Abstract:
Disclosed herein are an image forming apparatus and a color registration method thereof. The color registration method includes checking a change in a status of an image forming apparatus, selecting color registration patterns having a different length according to the status change, and performing color registration by printing the color registration patterns having the different length onto a transfer belt.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2009-0002188, filed in the Korean Intellectual Property Office on Jan. 12, 2009, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Aspects of one or more embodiments relate to an electrophotographic image forming apparatus to perform color registration using different patterns in different operation modes and a color registration method thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    A traditional electrophotographic image forming apparatus (such as a laser printer, a digital copier, etc.) forms an electrostatic latent image on the surface of a photosensitive medium charged with a predetermined potential by scanning light onto the photosensitive medium, develops the electrostatic latent image into a visible image by applying a toner as a developing agent to the electrostatic latent image, and transfers and fixes the developed image onto a piece of paper. An electrophotographic color image forming apparatus creates a color image by supplying four colors of toners (black K, yellow Y, magenta M, and cyan C) to a photosensitive medium and overlapping the resulting different color images with one another. 
         [0006]    Two color image creation schemes are generally available for the electrophotographic color image forming apparatus: a single-pass type using four optical scanning units and four photosensitive media, and a multi-pass type using a single optical scanning unit and a single photosensitive medium. When the electrophotographic color image forming apparatus, especially the single-pass electrophotographic color image forming apparatus, fails to accurately overlap the images of different colors at intended positions, image quality is degraded, as is the case with blurs at the edges of a color image. As this occurs due to interaction between a plurality of factors including developer replacement, an increased number of printed papers, etc., color registration may be required to align the images of the respective colors with one another. In this context, the single-pass color image forming apparatus is configured to create high-quality color images through Auto Color Registration (ACR) that triggers color registration automatically, each time various composite factors occur, such as developer replacement, an increase in the number of pieces of printed papers, and the like. 
         [0007]    To implement the ACR, the single-pass color image forming apparatus prints ACR patterns of a predetermined length (offset correction patterns for black K, yellow Y, magenta M, and cyan C for use in color registration) on a transfer belt and receives light reflected from each color offset correction pattern through an ACR sensor, thereby recognizing the toner transfer position of each color. With respect to the position of one reference color (e.g. black), the positions of the other colors are adjusted based on the sensed toner transfer positions, such that the images of the respective colors are aligned overlapped at correction positions. However, the ACR patterns of the predetermined length should be printed on a transfer belt for each color registration, taking a large amount of toner. The resulting increased ACR process time does not satisfy user convenience. 
       SUMMARY OF THE INVENTION 
       [0008]    Aspects of one or more embodiments to provide a method to perform color registration using ACR patterns of different lengths in different operation modes in a single-pass color image forming apparatus. 
         [0009]    According to an aspect of one or more embodiments, a color registration method is provided. The method includes detecting a change in a status of an image forming apparatus, selecting color registration patterns having a different length according to the detected status change, and performing color registration by printing the selected color registration patterns having the different length onto a transfer belt. 
         [0010]    According to another aspect of one or more embodiments, the color registration method may further include determining a current operation mode for the color registration, wherein the current operation mode is one of a plurality of operation modes, the plurality of operation modes including a first operation mode in which the color registration is performed under a condition of replacement of a consumable part and a second operation mode in which the color registration is performed under a condition other than replacement of a consumable part. 
         [0011]    According to another aspect of one or more embodiments, the condition of replacement of the consumable part may be that a toner transfer position for each color is misaligned along an X-axis direction perpendicular to a transfer direction of the transfer belt by at least a predetermined number of dots due to replacement of a developer or the transfer belt. 
         [0012]    According to another aspect of one or more embodiments, the condition other than replacement of the consumable part may be that a toner transfer position for each color is misaligned along an X-axis direction perpendicular to a transfer direction of the transfer belt by less than a predetermined number of dots due to an increase in the number of printed papers, a temperature change of a set, or power on/off. 
         [0013]    According to another aspect of one or more embodiments, the color registration patterns may be offset correction patterns for a plurality of colors for use in the color registration. 
