Abstract:
An method of controlling a feeding velocity of a printing medium in an image forming apparatus, and an image forming apparatus to perform the method, the method including sensing a time between two points on a feeding path on which the printing medium is moving, and regulating the feeding velocity of the printing medium according to the sensed time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Patent Application No. 2004-88565, filed Nov. 3, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an image forming apparatus. More particularly, the present invention relates to a method of controlling a top margin to uniformly regulate a position of an image on a printing medium, and an image forming apparatus employing the method.  
         [0004]     2. Description of the Related Art  
         [0005]     A top margin of a printing medium refers to a non-image area on the printing medium. More specifically, the top margin refers to a portion from a leading end to a position where an image starts to be transferred. Therefore, control of the top margin of the printing medium refers to the control of the transferring position of the image on the printing medium.  
         [0006]      FIG. 1  is a sectional view schematically illustrating a conventional color image forming apparatus. Referring to  FIG. 1 , the color image forming apparatus comprises four photoconductive drums  10  whereon electrostatic latent images are formed, a laser scanning unit  12  to generate the electrostatic latent images on the respective photoconductive drums  10 , four developing units  14  to change the electrostatic latent images into visible images, four transferring units  16  to transfer the visible images onto a printing medium, and a conveying belt  18  disposed between the photoconductive drums  10  and the transferring unit  16  to move the printing medium.  
         [0007]     In the conventional image forming apparatus, upon receiving a printing command from a host device such as a computer, a controller (not shown) drives a pickup roller  20  to pick up the printing medium from a paper feeding cassette  24  or a multifunction paper feeding unit  26 . The picked-up printing medium is conveyed between the conveying belt  18  and the photoconductive drums  10  by a feeding roller  22  at a constant velocity so as to have an image transferred thereupon. While being fed, the printing medium is detected by a registration sensor  28  mounted on a printing medium feeding path before entrance to the conveying belt  18 . Also, the laser scanning unit  12  exposes the photoconductive drums  10  to a laser beam after a preset time from a point of detecting the printing medium. The preset time is determined as a difference between a time obtained through a distance from the registration sensor  28  to a transfer position TP and a feeding velocity of the printing medium, and a time taken from a laser-scanning position to the transfer position TP. Therefore, a setup of a transfer location on the printing medium depends on the predetermined feeding velocity and feeding distance of the printing medium.  
         [0008]     However, since the pickup roller  20  or the feeding roller  22  can be abraded or stained with impurities, when the transfer position TP is determined as explained above, radiuses of the rollers  20  and  22  may change, thereby generating an error in the feeding velocity. Also, slips between the printing medium and the rollers  20  and  22  may cause an error in the feeding velocity of the printing medium. Due to the error, the transfer position TP for the image on the printing medium may be incorrectly determined. Furthermore, in a case in which the error is considerable, some images may miss being transferred onto the printing medium, thereby causing a loss of the information.  
       SUMMARY OF THE INVENTION  
       [0009]     An aspect of the present invention is to solve the above and/or other problems and/or disadvantages, and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an image forming apparatus capable of starting transfer of image on a preferable position of a printing medium, and a method of controlling a top margin of the printing medium in the image forming apparatus.  
         [0010]     Another aspect of the present invention is to provide an image forming apparatus capable of improving image quality, and a method of controlling a top margin of a printing medium in the image forming apparatus.  
         [0011]     Yet another aspect of the present invention is to provide a color image forming apparatus capable of rapidly forming a monochromatic image, and a method of controlling a top margin of a printing medium in the color image forming apparatus.  
         [0012]     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention.  
         [0013]     In order to achieve the above-described and/or other aspects of the present invention, there is provided an image forming apparatus comprising a feeding roller; a feeding motor to drive the feeding roller to move a picked-up printing medium; and a controller to control the feeding motor so as to regulate a feeding velocity of the printing medium according to a feeding time T 12  taken to move the printing medium within a predetermined section of a printing medium feeding path.  
         [0014]     The image forming apparatus may further comprise a memory to store a lookup table comprising detailed values of a variation ΔP of a predetermined motor driving frequency corresponding to a comparison between the feeding time T 12  and a reference time T R12 , wherein the controller drives the feeding motor at the predetermined driving frequency during the feeding time T 12  and varies the predetermined motor driving frequency by adding or subtracting the variation ΔP, selected from the lookup table based on the comparison between the feeding time T 12  and the reference time T R12 , with respect to the predetermined driving frequency P.  
         [0015]     According to another aspect of the present invention, there is provided an image forming apparatus comprising a feeding roller; a feeding motor to drive the feeding roller to move a picked-up printing medium; and a controller to control a transfer position of an image on the printing medium by intermitting power transmitted from the feeding motor to the feeding roller during color printing, and to control the transfer position on the printing medium during monochromatic printing by regulating a feeding velocity of the printing medium according to a feeding time t 12  taken to move the printing medium with a predetermined section of the printing medium feeding path.  
