toner images of the four colors (Y, M, C and K) are shifted, the color image becomes blurring. The registration adjustment to correct the shift of the toner images of the respective colors is required. Next, the outline of the image quality adjustment and the registration adjustment of the color image will be described. 
     (I) Outline of the Image Quality Adjustment of the Image; 
     (A) In the color image forming apparatus, the charging device applies electric charge and the photoconductor is charged to a surface potential V 0 . When exposure light is irradiated according to image information, an electrostatic latent image with residual potential Ver is formed on the photoconductor. Toner is supplied to the portion of the residual potential Ver by the developing device to develop the electrostatic latent image. When the development bias Vb is applied to the developing device, the toner adhesion amount of the photoconductor is changed according to the value of |Vb−Ver|. (|Vb−Ver| is called development contrast Vc.) 
     (B) (a) The charge amount of a developer is changed according to the humidity. There is a tendency that when the humidity is changed to a low humidity side, the image density becomes low, and when the humidity is changed to a high humidity side, the image density becomes high. In order to put the image density within a specified range, it is necessary that when the humidity is changed to the low humidity side, the development contrast Vc is set to be large, and when the humidity is changed to the high humidity side, the development contrast Vc is set to be low. 
     (b) The charge amount of the developer is changed according to the drive time (aging change occurs). As the drive time of the developing device elapses, the charge amount of the developer is lowered, and there is a tendency that the image density becomes high when the development contrast is the same as the initial one. In order to put the image density within the specified range, it is necessary that the development contrast Vc is set to become gradually low according to the temporal change. 
     (c) When |V 0 −Vb| of the developing device (|V 0 −Vb| is called background potential Vbg) is set to be low, there occurs fogging in which unnecessary toner is adhered to the white background of the photoconductor. When |V 0 −Vb| is set to be high, the carrier of the developer adheres to the photoconductor, and the developer is gradually decreased. It is necessary to set |V 0 −Vb|=Vbg within a suitable range. 
     (d) The electrostatic characteristic of the photoconductor is changed by temperature. When the temperature is changed to a low temperature side, the residual potential Ver rises. Even if the exposure amount is raised, the residual potential Ver hardly changes. 
     As in (b) to (d), the image quality is changed by the environmental characteristics and the aging characteristics. It is preferable to perform the image quality adjustment periodically. Like the case where the power of the image forming apparatus is turned ON in morning, when printing is started after the image forming apparatus is left for a long time, there is a high-possibility that the environmental characteristics are significantly changed, and the image quality adjustment of the toner image is especially desired. 
     (II) Outline of the Registration Adjustment of the Image 
     In the color image forming apparatus, for example, the temperature in the machine body when power is turned ON in morning is close to the room temperature (significantly different from the temperature in the machine body during print operation). In the optical system of the laser exposure device, a variation characteristic occurs by a variation in the temperature in the machine body of the color image forming apparatus. When the characteristics of the optical system vary, a position shift relatively occurs among the toner images of the four colors. The image registration adjustment corrects the position shift of toner images caused by the environmental characteristics. 
     In this embodiment, as the image adjustment, the image registration adjustment is performed, and then, the image quality adjustment is performed. In the image registration adjustment, patterns as pattern toner images written on the transfer belt  10  are detected, and the writing position of the pattern is adjusted. 
     An image density in an image formation condition when the patterns are written is made a registration density. The registration density is set separately from an image density in an image formation condition at the time of normal image formation (at the time of image formation mode). It is necessary that the registration density is suitably set. For example, when the same image patterns are formed, when the image density of the image pattern becomes low, the line width of the obtained image pattern becomes thin as compared with the case where the image density is high. When the patterns are written on the transfer belt  10 , when the image density is not stabilized, variation occurs in the line widths of the obtained patterns. When the line width of the pattern varies, an error occurs in the registration adjustment. Thus, the registration density is made the density within the range where a problem does not occur in the registration adjustment. That is, the registration density may not fall within the target range of the image density for image formation. 