         [0014]    According to another aspect of one or more embodiments, the offset correction patterns may include a color registration pattern for X-axis offset correction in the first operation mode and a color registration pattern for X-axis offset correction in the second operation mode. 
         [0015]    According to another aspect of one or more embodiments, the color registration patterns for the first and second operation modes may have different horizontal lengths. 
         [0016]    According to another aspect of one or more embodiments, the horizontal length of the color registration pattern of the first operation mode may be set according to the X-axis offset deviation prior to the color registration. 
         [0017]    According to another aspect of one or more embodiments, the horizontal length of the color registration pattern of the second operation mode may be set according to the X-axis offset deviation after the color registration. 
         [0018]    According to another aspect of one or more embodiments, the horizontal length of the color registration pattern of the second operation mode may be set to be shorter than the horizontal length of the color registration pattern of the first operation mode. 
         [0019]    According to another aspect of one or more embodiments, the color registration may include correcting positions of other colors with respect to a position of one reference color among a plurality of colors and aligning images of the colors to overlap with one another at the corrected positions. 
         [0020]    According to another aspect of one or more embodiments, the color registration method may further include measuring a velocity change of the transfer belt, and the color registration may include changing positions of the color registration patterns according to the velocity change of the transfer belt. 
         [0021]    According to another aspect of one or more embodiments, the color registration method may further include changing a length of the color registration patterns for each of the plurality of operation modes, and the color registration may include printing the color registration patterns of the changed length according to the current operation mode. 
         [0022]    According to another aspect of one or more embodiments, an image forming apparatus is provided. The image forming apparatus includes a storage unit to store different color registration patterns having different lengths for a plurality of operation modes for color registration, a controller to select color registration patterns of a different length according to a current operation mode, and a printer to print the selected color registration patterns onto a transfer belt so as to perform the color registration. 
         [0023]    According to another aspect of one or more embodiments, the controller may measure a velocity change of the transfer belt and perform the color registration by changing positions of the color registration patterns according to the velocity change of the transfer belt. 
         [0024]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0026]      FIG. 1  illustrates the configuration of a color image forming apparatus according to an embodiment; 
           [0027]      FIG. 2  is a control block diagram of the color image forming apparatus to perform color registration according to an embodiment; 
           [0028]      FIG. 3  illustrates ACR patterns with which Y-axis offsets are corrected in a first operation mode in the color image forming apparatus according to an embodiment; 
           [0029]      FIG. 4  illustrates ACR patterns with which X-axis offsets are corrected in the first operation mode in the color image forming apparatus according to an embodiment; 
           [0030]      FIG. 5  illustrates ACR patterns with which Y-axis offsets are corrected in a second operation mode in the color image forming apparatus according to an embodiment; 
           [0031]      FIG. 6  illustrates ACR patterns with which X-axis offsets are corrected in the second operation mode in the color image forming apparatus according to an embodiment; 
           [0032]      FIG. 7  is a graph illustrating variations in the velocity of a transfer belt in the color image forming apparatus according to an embodiment; 
           [0033]      FIG. 8  is a flowchart illustrating a color registration method in the color image forming apparatus according to an embodiment; and 
           [0034]      FIG. 9  illustrates ACR patterns with which X-axis offsets are corrected based on a change in the velocity of the transfer belt in the color image forming apparatus according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0035]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
         [0036]      FIG. 1  illustrates the configuration of a color image forming apparatus  1  according to an embodiment. As shown, the single-pass color image forming apparatus  1  to create a color image by sequentially transferring toner images of different colors overlappingly onto a piece of paper P. The single-pass color image forming apparatus  1  includes, within a body  10  forming its exterior, a paper feeding unit  20 , optical scanning units  30 , a development unit  40 , a transfer unit  50 , a fixing unit  60 , a paper discharge unit  70 , and a sensor unit  80 . According to other aspects of one or more embodiments, the single-pass color image forming apparatus  1  may include additional and/or different units. Similarly, the functionality of two or more of the above units may be integrated into a single component. Moreover, aspects of one or more embodiments can be implemented using other types of color image forming apparatuses, such as a multi-purpose type color image forming apparatus 
         [0037]    The paper feeding unit  20  is provided with a paper feeding cassette  21  detachably mounted to a bottom of the body  10 , a paper pressing plate  22  on which the paper P is stacked, an elastic member  23  under the paper pressing plate  22 , and a pick-up roller  24  positioned at a leading end of the paper P stacked on the paper pressing plate  22 . The paper pressing plate  22  is rotatable upward and downward within the paper feeding cassette  21 . The elastic member  23  elastically supports the paper pressing plate  22 . The pick-up roller  24  picks up the paper P from the paper pressing plate  22 . 