         [0016]     The image forming apparatus may comprise first and second photoconductive drums; first and second laser scanning units forming an electrostatic latent image on the first and the second photoconductive drums, respectively; a first sensor mounted on the printing medium feeding path; a second sensor mounted on the printing medium feeding path at a predetermined distance from the first sensor; a clutch connecting or disconnecting the feeding roller with respect to the feeding motor; a memory which stores a lookup table comprising detailed values of a variation ΔP of the predetermined motor driving frequency corresponding to a comparison between the feeding time t 12  taken to move the printing medium within the predetermined section and the reference time t R12  and variation Δt R2  of a time t R2  to restore the predetermined motor driving frequency; and a controller.  
         [0017]     When implementing color printing, the controller may disconnect the clutch upon detection of the printing medium by the second sensor. When the final laser scanning is performed by the first unit, the controller may change the reference time t RL  to a first reference time t RL1  which is determined by a distance between the first laser scanning unit and the transfer position TP, and connect the clutch after the first reference time t RL1 , and when the final laser scanning is performed by the second laser scanning unit, may change the reference time t RL  to a second reference time t RL2  which is determined by a distance between the second laser scanning unit and the transfer position TP, and connect the clutch after the second reference time t RL2 .  
         [0018]     When implementing monochromatic printing, the controller may drive the feeding motor at the predetermined motor driving frequency during the feeding time ‘t 12 ’ and vary the predetermined motor driving frequency by adding or subtracting the variation Δp, selected from the lookup table based on the comparison between the feeding time ‘t 12 ’ and the reference time ‘t R12 ’, with respect to the predetermined driving frequency ‘p’. The controller may vary the reference time by adding or subtracting the variation Δp, selected from the lookup table based on the result of comparison between the feeding time ‘t 12 ’ and the reference time ‘t R12 ’, and in the varied reference time after a point of detecting the printing medium by the sensor, restore the predetermined motor driving frequency after the varied reference time from when the second sensor detects the printing medium. Here, the feeding time ‘t 12 ’ may refer to a time from when the first sensor detects the printing medium to when the second sensor detects the printing medium. However, the feeding time ‘t 12 ’ may be a time taken from when the printing medium is picked up to when the printing medium is detected by a sensor mounted on the printing medium feeding path.  
         [0019]     According to another aspect of the present invention, there is provided a method of controlling a top margin on a printing medium in an image forming apparatus, the method comprising feeding the printing medium at a constant velocity V within a predetermined section of a printing medium feeding path; and varying a predetermined feeding velocity of the printing medium according to an actual time T 12  taken to move the printing medium within the predetermined section.  
         [0020]     According to another aspect of the present invention, there is provided a method of controlling a top margin in an image forming apparatus, the method comprising determining whether color printing is requested; regulating, during color printing, a point of reaching a transfer position TP on a printing medium by intermitting transmission of power to a feeding roller; and regulating, during monochromatic printing, the point of reaching the transfer position TP on the printing medium by varying a predetermined rotating velocity of the feeding roller. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     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:  
         [0022]      FIG. 1  is a schematic sectional view illustrating a conventional image forming apparatus;  
         [0023]      FIG. 2  is schematic sectional view illustrating a monochromatic image forming apparatus according to an embodiment of the present invention;  
         [0024]      FIG. 3  is a control block diagram illustrating the monochromatic image forming apparatus of  FIG. 2 ;  
         [0025]      FIG. 4  is a flowchart illustrating a method of controlling a top margin of a printing medium in the monochrome image forming apparatus of  FIG. 3 ;  
         [0026]      FIG. 5  is a flowchart illustrating a method of varying a feeding motor frequency P of  FIG. 4 ;  
         [0027]      FIG. 6  is a sectional view schematically illustrating a color image forming apparatus according to another embodiment of the present invention;  
         [0028]      FIG. 7  is a control block diagram illustrating a color image forming apparatus of  FIG. 6 ;  
         [0029]      FIG. 8  is a flowchart illustrating a method of controlling a top margin of a printing medium of the color image forming apparatus of  FIG. 6 ; and  
         [0030]      FIG. 9  is a flowchart illustrating a method of varying a transfer motor frequency P of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     Reference will now be made in detail to the 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 to explain the present invention by referring to the figures.  
         [0032]     In the following description, drawing reference numerals may be repeated to describe repeated elements in different drawings. Some matters may be defined in the description along with a detailed construction, however, these elements are merely provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention may be carried out without those particularly described elements. Also, well-known functions or constructions may not be described in detail, so as to not obscure the invention in unnecessary detail.  
         [0033]     Referring to  FIGS. 2 and 3 , an image forming apparatus comprises a controller  100 , a laser scanning unit driver  110 , a developing unit driver  120 , a photoconductive drum driver  130 , a power supply unit  140 , a feeding motor  150 , a memory  160 , and first and second sensors  170  and  175 .  