     The registration density is determined based on, for example, 
     (1) the environmental characteristics and aging characteristics of the color copy machine  1 . 
     The registration density is determined based on, for example, 
     (2) the immediately prior density adjustment value. 
     In  FIG. 9 , the patterns are written on the transfer belt  10  by using the registration density determined based on the environmental characteristics and the aging characteristics of the color copy machine  1  of (1). The detection result of the temperature and humidity sensor  38  is acquired by the image adjustment start. The drive times of the developing rollers of the respective developing devices  14 Y,  14 M,  14 C and  14 K are acquired from the count results of the counters  40 . The drive times of the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K are acquired (Act  200 ). 
     An image formation condition A (development bias, photoconductor surface potential, laser light amount), as density adjustment values, is calculated which is for obtaining the registration density when the patterns of the respective color components are written on the transfer belt  10  (Act  201 ). At Act  201 , the development contrast Vc is obtained by referring to  FIG. 4  from the humidity classification of the color copy machine  1  acquired at Act  200 . The contrast coefficient is obtained by referring to  FIG. 5  from the drive time of the developing roller acquired at Act  200 . The desired development contrast Vc is obtained by multiplying the development contrast Vc obtained by referring to  FIG. 4  by the contrast coefficient obtained by referring to  FIG. 5 . The desired development contrast Vc obtained by multiplying the contrast coefficient is made the value for the registration density and the development contrast Vc used when the pattern is first printed. For example, when the humidity in the main body of the color copy machine  1  is 45% RH and the drive time of the developing roller is close to the life end, the development contrast becomes Vc=320×0.5=160 (V). 
     At Act  201 , the residual potential Ver is determined by referring to  FIG. 6  from the temperature of the color copy machine  1  acquired at Act  200 . The laser coefficient is obtained by referring to  FIG. 7  from the number of rotations of the photoconductive drum acquired at Act  200 . The laser light amount Lp is determined by multiplying the initial laser light amount Lp (ini) by the laser coefficient obtained by referring to  FIG. 7 . The photoconductor surface potential V 0  is obtained by adding the development contrast Vc and the background potential Vbg (The background potential Vbg is fixed value. For example, the background potential Vbg is 120 V) to the residual potential Ver. That is, the photoconductor surface potential is V 0 =Ver+Vc+Vbg. 
     The patterns of the specified condition are printed on the transfer belt  10  under the image formation condition A (development bias Vb, photoconductor surface potential V 0 , laser light amount Lp) calculated at Act  201 . The interval between the printed patterns is detected (Act  202 ). At Act  202 , as shown in  FIG. 10 , the wedge-shaped patterns of the four colors of Y, M, C and K are printed on the transfer belt  10 . The four colors of Y, M, C and K are made one set, and eight sets of front side patterns  50 Y,  50 M,  50 C and  50 K and eight sets of rear side patterns  51 Y,  51 M,  51 C and  51 K are printed on the transfer belt  10 . 
     At Act  202 , each of the intervals between Y-M, M-C and C-K of the patterns is made an interval of 320 [dot] of a specified value. The pattern pitch between the sets is made a specified value of 128.4 [mm]. The interval of the front side patterns  50 Y,  50 M,  50 C and  50 K is measured plural times by the front side first pattern sensor  36 . The interval of the rear side patterns  51 Y,  51 M,  51 C and  51 K is measured plural times by the rear side second pattern sensor  37 . 
     The adjustment value of the registration adjustment is calculated from the average value of the intervals of the patterns measured at Act  202  (Act  203 ). The arithmetic unit  103  sets the adjustment value based on the measurement result. The setting of the adjustment value is well known (see, for example, JP-A-8-278680), and various well-known methods can be adopted. 