         [0038]    The optical scanning units  30  (or  30 K,  30 Y,  30 M and  30 C) scans light corresponding to image information of different colors, for example, black K, yellow Y, magenta M and cyan C onto the development unit  40 . A Laser Scanning Unit (LSU) using a laser diode as a light source may be used for the optical scanning units  30 . As shown, the apparatus  1  uses four colors, but aspects of one or more embodiments are not limited to the shown colors, and is also usable with different numbers of colors. 
         [0039]    The development unit  40  includes four developers  40 K,  40 Y,  40 M and  40 C to accommodate toners of different colors (for example, black K, yellow Y, magenta M, and cyan C toners) therein. The developers  40 K,  40 Y,  40 M and  40 C respectively include photosensitive media  41 K,  41 Y,  41 M and  41 C on which electrostatic latent images are formed by the optical scanning units  30 . While the photosensitive media  41 K,  41 Y,  41 M and  41 C are installed in the developers  40 K,  40 Y,  40 M and  40 C in the illustrated case of  FIG. 1 , they may be provided separately from the developers  40 K,  40 Y,  40 M and  40 C in the body  10 . 
         [0040]    Each of the developers  40 K,  40 Y,  40 M and  40 C has a toner storage  42  having toner, a charge roller  43 , a development roller  44  to develop an electrostatic latent image formed on a photosensitive medium to a toner image, and a supply roller  45  to supply the toner to the development roller  44 . 
         [0041]    The transfer unit  50  transfers toner images developed on the photosensitive media  41 K,  41 Y,  41 M, and  41 C onto the paper P. The transfer unit  50  is provided with a Paper Transfer Belt (PTB)  51  that goes around in contact with the photosensitive media  41 K,  41 Y,  41 M and  41 C, a driving roller  52  that drives the PTB  51 , a support roller  53  that maintains the tensile force of the PTB  51 , and four transfer rollers  54  that transfer the toner images from the photosensitive media  41 K,  41 Y,  41 M and  41 C to the paper P. 
         [0042]    The fixing unit  60  fixes the toner images onto the paper P by heat and pressure. The fixing unit  60  includes a heating roller  61  and a pressing roller  62 . The heating roller  61  has a heating source to heat the paper P with the toner transferred thereon. The pressing roller  62  faces the heating roller  61  to maintain a fixing pressure at a predetermined level with respect to the heating roller  61 . 
         [0043]    The paper discharge unit  70  discharges the printed paper P outside the body  10 . The paper discharge unit  70  includes a discharge roller  71  and a back-up roller  72  that rotates along with the discharge roller  71 . 
         [0044]    The sensor unit  80  senses toner transfer positions of ACR patterns printed on the transfer belt  51 , for color registration. The sensor unit  80  includes an optical sensor including a light emitter and a light receiver. The optical sensor projects light toward the transfer belt  51  before the light emitter along an X-axis direction. The light receiver receives the light reflected from the transfer belt  51 . The sensor unit  80  detects the toner transfer positions of the ACR patterns by receiving the light reflected from toner layers of the ACR patterns (offset correction patterns for the respective colors) printed on the transfer belt  51 . Because color registration may differ in one end portion and the other end portion of the transfer belt  51  along a width direction of a color image due to the scanning skews of the optical scanners  30 , the light receiver is positioned at both ends of the transfer belt  51 . 
         [0045]      FIG. 2  is a control block diagram of the color image forming apparatus to perform color registration according to an embodiment. The color image forming apparatus includes an operation mode decider  100 , a storage unit  102 , a controller  104 , and a printer  106 . While not required in all aspects, the decoder  100  and controller  104  can be implemented on one or more processors and/or computers, and may be implemented using software and/or firmware stored on one or more computer readable media. 