         [0034]     The controller  100  is connected to a host device such as a computer, for signal transmission. The controller  100  receives printing data and drives the respective drivers  110 ,  120 ,  130 ,  140 , and  150  in accordance with the received printing data. Also, the controller  100  controls overall processes used in forming an image.  
         [0035]     The laser scanning unit driver  110  drives a laser scanning unit  112  according to a signal from the controller  100 . Accordingly, the laser scanning unit  112  scans a photoconductive drum  132  with a laser beam at a certain point of time to thereby generate an electrostatic latent image.  
         [0036]     The developing unit driver  120  drives a developing unit  122 , comprising a developing roller  124 , according to a signal from the controller  100  to thereby change the electrostatic latent image formed on the photoconductive drum  132  into a visible image.  
         [0037]     The photoconductive drum driver  130  drives the photoconductive drum  132  so that the laser scanning unit  112  and the developing unit  122  can perform exposing and developing, respectively, at an appropriate position.  
         [0038]     The power supply unit  140  supplies power to a transfer unit  142  according to a signal from the controller  100 , thereby transferring the visible image onto a printing medium such as a sheet of printing paper.  
         [0039]     The feeding motor  150 , according to a signal from the controller  100 , drives a pickup roller  154  and first and second feeding rollers  156  and  158 . The first and the second feeding rollers  156  and  158  are disposed on a printing medium feeding path at a predetermined distance from each other to move the printing medium to a transfer position TP.  
         [0040]     The memory  160  stores data received from the host (not shown) and various other information used to operate the controller  100 . For example, a lookup table used to vary frequency of the feeding motor  150  is stored to the memory  160 . The lookup table will be described in greater detail hereinafter.  
         [0041]     The first and the second sensors  170  and  175  are respectively disposed at the first and the second feeding rollers  156  and  158  to detect the printing medium being fed. Upon detection of the printing medium, the sensors  170  and  175  transmit appropriate information to the controller  100 .  
         [0042]     Hereinbelow, a method of controlling a top margin of the printing medium in the above image forming apparatus will be described in greater detail.  
         [0043]     Referring to  FIGS. 3 and 4 , printing data is input from a host device such as a computer (S 1000 ). The controller  100  drives the laser scanning unit  112  (S 1010 ), and drives the feeding motor  150  at frequency P (S 1020 ). When the feeding motor  150  rotates at frequency P, the printing medium is fed at a velocity V. Therefore, as the pickup roller  154  and the first and the second feeding rollers  156  and  158 , which are in connection with the feeding motor  150 , rotate, the printing medium is fed (S 1030 ). When the printing medium reaches the first sensor  170 , the first sensor  170  detects the printing medium and transmits information regarding this to the controller  100  (S 1040 ). Therefore, the controller  100  recognizes time T 1  (S 1050 ), the point at which the first sensor  170  detects the printing medium, and determines whether the time T 1  is equal to a preset reference time T R1  (S 1060 ). Here, the preset reference time T R1  refers to a time taken from when the first sensor  170  detects the printing medium to when a laser scanning operation begins. The reference time T R1  is obtained by a difference between a time obtained through a distance from the first sensor  170  to a transfer position TP and a feeding velocity of the printing medium, and a time taken for a laser-scanning position on the photoconductive drum  132  to arrive at the transfer position TP, passing through other processes such as developing.  
         [0044]     When the time T 1 , taken from when the first sensor  170  detects the printing medium, is equal to the preset reference time T R1 , the controller  100  controls the laser scanning unit  112  to start the laser scanning operation (S 1070 ). Then, when the printing medium reaches the second sensor  175  and is detected by the second sensor  175  (S 1080 ), the controller  100  recognizes time T 12  taken for the printing medium to be moved from the first sensor  170  to the second sensor  175 , and determines whether the time T 12  is equal to a preset reference time T R12  (S 1090 ). Here, the preset reference time T R12  refers to a preferable time required for the printing medium to be moved from the first sensor  170  to the second sensor  175  when the feeding motor  150  is driven at frequency P and the printing medium is fed at a constant velocity V. However, discrepancy may occur between the time T 12  and the preset reference time T R12 , the time T 12  being the time actually required for the printing medium to be moved from the first sensor  170  to the second sensor  175 , due to abrasion of the feeding rollers  156  and  158  and/or slip between the feeding rollers  156  and  158  and the printing medium. Hereinbelow, the time T 12  is referred to as ‘actual time T 12 ’.  
         [0045]     In the case in which the actual time T 12  is equal to the preset reference time T R12 , the controller  100  does not vary the driving frequency P of the feeding motor  150 . Therefore, the printing medium moved up to the second sensor  175  is moved to the transfer position TP at the constant velocity V (S 1130 ). Then, the controller  100  performs a transferring operation at the transfer position TP (S 1140 ), determines whether the printing operation is completed (S 1150 ), and finishes the printing processes if the printing operation is completed. If the processes are not finished, the controller  100  drives the laser scanning unit  112  again to repeat the image forming processes with another sheet of a printing medium.  