     For example, as shown in  FIG. 11 , it is assumed that the output timing of the front side pattern  50 K of black (K) is shifted from that of the rear side pattern  51 K by Δt 1 . As shown in  FIG. 12 , the arithmetic unit  103  determines that an axis  113 K of the photoconductive drum  12 K of black (K) is inclined with respect to the scanning direction of a laser beam  114 K of the laser oscillator  111 K of black (K). When image formation is performed without adjustment, as shown in  FIG. 13A , a black toner image formed on the sheet P becomes an inclined shift toner image  117  indicated by a solid line with respect to an appropriate position  116  indicated by a chain line. In order to adjust the inclination, the arithmetic unit  103  sets the inclination amounts of the tilt mirrors  112 Y,  112 M,  112 C and  112 K as the adjustment values according to the inclination amount. As shown in  FIG. 13B , tilt motors  312 Y,  312 M,  312 C and  312 K are driven by the mirror driver  112  as the need arises, and the inclinations of the tilt mirrors  112 Y,  112 M,  112 C and  112 K are adjusted in an arrow direction. Respective scanning lines  114 Y,  114 M,  114 C and  114 K are shifted in an arrow t direction and the inclination is adjusted. 
     For example, as shown in  FIG. 14 , it is assumed that the interval T 1  between the pattern  50 C,  51 C of cyan (C) and the pattern  50 K,  51 K of black (K) is shifted from the interval T 2  between other patterns. As shown in  FIG. 15 , the arithmetic unit  103  determines that a position  118 K of the pattern  50 K,  51 K of black (K) is shifted in the sub-scanning direction by Δt 2  as the difference between the interval T 1  and the interval T 2  with respect to an original position  119 K. When the image formation is performed without adjustment, as shown in  FIG. 16 , the black toner image formed on the sheet P becomes a sub-scanning position shift toner image  122  indicated by a solid line with respect to an appropriate position  121  indicated by a chain line. 
     In order to adjust the shift in the sub-scanning direction, the arithmetic unit  103  sets the difference between output timings of the image data corresponding to Δt 2  as the adjustment value. The laser control unit  110  adjusts the delay amount between the drums. Incidentally, the adjustment value obtained by combining the inclination amount in  FIG. 11  and the shift amount of the pattern interval in the sub-scanning direction in  FIG. 14  may be set as the adjustment value. 
     For example, as shown in  FIG. 17 , it is assumed that detection lengths ΔK 1 , ΔC 1 , ΔM 1  and ΔY 1  of the front side patterns  50 Y,  50 M,  50 C and  50 K are shifted from each other. As shown in  FIG. 18 , the arithmetic unit  103  determines that a position of each color component  123  is shifted from an original position  124  in the main scanning direction by α. When the image formation is performed without adjustment, as shown in  FIG. 19 , a toner image formed on the sheet P becomes a main scanning position shift toner image  127  indicated by a solid line with respect to an appropriate position  126  indicated by a chain line. The amount of the position shift of the image in the main scanning direction is determined from the respective differences of the detection lengths ΔK 1 , ΔC 1 , ΔM 1  and ΔY 1 . 
     The arithmetic unit  103  sets the shift amounts of image data corresponding to the detection lengths ΔK 1 , ΔC 1 , ΔM 1  and ΔY 1  as the adjustment values in order to adjust the shift in the main scanning direction. The adjustment values are set so as to establish ΔK 1 =ΔC 1 =ΔM 1 =ΔY 1 . The laser control unit  110  adjusts the timing of the start of main scanning printing-out. 