         [0046]    The operation mode decider  100  selects an operation mode in which color registration is to be performed in the single-pass color image forming apparatus  1 . The operation mode may be a first operation mode or a second operation mode. In the first operation mode, ACR is performed under the condition that the toner transfer position for each color is greatly out of alignment by at least a predetermined number of dots due to replacement of a consumable part, such as the developers  40 K,  40 Y,  40 M and  40 C or the transfer belt  51 . In the second operation mode, ACR is performed under the condition that the toner transfer position for each color is slightly out of alignment by fewer than a predetermined number of dots due to variations in set conditions other than replacement of a consumable part, such as an increase in the number of printed papers, a temperature change of a set, power on/off, and the like. 
         [0047]    The storage  102  sets and stores ACR patterns of a different horizontal length according to the ACR operation mode used. The storage  102  sets the horizontal length of the ACR patterns (the X-axis length of offset correction patterns) shorter in the second operation mode than in the first operation mode. The reason for using the shorter ACR patterns in the second operation mode is to reduce toner consumption and an ACR process time by changing the horizontal length of the ACR patterns according to the used operation mode, considering the fact that the misalignment of the toner transfer positions is less in the second operation mode than in the first operation mode. While not required in all aspects, the storage  102  can be magnetic and/or optical media, and can be rewritable as in the case that the ACR patterns are updated. 
         [0048]    The ACR patterns are offset correction patterns corresponding to the four colors, black K, yellow Y, magenta M and cyan C for color registration. These ACR patterns may take various shapes. According to aspects of one or more embodiments, the ACR patterns are set to correct offset deviations in X-axis and Y-axis directions, taking into account X-axis and Y-axis misalignments of the toner transfer positions depending on whether a consumable part (such as the developer  40 K,  40 Y,  40 M, or  40 C, or the transfer belt) is replaced. To this end, the horizontal of the ACR patterns is changed depending on whether the consumable part is replaced with a new one, which will be described later with reference to  FIGS. 3 and 4 . As shown, the X axis is horizontal, and the Y axis is parallel to a moving direction of the paper. 
         [0049]    The controller  104  selects the ACR patterns for use in X-axis and Y-axis offset correction from the storage  102  to perform color registration according to the operation mode of the color image forming apparatus  1  decided by the operation mode decider  100 . The controller  104  provides the selected ACR patterns to the printer  106  so that the printer  106  prints the ACR patterns. 
         [0050]    The printer  106  prints the selected ACR patterns on the transfer belt  51 . The sensor unit  80  senses the toner transfer positions of the ACR patterns and notifies the controller  104  of the sensed toner transfer positions. The controller  104  performs the ACR according to the toner transfer positions of the ACR patterns to calibrate color registration by controlling the optical scanners  30 K,  30 Y,  30 M and  30 C such that images of the respective colors are overlapped at accurate positions. 
         [0051]      FIG. 3  illustrates ACR patterns with which Y-axis offsets are corrected in the first operation mode in the color image forming apparatus according to an embodiment.  FIG. 4  illustrates ACR patterns with which X-axis offsets are corrected in the first operation mode in the color image forming apparatus according to an embodiment. 
         [0052]    Referring to  FIGS. 3 and 4 , the horizontal lengths D of the ACR patterns for use in X-axis and Y-axis offset correction are equal. The horizontal length D of the ACR patterns for the first operation mode may be computed by 
         [0000]        D/ 2 =A+B+C   [Equation 1] 
         [0000]    where A denotes a left margin deviation (generally about 1.5 mm) along a main scanning direction of a reference color (e.g. black) among the four colors, black K, yellow Y, magenta M and cyan C, with respect to a maximum deviation that may occur in the X-axis direction (i.e. the main scanning direction) when the developers  40 K,  40 Y,  40 M and  40 C are mounted initially, B denotes a pre-ACR correction X-axis offset deviation (generally about 2.5 mm) between the reference color (black) and the other colors (e.g. yellow, magenta, and cyan), and C denotes the beam diameter (generally about 1.5 mm) of the optical sensor being the optical sensor unit. 