         [0046]     Conversely, when the actual time T 12  is different from the preset reference time T R12 , the controller  100  varies the driving frequency P of the feeding motor  150  to thereby vary the feeding velocity V of the printing medium (S 1100 ). By varying the feeding velocity V of the printing medium, the actual time T 12 , taken for the printing medium to reach the second sensor  175 , is compensated to approximate the preset reference time T R12 . Therefore, the image transferring position on the printing medium is set by varying the actual time T 12 .  
         [0047]     Referring to  FIG. 5 , the processes of varying the feeding motor frequency P will now be described. First, it is determined whether the actual time T 12 , which is the time actually taken for the printing medium to move from the first sensor  170  to the second sensor  175 , is greater than the preset reference time T R12  (S 1110 ).  
                                                   TABLE 1                           Lookup Table                        Variation                   (ΔT R2 )                   of preset time               Variation (ΔP)   for restoration               of feeding   of feeding       Recogni-       motor driving   motor driving       tion No.   |T 12  − T R12 |   frequency (P)   frequency (P)                    1   0 &lt; |T 12  − T R12 | ≦ ΔT 1     ΔP 1     ΔT R21         2   ΔT 1  &lt; |T 12  − T R12 | ≦ ΔT 2     ΔP 2     ΔT R22         3   ΔT 2  &lt; |T 12  − T R12 | ≦ ΔT 3     ΔP 3     ΔT R23         4   ΔT 3  &lt; |T 12  − T R12 | ≦ ΔT 4     ΔP 4     ΔT R24         5   ΔT 4  &lt; |T 12  − T R12 | ≦ ΔT 5     ΔP 5     ΔT R25         6   ΔT 5  &lt; |T 12  − T R12 | ≦ ΔT 6     ΔP 6     ΔT R26         7   ΔT 6  &lt; |T 12  − T R12 | ≦ ΔT 7     ΔP 7     ΔT R27         8   ΔT 7  &lt; |T 12  − T R12 | ≦ ΔT 8     ΔP 8     ΔT R28         9   ΔT 8  &lt; |T 12  − T R12 | ≦ ΔT 9     ΔP 9     ΔT R29         10   ΔT 9  &lt; |T 12  − T R12 | ≦ ΔT 10     ΔP 10     ΔT R210                    
 
         [0048]     When the actual time T 12  is greater than the preset reference time T R12 , the feeding velocity needs to be increased from the second sensor  175  to the transfer position TP because the actual time T 12  is too long for the proper transfer of an image to the printing medium. Data T 12 -T R12  is calculated to increase the feeding velocity V (S 1111 ). According to a result of the caculation, a corresponding recognition number shown in Table 1, illustrating a lookup table, is selected (S 1112 ). Although Table 1 shows the recognition number of 1 to 10 as an example, more numbers may be employed to sub-divide control of the feeding velocity V. Upon selection of the recognition number, variation ΔP of the feeding motor driving frequency P is determined according to the selected recognition number. Therefore, an updated feeding motor driving velocity frequency P′ is determined by adding the variation ΔP to the feeding velocity frequency P (S 1113 ). Additionally, according to the selected recognition number, the variation ΔT R2  of preset reference time T R2  is selected to determine a point of time to restore the updated feeding motor driving frequency P′ to the original feeding motor frequency P. Then, an updated preset restoration reference time T R2 ′ is calculated by subtracting the variation ΔT R2  from the preset restoration reference time T R2  (S 1113 ).  
         [0049]     After completion of the calculations, the printing medium is fed according to the updated feeding motor frequency P′ (S 1114 ). In this case, the feeding velocity V of the printing medium increases in proportion to the variation ΔP of the frequency. Therefore, the printing medium is able to correctly reach the transfer position TP by compensating the actual time the printing medium takes to move from the first sensor  170  to the second sensor  175 .  
         [0050]     However, in order to transfer the visible image from the transfer position TP of the photoconductive drum  132  onto the printing medium, the varied feeding velocity V′ needs to be restored to the original feeding velocity V of the printing medium, because a rotational velocity of the photoconductive drum  132  and transferring velocity of the transfer unit  142  are set to the original feeding velocity V of the printing medium.  
         [0051]     Therefore, the controller  100  recognizes an actual time T 2  taken from when the second sensor  175  detects the printing medium (S 1115 ), and compares the actual time T 2  with a reference time T R2 ′ to restore the varied feeding velocity V′ of the printing medium (S 1116 ). If the actual time T 2  is different from the reference time T R2 ′, the controller  100  recognizes the actual time T 2  again (S 1115 ). When the actual time T 2  is equal to the reference time T R2 ′, the controller  100  changes the varied feeding motor frequency P′ to the original feeding motor frequency P so as to restore the varied feeding velocity V′ to the original feeding velocity V (S 1117 ). After the transferring operation, the controller  100  determines whether the printing operation is completed (S 1150 ). When the printing operation is not completed, the controller  100  restarts the laser scanning operation (S 1010 ) to keep the printing operation with a following sheet of the printing medium. Otherwise, the controller  100  finishes all the processes.  