     For example, as shown in  FIG. 20 , it is assumed that detection lengths ΔK 2 , ΔC 2 , ΔM 2  and ΔY 2  of the front side patterns  50 Y,  50 M,  50 C and  50 K of the respective color components and detection lengths ΔK 3 , ΔC 3 , ΔM 3  and ΔY 3  of the rear side patterns  51 Y,  51 M,  51 C and  51 K are respectively shifted. As shown in dot units in  FIG. 21 , the arithmetic unit  103  determines that the number of dots of each color component  128  is different from the number of dots of an original pattern  129 , and a magnification shift occurs in the main scanning direction. When the image formation is performed without adjustment, as shown in  FIG. 22 , the toner image formed on the sheet P becomes a main scanning magnification shift toner image  132  indicated by a solid line with respect to an appropriate image  131  indicated by a chain line. With respect to the amount of the magnification shift of the image in the main scanning direction, the adjustment value is set from the value obtained by adding the front side detection lengths ΔK 2 , ΔC 2 , ΔM 2  and ΔY 2  of the respective color components and the rear side detection lengths ΔK 3 , ΔC 3 , ΔM 3  and ΔY 3 . When (ΔK 2 +ΔK 3 )=(ΔC 2 +ΔC 3 )=(ΔM 2 +ΔM 3 )=(ΔY 2 +ΔY 3 ) is established, it is determined that the image magnifications of the respective color components in the main scanning direction are constant. 
     In order to adjust the magnification shift in the main scanning direction, the arithmetic unit  103  sets the speed of an image clock as the adjustment value. The laser control unit  110  adjusts the magnifications of clock frequencies of the laser oscillators  111 Y,  111 M,  111 C and  111 K. 
     The respective adjustment values calculated at Act  203  are decided as the registration adjustment values (Act  204 ). The respective decided adjustment values are stored in the memory  102 . The image registration adjustment is completed. 
     By the decided registration adjustment values, patch data for image quality adjustment is used, and patches as toner images for image quality are printed on the transfer belt  10 . The patches are printed under the image formation condition A. The densities of the printed patches are detected (Act  206 ). As shown in  FIG. 23 , patches  134 Y,  134 M,  134 C and  134 K of the respective color components are printed on the transfer belt  10 . With respect to the patches  134 Y,  134 M,  134 C and  134 K, printing is started simultaneously for all colors. When printing is performed by the length of the interval between the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the printing is stopped simultaneously for all the colors. The patches  134 Y,  134 M,  134 C and  134 K of the four colors are arranged on the transfer belt  10  without gap. The patches  134 Y,  134 M,  134 C and  134 K of the four colors are made one set. 
     Each of the patches  134 Y,  134 M,  134 C and  134 K of the four colors include a solid patch (B) and a halftone patch (H) composed of specified pattern dots. The density sensor  42  detects the toner adhesion amount at 12 points for each of the solid patch (B) and the halftone patch (H) 
     The arithmetic unit  103  calculates an average of the detection values of the density sensor  42  and decides a detection value of the density sensor  42 . The arithmetic unit  103  calculates a difference between the target value of the image density of each color component and the decided detection value of the density sensor  42  (Act  207 ).  FIG. 24  shows a relation among the detection value of the density sensor  42 , the toner adhesion amount on the transfer belt  10 , and the image density. A solid line (w) represents the detection value of the density sensor  42 , and a solid line (x) represents the toner adhesion amount on the transfer belt  10 . Reference is made to  FIG. 24 , and the range of the detection value of the density sensor  42  is determined according to the target range of the image density. For example, when the target range of the image density of the halftone patch (H) is (C), the range of the detection value of the density sensor  42  is determined to be (γ). When the target range of the image density of the solid patch (B) is (D), the range of the detection value of the density sensor  42  is determined to be (δ). 
     The CPU  101  determines from the calculation result of Act  207  whether a difference between the image densities of the respective color components is within a specified range (Act  208 ). When the difference between the image densities of the respective color components is within the specified range, advance is made to Act  210 , and the image formation condition is decided. The image formation condition decided at Act  210  is decided as an image formation condition B at the time of the image formation. The image adjustment is ended. 