         [0053]    Referring to  FIG. 4 , as ACR patterns for X-axis offset correction, a bar pattern along a horizontal direction and a slant pattern inclined from the horizontal direction by a predetermined angle are formed for each color. Therefore, the toner transfer position of each color may be adjusted by as many X-axis dots as misaligned based on the differences between the bar-slant pattern interval of the reference color (black) and the bar-slant pattern intervals of the other colors (yellow, magenta, and cyan). 
         [0054]      FIG. 5  illustrates ACR patterns with which Y-axis offsets are corrected in the second operation mode in the color image forming apparatus according to an embodiment.  FIG. 6  illustrates ACR patterns with which X-axis offsets are corrected in the second operation mode in the color image forming apparatus according to an embodiment. 
         [0055]    Referring to  FIGS. 5 and 6 , the horizontal lengths E of the ACR patterns for use in X-axis and Y-axis offset correction are equal. The horizontal length E of the ACR patterns for the second operation mode may be computed by 
         [0000]        E/ 2 =A+B′+C   [Equation 2] 
         [0000]    where A denotes a left margin deviation (generally about 1.5 mm) along the main scanning direction of the reference color (e.g. black) among the four colors, black K, yellow Y, magenta M and cyan C, with respect to a maximum deviation that may occur in the X-axis direction (i.e. the main scanning direction) when the developers  40 K,  40 Y,  40 M and  40 C are mounted initially, B′ denotes an X-axis offset deviation (generally about 0.2 mm) between the reference color (black) and the other colors (e.g. yellow, magenta, and cyan), which may be caused by an increase in the number of printed papers and a temperature change of the set after ACR correction. C denotes the beam diameter (generally about 1.5 mm) of the optical sensor being the optical sensor unit  80 . The values of A, B, B′, and C are for purposes of example; aspects of one or more embodiments are not limited thereto. 
         [0056]    Referring to  FIG. 6 , as ACR patterns for X-axis offset correction in the second operation mode, a bar pattern along the horizontal direction and a slant pattern inclined from the horizontal direction by a predetermined angle are formed for each color. As noted, because the horizontal length E of the ACR patterns for the second operation mode is shorter than the horizontal length D of the ACR patterns for the first operation mode, the longitudinal length F of the slant patterns is also shortened. 
         [0057]    Because the positions of the other color images (yellow, magenta, and cyan) are corrected with respect to the position of the reference color image (black) by the ACR, the X-axis inter-set deviation A of the reference color (black) is not corrected even after the ACR. The X-axis deviations between the reference color (black) and the other colors (yellow, magenta, and cyan) include the main scanning-directional deviations B among the colors of the optical scanners  30 K,  30 Y,  30 M and  30 C when an ACR is initially performed, and are the deviations B′ caused by a change in set conditions since an ACR is performed based on previous correction values after the ACR. 
         [0058]    In a current set, B and B′ are roughly given as follows.
       B=2.5 mm and B′=0.2 mm (B&gt;B′)       
 
         [0060]    Therefore, the horizontal length of the ACR patterns may be decreased by
       (B−B′)×2=4.6 mm       
 
         [0062]    The horizontal length E (about 6.4 mm) of the ACR patterns calculated by [equation 2] in the second operation mode is about 58.2% shorter than the horizontal length D (about 11 mm) of the ACR patterns calculated by [equation 1] in the first operation mode. 
         [0063]    The ACR is performed mostly in the second operation mode in the color image forming apparatus  1 . The use of the shorter horizontal length E of the ACR patterns leads to the reduction of toner consumption for each color during ACR and also to the decrease of the longitudinal length F of the slant patterns for X-axis color registration (refer to  FIG. 6 ). The resulting decrease of the total Y-axis length of the ACR patterns shortens the total ACR process time. With respect to the X-axis offset correction patterns, since the sensing distance between a bar pattern and a slant pattern is reduced, the influence of a Y-axis velocity of the transfer belt  51  that is generated during rotation of the transfer belt  51  may be minimized, as illustrated in  FIG. 7 . 