         [0052]     When the actual time T 12  taken from the first sensor  170  to the second sensor  175  is not greater than the preset reference time T R12 , which is contrary to the case in which the actual time T 12  is greater than the reference time T R12 , the recognition number is selected (S 1122 ) by subtracting the actual time T 12  from the reference time T R12  (S 1121 ). The variation ΔP of the feeding motor driving frequency P is selected in accordance with the selected recognition number, and an updated driving frequency P″ is calculated by subtracting the variation ΔP from the original feeding motor driving frequency P (S 1123 ). Furthermore, in order to change the updated feeding motor driving frequency P″ to the original feeding motor driving frequency P, variation ΔT R2  of the preset reference time T R2  is selected according to the recognition number, and an updated reference time T R2 ″ is calculated by adding the variation ΔT R2  to the original preset reference time T R2  (S 1123 ). Also, the printing medium is fed by driving the feeding motor  150  according to the updated feeding motor driving frequency P″ (S 1124 ), and the actual time T 2 , which is taken from when the second sensor  175  detects the printing medium, is recognized (S 1125 ). When the actual time T 2  is equal to the updated reference time T R2 ″ as a result of comparison (S 1126 ), the updated feeding motor driving frequency P″ is changed to the original feeding motor driving frequency P (S 1127 ) for the transferring operation (S 1150 ). After the transferring operation, it is determined whether the printing operation has been completed.  
         [0053]     In the present embodiment, the method of controlling the transfer position TP on the printing medium has been described, including the first and the sensors  170  and  175  mounted on the printing medium feeding path. However, the same effects as in the above embodiment can be acquired by changing the feeding velocity from the second sensor  175  to the transfer position TP based on time taken from a point of picking up the printing medium to a point of the printing medium reaching the second sensor  175 , with the first sensor  170  omitted.  
         [0054]      FIGS. 6 and 7  are views illustrating the structure of a color image forming apparatus according to another embodiment of the present invention. Referring to  FIGS. 6 and 7 , the color image forming apparatus comprises a controller  200 , a laser scanning unit  210 , a developing unit driver  220 , a driver  230  for a photoconductive drum and intermediate transfer belt (ITB), a power supply unit  240 , a feeding motor  250 , a memory  260 , and first and second sensors  270  and  275 .  
         [0055]     The controller  200  is connected with a host device such as a computer, for signal transmission, in the same manner as the previously described embodiment of the present invention. Therefore, the controller  200  receives printing data, thereby driving the respective drivers  210 ,  220 ,  230 ,  240 , and  250 , and regulates the overall processes for image formation.  
         [0056]     The laser scanning unit driver  210  comprises a first laser scanning unit driver  212  to expose a first photoconductive drum  232  to develop yellow (Y) and cyan (C) developers, and a second laser scanning unit driver  214  to expose a second photoconductive drum  234  to develop magenta (M) and black (K) developers. The first and second laser scanning units drivers  212  and  214  respectively drive first and second laser scanning units  213  and  215 , according to a signal of the controller  200 , so that the first and the second laser scanning units  213  and  215  project a laser beam onto the first and the second photoconductive drums  232  and  234 . Thereby, an electrostatic latent image is generated.  
         [0057]     The developing unit driver  220 , according to a signal from the controller  200 , drives a developing unit to thereby change the electrostatic latent image formed on the first and the second photoconductive drums  232  and  234  into a visible image using yellow (Y), magenta (M), cyan (C), and black (K) developers.  
         [0058]     The driver  230  for the photoconductive drum and intermediate transfer belt drives the first and second photoconductive drums  232  and  234 , and the intermediate transfer belt  236 , according to a signal from the controller  200 .  
         [0059]     The power supply unit  240 , according to a signal from the controller  200 , transfers onto the intermediate transfer belt  236  the visible image developed on the photoconductive drums  232  and  234 . In addition, the power supply unit  240  supplies electric power to the first transfer unit  242 , comprising first and second transfer rollers  242   a  and  242   b,  corresponding to the first and the second photoconductive drums  232  and  234 , and to the second transfer unit  242  transferring to the printing medium the image transferred onto the first transfer unit.  
         [0060]     The feeding motor  250  drives the pickup roller  254  and first and second feeding rollers  256  and  258 , according to a signal from the controller  200 . Clutches  252  are interposed among the feeding motor  250 , the first and the second feeding roller  256  and  258 , and the pickup roller  254  in order to connect and disconnect the feeding motor  250 , with respect to the first and the second feeding rollers  256  and  258  and the pickup roller  254 , under the control of the controller  200 . By this, timing for moving the printing medium to the transfer position TP is enabled.  
         [0061]     The memory  260  stores data received from the host device (not shown) and various other information used to operate the controller  200 . As in the previously discussed embodiment, the lookup table is stored to the memory  260 .  