     When the difference between the image densities of the respective color components exceeds the specified range, advance is made to Act  211 . At Act  211 , the image formation condition is adjusted for each color component. The development contrast Vc is adjusted so that the image density of the solid patch (B) is within the target range. The laser light amount Lp is adjusted so that the image density of the halftone patch (H) is within the target range. From a difference from the range (δ) of the detection value calculated in the arithmetic unit  103 , the print control unit  120  adjusts the development bias so that the image density of the solid patch (B) is within the target range (range (δ) of the detection value). The laser driver  111  adjusts the laser light amount Lp so that the image density of the halftone patch (H) falls within the target range (range (γ) of the detection value). The image density is adjusted at the two points of the solid patch (B) and the halftone patch (H) and the image density is more finely adjusted. 
     The solid patch (B) and the halftone patch (H) are printed on the transfer belt  10  by using the patch data for density detection under the image formation condition adjusted at Act  211 . The density of the printed pattern is detected (Act  212 ). Advance is made to Act  207 , the average of the detection values of the density sensor  42  is calculated, and the detection value of the density sensor  42  is decided. The arithmetic unit  103  calculates a difference between the target value of the image density of each color component and the decided detection value of the density sensor  42 . At Act  208 , when the difference between the image densities of the respective color components is within the specified range, advance is made to Act  210 , and the image formation condition B at the time of the image formation is decided. The decided image formation condition B is stored in the memory  102 . 
     When the difference between the image densities of the respective color components exceeds the specified range (Act  211 ), Act  212 , Act  207  and Act  208  are repeated. The development bias and the laser amount Lp are adjusted so that the output of the density sensor  42  falls within the target range. 
     In the color copy machine  1 , when the image adjustment including the registration adjustment and the image quality adjustment is completed in accordance with the flowchart of  FIG. 9 , a belt cleaner  19  removes the patterns and the patches on the transfer belt  10 . After the image adjustment is completed, the color copy machine  1  performs desired printing on the sheet P according to image data. 
     In the first embodiment, when the power of the color copy machine  1  is turned ON, the registration adjustment is started, and when the registration adjustment is completed, the image quality adjustment is performed. The developer of the developing devices  14 Y,  14 M,  14 C and  14 K of the color copy machine  1  exhibits a charge characteristic shown in  FIG. 25  when the driving is started by power-on. In the developer of the developing devices  14 Y,  14 M,  14 C and  14 K, the charge amount does not become stable until, for example, two minutes pass from the power-on. When the density adjustment is performed before the charge amount of the developer becomes stable, a high-precision adjustment value can not be obtained. In the first embodiment, the registration adjustment is performed before two minutes pass after the power is turned ON, that is, during a period (E) in which the charge amount of the developer does not become stable. During the period (E) in which the registration adjustment is performed, the developing devices  14 Y,  14 M,  14 C and  14 K are driven, and the charge amount of the developer gradually becomes stable. The image quality adjustment is performed during a period (F) after the registration adjustment is completed and after the charge amount of the developer becomes stable. After the power is turned ON, the registration adjustment and the image quality adjustment are all completed at development drive time (t 2 ). 
     For example, as image adjustment of a comparison example, it is assumed that when the power of the color copy machine  1  is turned ON, the density adjustment is started, and after the density adjustment is completed, the registration adjustment is performed. When the power of the color copy machine is turned ON, even if the density adjustment is started, the density adjustment does not become stable until the charge amount of the developer becomes stable. The substantial density adjustment is obtained after (t 1 ) when the charge amount of the developer becomes stable. In the comparison example, the substantial density adjustment is performed in a period (Q) from (t 1 ), and after the density adjustment is completed, the registration adjustment is performed during a period (R). In the image adjustment, after the power is turned ON, the registration adjustment and the density adjustment are all completed at development drive time (t 3 ). In the first embodiment, after the power is turned ON, the density adjustment is completed at (t 2 ). In the comparison example, after the power is turned ON, the density adjustment is completed at (t 3 ). 
     The image adjustment uses, for example, the timing when the power of the color copy machine  1  is turned on, the timing when warm up is performed after a jam removal process, or the timing between the sheets P during printing of image data. 
     The registration adjustment of the image adjustment may not be performed when the power is turned ON immediately after the power of the color copy machine  1  is turned OFF (the temperature in the machine does not change very much). When the power of the color copy machine  1  is turned ON, the temperature of the heat roller of the fixing device  22  or the like is measured by, for example, a thermistor, and the registration adjustment may be performed according to the measurement result as the need arises. For example, only when the measurement result is lower than an allowable temperature (60° C.), the registration adjustment may be performed at the time of warming-up. 
     In the first embodiment, after the power is turned ON, before the charge amount of the developer of the developing devices  14 Y,  14 M,  14 C and  14 K becomes stable, the patterns for registration are formed on the transfer belt  10  at the registration density obtained under the image formation condition A. The registration adjustment is first completed by using the formed patterns. After the power is turned ON, the charge amount of the developer becomes stable before the registration adjustment is completed. After the charge amount of the developer becomes stable, the image quality adjustment is performed. After the power is turned ON, the registration adjustment is first performed without waiting for stabilization of the charge amount of the developer. Thus, the time before the charge amount of the developer becomes stable can be effectively used. Even if the charge amount of the developer does not become stable, the patterns for registration are formed by using the calculated image formation condition A. Accordingly, at the time of the registration adjustment, there does not occur a problem due to the instability of the density of the pattern, and the high-precision registration adjustment is obtained. The waiting time of the user until the image adjustment including the image quality adjustment and the registration adjustment is completed can be shortened, and the speed of printing can be increased. By the shortening of the waiting time until the image adjustment is completed, services to the user can be improved. 
     Next, a second embodiment will be described. In the second embodiment, registration density when patterns for registration adjustment are written is different from that of the first embodiment. The others are constructed similarly to the first embodiment, and with respect to the same structure, the explanation is common to the first embodiment and the second embodiment. In the second embodiment, (2) the patterns are written on the transfer belt  10  by using the registration density determined based on the immediately prior density adjustment value. 
     After the image adjustment is completed, the color copy machine  1  performs desired printing corresponding to image data. For example, when the image adjustment is performed using the timing between sheets P during printing of the image data, the image formation condition B at the time of the image formation decided at Act  210  of  FIG. 9  is made the immediately prior density adjustment value. 
     For example, the image quality adjustment of the color copy machine  1  is performed at the timing between the sheets P during the printing of the image data. In the second embodiment, it is not necessary to perform the operation for setting, based on the environmental characteristics and the aging characteristics, the image formation condition A under which the registration pattern is formed. In the second embodiment, when the image adjustment is started, the immediately prior density adjustment value stored in the memory  102  (image formation condition B) is used as the image formation condition A of Act  202  of  FIG. 9 . Patterns of the specified condition are printed on the transfer belt  10  at the immediately prior density adjustment value (image formation condition B). The interval between the printed patterns is detected. Thereafter, similarly to the first embodiment, the image adjustment is performed by Act  203 , Act  204 , Act  206  to Act  208 , and Act  210  to Act  212 . At Act  210 , the image formation condition B is newly decided, and the memory  102  is written. 
     In the image adjustment when the power of the color copy machine  1  is turned ON, for example, the image formation condition B at the time of image formation immediately before the power of the color copy machine  1  is turned off on the preceding day is used as (2) the registration density determined based on the immediately prior density adjustment value. 
     In the second embodiment, similarly to the first embodiment, after the power is turned ON, the registration adjustment is completed, and then, the image quality adjustment is performed. After the power is turned ON, the registration adjustment can be first performed without waiting for stabilization of the charge amount of the developer, and the time before the charge amount of the developer becomes stable can be effectively used. Even if the charge amount of the developer does not become stable, the patterns are formed at the registration density while the immediately prior density adjustment value (image formation condition B) is made the image formation condition A. The calculation of the image formation condition A becomes unnecessary. There does not occur a problem due to instability of pattern density at the time of registration adjustment, and the registration adjustment can be performed at high precision. The waiting time until the image adjustment of performing the image quality adjustment and the registration adjustment is completed can be shortened, and the speed of printing can be increased. Services to the user can be improved by shortening of the waiting time until the completion of the image adjustment. 
     The invention is not limited to the above embodiment, but various modifications can be made within the scope of the invention. For example, the structure of the image forming apparatus is not limited. In the case of the image forming apparatus of the train-of-four tandem system, the apparatus may be such that color printing is performed using an intermediate transfer belt.  FIG. 26  and  FIG. 27  show a main part of a color copy machine  30  of a first modified example. The color copy machine  30  uses an intermediate transfer belt  31  as an intermediate transfer medium. Four sets of image forming stations  35 Y,  35 M,  35 C and  35 K including photoconductive drums  31 Y,  31 M,  31 C and  31 K of yellow (Y), magenta (M), cyan (C) and black (K) are disposed in parallel along the lower side of the intermediate transfer belt  31 . First and second pattern sensors  36  and  37  and a density sensor  42  are provided downstream of the image forming station  31 K of black (K) in the intermediate transfer belt rotating in an arrow j direction. A detection position of the intermediate transfer belt  31  by the first and the second pattern sensors  36  and  37  and the density sensor  42  is covered with a shutter  45  having openings  45   a.    
     In the first modified example, at the time of image adjustment, front side patterns  50 Y,  50 M,  50 C and  50 K, rear side patterns  51 Y,  51 M,  51 C and  51 K, and patches  134 Y,  134 M,  134 C and  134 K of respective color components are printed on the intermediate transfer belt  31 . The patterns and the patches are respectively read through the openings  45   a  of the shutter  45 , and the image adjustment is performed. After the image adjustment, an intermediate transfer belt cleaner  46  removes the patterns and the patches. At the time of an image formation mode, toner images on the photoconductive drums  31 Y,  31 M,  31 C and  31 K rotating in an arrow i direction are primarily transferred to the intermediate transfer belt  31 . The toner images on the intermediate transfer belt  31  are collectively secondarily transferred to a sheet P at the position of a secondary transfer roller  47 . The toner image on the sheet P is fixed by a fixing device  48 , and the fixed toner image is obtained on the sheet P. 
     The image forming apparatus may be an image forming apparatus of four-rotation intermediate transfer system.  FIG. 28  shows a main part of a color copy machine  60  of a second modified example of the four-rotation intermediate transfer system. The color copy machine  60  includes one photoconductive drum  61  and an intermediate transfer belt  67  as an intermediate transfer medium. The color copy machine  60  includes a revolver developing unit  62  rotating and holding developing devices  62 Y,  62 M,  62 C and  62 K of yellow (Y), magenta (M), cyan (C) and black (K). First and second pattern sensors  36  and  37  and a density sensor  42  are provided around the intermediate transfer belt  67  rotating in an arrow q direction and downstream of the photoconductive drum  61 . 
     In the second modified example, at the time of image adjustment, patterns and patches formed on the intermediate transfer belt  67  are read, and the image adjustment is performed. After the image adjustment, an intermediate transfer belt cleaner  63  removes the patterns and the patches. At the time of an image formation mode, for example, at the first rotation of the photoconductive drum  61 , an yellow (Y) toner image is formed on the photoconductive drum  61 . The yellow (Y) toner image on the photoconductive drum  61  is primarily transferred to the intermediate transfer belt  67  by a primary transfer roller  64 . At the second rotation of the photoconductive drum  61 , a magenta (M) toner image is formed on the photoconductive drum  61 . The magenta (M) toner image on the photoconductive drum  61  is superimposed on the yellow (Y) toner image on the intermediate transfer belt  67  and is primarily transferred by the primary transfer roller  64 . The photoconductive drum  61  is rotated four times, and a color toner image in which the yellow (Y), magenta (M), cyan (C) and black (K) toner images are superimposed on one another is formed on the intermediate transfer belt  67 . The color toner image on the intermediate transfer belt  67  is collectively secondarily transferred to a sheet P at a position of a secondary transfer roller  65 . The toner image on the sheet P is fixed by a fixing device  66 , and the fixed toner image is obtained on the sheet P. 
     The image forming apparatus may be an image forming apparatus of multi-transfer (transfer drum) system.  FIG. 29  shows a main part of a color copy machine  70  of a third modified example of the multi-transfer (transfer drum) system. The color copy machine  70  includes one photoconductive drum  71  and a transfer drum  77  as an intermediate transfer medium. The color copy machine  70  includes a revolver developing unit  72  rotating and holding developing devices  72 Y,  72 M,  72 C and  72 K of yellow (Y), magenta (M), cyan (C) and black (K). First and second pattern sensors  36  and  37  and a density sensor  42  are provided around the transfer drum  77  rotating in an arrow r direction and downstream of the photoconductive drum  71 . 
     In the third modified example, at the time of image adjustment, patterns and patches formed on the transfer drum  77  are respectively read, and the image adjustment is performed. After the image adjustment, a transfer drum cleaner  73  removes the patterns and the patches. At the time of an image formation mode, for example, at the first rotation of the photoconductive drum  71 , an yellow (Y) toner image is formed on the photoconductive drum  71 . The yellow (Y) toner image on the photoconductive drum  71  is primarily transferred to the transfer drum  77 . At the second rotation of the photoconductive drum  71 , a magenta (M) toner image is formed on the photoconductive drum  71 . The magenta (M) toner image on the photoconductive drum  71  is superimposed on the yellow (Y) toner image on the transfer drum  77  and is primarily transferred. The photoconductive drum  71  is rotated four times, and a color toner image in which yellow (Y), magenta (M), cyan (C) and black (K) toner images are superimposed on one another is formed on the transfer drum  77 . The color toner image on the transfer drum  77  is collectively secondarily transferred to the sheet P traveling between the photoconductive drum  71  and the transfer drum  77 . The toner image on the sheet P is fixed by a fixing device  76 , and the fixed toner image is obtained on the sheet P. 
     The image forming apparatus may be an image forming apparatus of collective transfer (multi-development) system.  FIG. 30  shows a main part of a color copy machine  80  of a fourth modified example of the collective transfer (multi-development) system. The color copy machine  80  includes one photoconductive drum  81  and a transfer belt  87 . The color copy machine  80  includes developing devices  82 Y,  82 M,  82 C and  82 K of yellow (Y), magenta (M), cyan (C) and black (K) around the photoconductive drum  81 . First and second pattern sensors  36  and  37  and a density sensor  42  are provided around the photoconductive drum  81  rotating in an arrow u direction and downstream of the transfer belt  87 . 
     In the fourth modified example, at the time of image adjustment, patterns and patches formed on the photoconductive drum  81  are read, and the image adjustment is performed. After the image adjustment, a photoconductive cleaner  83  removes the patterns and the patches. At the time of an image formation mode, for example, at the first rotation of the photoconductive drum  81 , an yellow (Y) toner image is formed on the photoconductive drum  81 . At the second rotation of the photoconductive drum  81 , a magenta (M) toner image is formed to be superimposed on the yellow (Y) toner image on the photoconductive drum  81 . The photoconductive drum  81  is rotated four times, and a color toner image in which yellow (Y), magenta (M), cyan (C) and black (K) toner images are superimposed on one another is formed on the photoconductive drum  81 . The color toner image on the photoconductive drum  81  is collectively primarily transferred to the sheet P traveling on the transfer belt  87 . The toner image on the sheet P is fixed by a fixing device  86 , and the fixed toner image is obtained on the sheet P. 
     The pattern shape or the pattern interval when the registration adjustment is performed is not limited. The pattern of dots of the halftone patch when the image quality adjustment is performed is not limited.