         [0064]      FIG. 8  is a flowchart of a color registration method in the color image forming apparatus according to an embodiment. In operation  200 , the single-pass color image forming apparatus  1  determines a current operation mode for color registration through the operation mode decider  100  by checking a status change caused by replacement of a consumable part (a developer or a transfer belt) or a change in set conditions. The operation mode may be a first operation mode in which an ACR is performed under the condition that the toner transfer position of each color is greatly misaligned by a predetermined number of or more dots because of replacement of a consumable part (such as the developers  40 K,  40 Y,  40 M and  40 C or the transfer belt  51 ), or a second operation mode in which an ACR is performed under the condition that the toner transfer position of each color is slightly misaligned by fewer than a predetermined number of dots because of operation errors or the like, without replacement of a consumable part. 
         [0065]    In operation  202 , the controller  104  determines whether the current operation mode is the first operation mode. In the case of the first operation mode, the controller  104  selects ACR patterns corresponding to the first operation mode as illustrated in  FIGS. 3 and 4  from the storage  102  in operation  204 . In operation  206 , the controller  104  provides the first-operation mode ACR patterns to the printer  106  and controls the printer  106  to print the patterns onto the transfer belt  51 . The optical sensor unit  80  at both end portions of the transfer belt  51  senses the toner transfer positions of the ACR patterns and notifies the controller  104  of the sensed toner transfer positions. 
         [0066]    In operation  208 , the controller  104  performs an ACR by controlling the optical scanning units  30  to overlap the images of the respective colors at correct positions according to the toner transfer positions of the ACR patterns, thereby calibrating color registration. 
         [0067]    If the current operation mode is not the first operation mode in operation  202 , the controller  104  determines whether the current operation mode is the second operation mode in operation  210 . In the case of the second operation mode, the controller  104  selects ACR patterns corresponding to the second operation mode as illustrated in  FIGS. 5 and 6  from the storage  102  in operation  212 . In operation  214 , the controller  104  provides the second-operation mode ACR patterns to the printer  106  and controls the printer  106  to print the patterns onto the transfer belt  51 . The pair of sensor units  80  at both end portions of the transfer belt  51  sense the toner transfer positions of the ACR patterns and notifies the controller  104  of the sensed toner transfer positions. 
         [0068]    In operation  208 , the controller  104  performs an ACR by controlling the optical scanning units  30  to overlap the images of the respective colors at correct positions according to the toner transfer positions of the ACR patterns, thereby calibrating color registration. 
         [0069]    As described above, different ACR operation modes may employ ACR patterns of different horizontal lengths according to the misalignment degrees of toner transfer positions caused by replacement or non-replacement of a consumable part. While shown with only two modes for purposes of simplicity, further modes can be defined to account for different color registration problems caused by specific events. According to other aspects of one or more embodiments, ACR patterns may be changed, taking into further account the velocity change of the transfer belt  51 . This method will be described below with reference to  FIG. 9 . 
         [0070]      FIG. 9  illustrates ACR patterns with which X-axis offsets are corrected according to a change in the velocity of the transfer belt in the color image forming apparatus according to an embodiment. As shown in  FIG. 9 , a bar pattern and a slant pattern for the reference color (black) are formed with a minimal distance to bar and slant patterns for the other colors (yellow, magenta and cyan). 
         [0071]    The distance G between each black pattern and any other color pattern is determined based on the beam diameter of the sensor unit  80  and Y-axis offset deviations among the colors. When the reference-color patterns (the black patterns) are close to specific-color patterns (e.g. yellow patterns) in X-axis offset correction of yellow, the influence of the Y-axis velocity change of the transfer belt  51  caused by its rotation may be reduced. This is because the velocity change of the transfer belt  51  may need to be considered and the impact of the velocity change of the transfer belt  51  may need to be avoided as well, for Y-axis offset correction. 
         [0072]    As is apparent from the above description, the single-pass color image forming apparatus performs color registration using ACR patterns of a different length according to an operation mode used. Therefore, toner consumption and an ACR process time are reduced. Also, the accuracy of the color registration is improved by changing the positions of the correction patterns according to a velocity change of the transfer belt when an ACR is performed. 
         [0073]    Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.