         [0062]     The first and the second sensors  270  and  275  are respectively disposed at the first and the second feeding rollers  256  and  258  to detect the printing medium being fed. Upon detection of the printing medium, the sensors  270  and  275  transmit appropriate information to the controller  200 . A paper-jam sensor to detect occurrence of a paper jam is used as the first sensor  270 , while a registration sensor used to arrange leading ends of the paper is used for the second sensor  275 .  
         [0063]     Hereinbelow, a method of controlling the transfer position on the printing medium, in a color image forming apparatus having the above described structure, will be described in greater detail.  
         [0064]     Referring to  FIGS. 7 and 8 , when the printing data is input from the host device such as a computer (S 2000 ), the controller  200  determines whether the printing data is for color printing (S 2100 ).  
         [0065]     When implementing color printing, the controller  200  choose developers (Y, M, C, and K) for printing (S 2200 ). The controller  200  checks up the number and order (1, 2, . . . n-1, n) of performing the laser scanning operation for the respective developers as chosen (S 2210 ). The present embodiment takes 4 for ‘n’ as an example; however, ‘n’ may be more than 4 if the number of developer colors is more than 4. The controller  200  drives the laser scanning units  213  and  215  (S 2220 ), drives the feeding motor  250  at driving frequency P (S 2230 ) to supply the printing medium into the main body of the image forming apparatus, and feeds the printing medium at the printing medium feeding velocity V corresponding to the diving frequency P of the feeding motor  150  (S 2240 ). Therefore, the first sensor  270  detects the printing medium being fed (S 2250 ), recognizes the actual time ‘t’ taken to receive the printing medium from the pickup point (S 2260 ), and compares the actual time ‘t’ to a reference time ‘t R ’ for determining occurrence of a paper jam. When the actual time ‘t’ is greater than the reference time ‘t R ’, the controller  200  determines that a paper jam is generated, and converts to a paper-jam mode (S 2280 ) to finish the processes. When the actual time ‘t’ is equal to or smaller than the reference time ‘t R ’, the controller  200  performs the laser scanning (S 2290 ) and performs first transfer with respect to the intermediate transfer belt  236  (S 2300 ). Here, the printing medium, being fed and passing through the first sensor  270 , is detected by the second sensor  275  (S 2310 ). Upon detection of the printing medium, or in a predetermined time after the detection, the controller  200  disconnects the clutches  252  (S 2320 ) so that the power of the feeding motor  250  is not transmitted to the first and the second feeding rollers  256  and  258 . Therefore, the printing medium stays on the second feeding roller  258 . This is because more time is required to form a color image and perform secondary transfer than to pick up the printing medium and feed the printing medium to the transfer position TP.  
         [0066]     In a state in which the printing medium stays on the second transfer roller  258 , the controller  200  determines whether to perform a final laser scanning (S 2330 ). If it is not a stage for the final laser scanning yet, the controller  200  repeats the processes for the laser scanning (S 2290 ). If it is a stage for the final laser scanning, the controller  200  performs the final laser scanning (S 2340 ). Then, the controller  200  determines whether the final laser scanning is performed by the first laser scanning unit  213  (S 2350 ). If so, the controller  200  changes a preset reference time ‘t RL ’ for determining a point of reconnecting the clutches  252  to a preset reference time ‘t RL1 ’ required for laser scanning by the first laser scanning unit  213  (S 2360 ). If not, the controller  200  determines that the final laser scanning is performed by the second laser scanning unit  215  and therefore changes the preset reference time ‘t RL ’ to a preset reference time ‘t RL2 ’ required for laser scanning by the second laser scanning unit  215  (S 2370 ).  
         [0067]     Here, the time ‘t RL1 ’ denotes a reference time taken from the start of laser scanning by the first laser scanning unit  213  to connection of the clutches  252 . More specifically, the time ‘t RL1 ’ is calculated by subtracting the time taken from when the first laser scanning unit  213  starts laser scanning to when the image transferred on the intermediate transfer belt  236  reaches the transfer position TP, from the time taken from when the printing medium, detected by the second sensor  275  and staying on the second feeding roller  258 , starts being fed to when the printing medium reaches the transfer position TP. The time ‘t RL2 ’ denotes a reference time taken from the start of laser scanning by the second laser scanning unit  215  to connection of the clutches  252 . More specifically, the time ‘t RL2 ’ is calculated by subtracting the time taken from when the second laser scanning unit  215  starts laser scanning to when the image transferred on the intermediate transfer belt  236  reaches the transfer position TP, from the time taken from when the printing medium, detected by the second sensor  275  and staying on the second feeding roller  258 , starts being fed to when the printing medium reaches the transfer position TP. The times ‘t RL1 ’ and ‘t RL2 ’ are determined through the feeding motor driving frequency P, a distance from the second sensor  275  to the transfer position TP, a driving velocity of the intermediate transfer belt  236 , and distances from laser scanning positions on the photoconductive drums  232  and  235  to the transfer position TP.  
         [0068]     The controller  200  recognizes the actual time t L  actually taken from the final laser scanning point (S 2380 ) and determines whether the actual time t L  is equal to the preset reference time t RL  (S 2390 ). When they are not equal to each other, the controller  200  repeats the processes of recognizing the actual time t L  (S 2380 ). When the actual time t L  is equal to the preset reference time t RL , the controller  200  connects the clutches  252  so that the first and second feeding rollers  256  and  258  can be powered by the feeding motor  250 . Having received this power, the second feeding roller  258  moves the printing medium to the transfer position TP. Here, the image transferred on the intermediate transfer belt  236  reaches the transfer position TP simultaneously with the printing medium, thereby enabling the secondary transfer of the image transferred on the intermediate transfer belt  236  on a correct position of the printing medium (S 2410 ). After the secondary transfer and further processes such as image fixing, one sheet of printing medium is completed with the printing operation. The controller  200  determines whether the printing operation is completed (S 2420 ) and, if not, restarts from input of the printing data (S 2000 ).  
         [0069]     Non-color printing refers to monochromatic printing in which a developer of only one color out of the respective color developing units is used for printing. In other words, printing is embodied with only one color among yellow (Y), magenta (M), cyan (C), and black (K). In this embodiment, black (K) is used as an example.  
         [0070]     When implementing non-color printing, the controller  200  operates the second laser scanning unit  215  (S 2500 ) and drives the feeding motor  250  at the driving frequency P (S 2510 ). Accordingly, the printing medium is supplied from the multifunction paper feeding unit  280  or the paper feeding cassette  285  into the main body of the image forming apparatus and fed at the feeding velocity V (S 2520 ). Therefore, the first sensor  270  detects the printing medium being fed, and the controller  200  recognizes the actual time ‘t’ actually taken from the pickup point to the detecting point by the first sensor  270  (S 2540 ) and compares the actual time ‘t’ with the preset reference time ‘t R ’ (S 2550 ). When the actual time ‘t’ is greater than the reference time ‘t R ’, the controller  200  converts to a paper-jam mode (S 2280 ). When the actual time ‘t’ is equal to or smaller than the reference time ‘t R ’, the controller  200  recognizes the actual time ‘t 1 ’ actually taken from when the first sensor  70  detects the printing medium (S 2560 ), and determines whether the actual time ‘t 1 ’ is equal to the reference time ‘t R1 ’ (S 2570 ). Here, the reference time ‘t R1 ’ denotes a time required from when the first sensor  270  detects the printing medium to when the laser scanning operation begins. The reference time ‘t R1 ’ is determined based on a distance between the laser scanning position to the transfer position TP, a driving velocity of the intermediate transfer belt  236 , a distance between the first sensor  270  to the transfer position TP, and a feeding velocity V of the printing medium. When the actual time ‘t 1 ’ is not equal to the reference time ‘t R1 ’ as a result of comparison, the controller  200  repeats the process of recognizing the actual time ‘t 1 ’ (S 2560 ). When the actual time ‘t 1 ’ and the reference time ‘t R1 ’ correspond, the controller  200  performs the laser scanning (S 2580 ) and the first transfer (S 2590 ). When the printing medium being fed is detected by the second sensor  275  (S 2600 ), the controller  200  recognizes an actual time ‘t 12 ’ taken from a point of detecting the printing medium by the first sensor  270  to a point of detecting the printing medium by the second sensor  275  (S 2610 ), and compares the actual time ‘t 12 ’ with the reference time ‘t R12 ’ (S 2620 ). The reference time ‘t R12 ’ refers to a time normally taken for the printing medium being fed at the feeding velocity V according to the feeding motor driving frequency P to be moved from the first sensor  270  to the second sensor  275 .  
         [0071]     When the actual time ‘t 12 ’ and the reference time ‘t R12 ’ are equal to each other, the printing medium is fed at a constant velocity ‘v’ without varying a feeding motor driving frequency ‘p’ (S 2630 ). As the printing medium being fed reaches the transfer position TP, the secondary transfer is performed (S 2730 ).  
         [0072]     When the actual time ‘t 12 ’ is different from the reference time ‘t R12 ’, the controller  200  varies the feeding velocity ‘v’ of the printing medium by varying the driving frequency ‘p’ of the feeding motor  250  (S 2700 ).  
         [0073]      FIG. 9  illustrates a detailed flowchart regarding the processes of varying the feeding motor driving frequency ‘p’.  
         [0074]     When the actual time ‘t 12 ’ is different from the reference time ‘t R12 ’, the controller  200  determines whether the actual time ‘t 12 ’ is greater than the reference time ‘t R12 ’ (S 2710 ). If so, the controller  200  varies the feeding motor driving frequency in the same manner as in the previously described embodiment of the present invention.  
                                                   TABLE 2                           Lookup Table                        Change (Δt R2 )                   of preset time               Change (Δp)   for restoration               of feeding   of feeding       Recognition       motor driving   motor driving       No.   |t 12  − t R12 |   frequency (p)   frequency (p)                    1   0 &lt; |t 12  − t R12 | ≦ Δt 1     Δp 1     Δt R21         2   Δt 1  &lt; |t 12  − t R12 | ≦ Δt 2     Δp 2     Δt R22         3   Δt 2  &lt; |t 12  − t R12 | ≦ Δt 3     Δp 3     Δt R23         4   Δt 3  &lt; |t 12  − t R12 | ≦ Δt 4     Δp 4     Δt R24         5   Δt 4  &lt; |t 12  − t R12 | ≦ Δt 5     Δp 5     Δt R25         6   Δt 5  &lt; |t 12  − t R12 | ≦ Δt 6     Δp 6     Δt R26         7   Δt 6  &lt; |t 12  − t R12 | ≦ Δt 7     Δp 7     Δt R27         8   Δt 7  &lt; |t 12  − t R12 | ≦ Δt 8     Δp 8     Δt R28         9   Δt 8  &lt; |t 12  − t R12 | ≦ Δt 9     Δp 9     Δt R29         10   Δt 9  &lt; |t 12  − t R12 | ≦ Δt 10     Δp 10     Δt R210                    
 
         [0075]     In greater detail, a recognition number is selected through calculating t 12− t R12  (S 2711 ) and the controller  200  selects the variation Δp of the feeding motor driving frequency ‘p’ according to the selected recognition number. Therefore,an updated feeding motor driving velocity frequency ‘p′’ is calculated by adding the variation Δp to the original feeding velocity frequency ‘p’ (S 2713 ). According to the selected recognition number, in addition, variation Δt R2  of preset reference time t R2  is selected to restore the updated feeding motor driving frequency ‘p′’ to the original feeding motor frequency ‘p’. Then, an updated reference time t R2 , is calculated by subtracting the variation Δt R2  from the present reference time t R2  (S 2713 ). The printing medium is fed by driving the feeding motor  250  according to the updated feeding motor driving frequency ‘p′’ (S 2714 ). The controller  200  recognizes the actual time ‘t 2 ’ taken from when the second sensor  275  detects the printing medium (S 2715 ) and compares the actual time ‘t 2 ’ with the updated reference time ‘t R2 ’ (S 2716 ). When the actual time ‘t 2 ’ and the reference time ‘t R2 ’ are equal to each other, the updated feeding motor driving frequency ‘p′’ is restored to the original feeding motor driving frequency ‘p’ (S 2717 ) to perform the transferring operation (S 2730 ). After the transfer, the controller  200  determines whether the printing operation is completed (S 2420 ) as described above.  
         [0076]     When the actual time ‘t 12 ’ is not greater than the reference time ‘t R12 ’, that is, when the actual time ‘t 12 ’ is less than the reference time ‘t R12 ’, the recognition number is selected (S 2722 ) by subtracting the actual time t 12  from the reference time t R12  (S 2721 ). Further, the variation Δp of the feeding motor driving frequency ‘p’ is selected, and the selected variation Δp is subtracted from the original feeding motor driving frequency ‘p’, thereby calculating an updated driving frequency ‘p″’ (S 2723 ). According to the selected recognition number, in addition, variation Δt R12  of preset reference time t R2  is selected to restore the updated feeding motor driving frequency ‘p″’ to the original feeding motor frequency ‘p’. Then, an updated reference time t R2 ″ is calculated by adding the variation Δt R2  to the preset reference time t R2  (S 2723 ). The printing medium is fed by driving the feeding motor  250  according to the updated feeding motor driving frequency ‘p″’ (S 2724 ). The controller  200  recognizes the actual time ‘t 2 ’ taken from when the second sensor  275  detects the printing medium (S 2725 ) and compares the actual time ‘t 2 ’ with the updated reference time ‘t R2 ’ (S 2726 ). When the actual time ‘t 2 ’ and the reference time ‘t R2 ’ are equal to each other, the updated feeding motor driving frequency ‘p″’ is restored to the original feeding motor driving frequency ‘p’ (S 2727 ) to perform the transferring operation (S 2730 ). After the transfer, the controller  200  determines whether the printing operation is completed (S 2420 ) as described above.  
         [0077]     As can be appreciated from the above description, according to embodiments of the present invention, discrepancy in feeding velocity of the printing medium, caused due to abrasion or slips between the feeding rollers and the printing medium, can be compensated for. Accordingly, loss of information to be printed can be prevented and, furthermore, transfer of the image can be correctly achieved on a desired position of the printing medium.  
         [0078]     Therefore, image forming quality can be enhanced.  
         [0079]     Also, when implementing monochromatic printing in a color image forming apparatus, the feeding velocity of the printing medium can be independently controlled, thereby enhancing the speed of the monochromatic printing.  
         [0080]     In addition, since a paper-jam sensor and a registration sensor may be used as the sensors detecting the printing medium, regulation of a top margin is enabled without adding dedicated parts.  
         [0081]     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. 
        What is claimed is: