Abstract:
A method of and apparatus for controlling a transfer voltage in an electro-photographic image forming apparatus. A transfer roller resistance between a transfer roller and a photosensitive drum is detected with a constant voltage applied to the transfer roller in a state where the photosensitive drum and the transfer roller form a transfer nip. A paper resistance among the transfer roller, a paper, and the photosensitive drum is detected when the constant voltage is applied to the transfer roller while the paper approaches the transfer nip of the transfer roller. A paper type is determined and a transfer voltage is selectively applied to the transfer roller based on the detected transfer roller and paper resistances. Reference tables are stored in a memory to aid in determining the paper type and the applied transfer voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2005-26505, filed on Mar. 30, 2005, 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]     Aspects of the present invention relate to a method of and an apparatus for controlling a transfer voltage in an image forming apparatus, and more particularly, to a method of and an apparatus for controlling a transfer voltage in an electrophotographic image forming apparatus in which a type of paper is determined using a transfer roller resistance and a paper resistance, and a proper transfer voltage is applied according to a transfer table for the determined type of paper.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, an image forming apparatus forms an image by charging a surface of a photosensitive medium using a charging device, and radiating light onto the photosensitive medium using a laser scanning unit according to a print signal to form an electrostatic latent image thereon, forming a toner image using toners supplied from developing units, and transferring the toner image onto paper.  
         [0006]     The image forming apparatus uses a method of contacting the photosensitive medium with the charging device to charge the surface of the photosensitive medium to a predetermined potential. Using a potential difference between the devices, a toner is attached to the electrostatic latent image formed on the photosensitive medium or the toner image is transferred from the photosensitive medium onto a transfer device or from the transfer device onto paper.  
         [0007]     In order to obtain a good quality image, an appropriate transfer voltage must be applied to a transfer roller to transfer the toner image formed on the photosensitive medium onto the paper.  
         [0008]     U.S. Pat. No. 5,682,575, entitled “Electrophotographic Recording Apparatus Having Transfer Voltage Control Device,” discloses a method of measuring a resistance of a transfer roller when a top end of paper passes through a transfer roller and supplying a transfer voltage to the transfer roller so that an appropriate voltage is supplied to the transfer roller according to the type of paper. In the method disclosed in U.S. Pat. No. 5,682,575, the resistance of the transfer roller is detected while the paper does not pass between the photosensitive medium and the transfer roller, a high voltage is applied to the transfer roller while a leading end of the paper passes between the photosensitive medium and the transfer roller to detect a combined resistance of the paper, the photosensitive medium, and the transfer roller, the resistance of the transfer roller is compared with the combined resistance, and an appropriate transfer voltage is applied to the transfer roller, thereby controlling the transfer voltage.  
         [0009]     However, since this method does not consider environmental changes of the transfer roller resulting from the application of a transfer voltage that varies according to the resistance of paper and any change caused by long term usage, a transfer failure may occur due to the application of an improper transfer voltage or an image may distort due to the application of an overvoltage.  
       SUMMARY OF THE INVENTION  
       [0010]     Aspects of the present invention provide a method of and an apparatus for controlling a transfer voltage by detecting a transfer roller resistance and a paper resistance, determining the type of a paper using a relationship between a variation in transfer roller resistance and a variation in paper resistance, and applying an appropriate transfer voltage to a transfer roller with reference to a transfer table for each type of paper.  
         [0011]     According to an aspect of the present invention, there is provided an apparatus for controlling a transfer voltage in an electrophotographic image forming apparatus using an electric transfer roller, the apparatus comprising: a transfer roller resistance detecting unit detecting a transfer roller resistance between a transfer roller and a photosensitive drum when a constant voltage is applied to the transfer roller in a state where the photosensitive drum and the transfer roller form a transfer nip; a paper resistance detecting unit detecting a paper resistance among the transfer roller, a paper, and the photosensitive drum when the constant voltage is applied to the transfer roller while the paper approaches the transfer nip of the transfer roller; a memory storing a paper resistance recognition table and a transfer table for each type of paper; and a transfer voltage controlling unit determining the type of the paper based on the transfer roller resistance and the paper resistance and outputting a transfer voltage with reference to the transfer table.  
         [0012]     The transfer voltage controlling unit may comprise: a paper type determining unit determining the type of the paper using the transfer roller resistance and the paper resistance with reference to the paper resistance recognition table; and a transfer voltage outputting unit outputting the transfer voltage with reference to the transfer table for the determined type of the paper if the type of the paper have been determined, and outputting the transfer voltage with reference to a general transfer table if the type of the paper have not been determined.  
         [0013]     According to another aspect of the present invention, there is provided a method of controlling a transfer voltage in an image forming apparatus, the method comprising: applying a constant voltage to a transfer roller in a state where a photosensitive drum and the transfer roller form a transfer nip and detecting a transfer roller resistance between the transfer roller and the photosensitive drum; applying the constant voltage to the transfer roller when paper approaches the transfer nip of the transfer roller and detecting a paper resistance among the transfer roller, the paper, and the photosensitive drum; determining a type of paper from a paper resistance recognition table based on the transfer roller resistance and the paper resistance; and applying the transfer voltage to the transfer roller with reference to the determined type paper and a transfer table to perform a transferring operation.  
         [0014]     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  
       [0015]     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:  
         [0016]      FIG. 1  is a schematic diagram of an image forming apparatus to which a method of controlling a transfer voltage according to an embodiment of the present invention is applied;  
         [0017]      FIG. 2  is a block diagram of an apparatus for controlling a transfer voltage in the image forming apparatus of  FIG. 1 ;  
         [0018]      FIG. 3A  shows an example of variation in a transfer roller resistance;  
         [0019]      FIGS. 3B and 3C  illustrate examples of variation in paper resistance;  
         [0020]      FIGS. 3D and 3E  illustrate examples of a paper resistance recognition table;  
         [0021]      FIGS. 3F and 3G  illustrate examples of a transfer table; and  
         [0022]      FIG. 4  is a flowchart illustrating a method of controlling a transfer voltage in an image forming apparatus according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     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.  
         [0024]      FIG. 1  is a schematic diagram of an image forming apparatus  100  to which a method of controlling a transfer voltage according to an embodiment of the present invention may be applied. The image forming apparatus  100  includes a photosensitive drum  110 , a transfer roller  120 , a charging roller  130 , a developing roller  140 , a controlling unit  150 , a high voltage power supply unit (HVPS)  160 , and a laser scanning unit (LSU)  170 .  
         [0025]     The HVPS  160  is controlled by the controlling unit  150  and applies a high voltage to the transfer roller  120 , the charging roller  130 , and the developing unit  140 . The charging roller  130 , which receives a high negative voltage, charges the photosensitive drum  110  to a constant negative potential. The LSU  170  radiates a laser beam onto a surface of the photosensitive drum  110 , causing a potential difference. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum  110 . A toner charged with a negative potential is adhered to the electrostatic latent image by the developing roller  140 . At a transfer nip N formed as the photosensitive drum  110  and the transfer roller  120  contact, the toner image on the photosensitive drum  110  is transferred to a paper P by the transfer roller  120  which receives a high positive voltage from the HVPS  170 . In the image forming apparatus  100 , the transfer roller  120  is an electric conductivity type ethylene propylene dimonomer (EPDM) sponge having a resistance between 60 MΩ and 260 MΩ.  
         [0026]      FIG. 2  is a block diagram of an apparatus  200  for controlling a transfer voltage in an image forming apparatus according to an embodiment of the present invention. The apparatus  200  includes a transfer roller resistance detecting unit  210 , a paper resistance detecting unit  220 , a transfer voltage controlling unit  230 , and a memory  240 .  
         [0027]     Referring to  FIGS. 1 and 2 , when a constant high voltage, for example, +1800V, is applied to the transfer roller  120  in a state where the photosensitive drum  110  and the transfer roller  120  form the transfer nip (N), the transfer roller resistance detecting unit  210  receives as a feedback a signal IN 1  representing an amount of current flowing in the transfer roller  120  and detects a combined resistance (hereinafter, referred to as a transfer roller resistance) of the transfer roller  120  and the photosensitive drum  110 .  
         [0028]     When printing paper is fed and approaches the transfer nip N, the paper resistance detecting unit  220  receives a signal IN 2  representing the amount of current flowing in the transfer roller  120  and detects a combined resistance of the transfer roller  120 , the paper P, and the photosensitive drum  110  (hereinafter, referred to as a paper resistance). At this time, the high voltage applied to the transfer roller  120  is adjusted to be equal to the voltage applied to the transfer roller  120  when detecting the transfer roller resistance. This is for reducing a deviation caused by overshoot, etc., which would occur when the voltage applied to obtain a paper resistance and the voltage applied to obtain a transfer roller resistance are different.  
         [0029]     The transfer voltage controlling unit  230  determines the type of paper using a relationship between the transfer roller resistance and the paper resistance and outputs a transfer voltage OUT corresponding to the transfer roller resistance with reference to a transfer table for the determined type of paper. The transfer voltage controlling unit  230  includes a paper type determining unit  231  and a transfer voltage outputting unit  232 .  
         [0030]     A paper resistance recognition table  241  which is used to determine the type of paper and a transfer table  242  which provides a transfer voltage corresponding to the transfer roller resistance are stored in the memory  240 .  
         [0031]     The paper determining unit  231  determines the type of paper from the paper resistance recognition table  241  based on the transfer roller resistance and the paper resistance.  
         [0032]     The transfer voltage outputting unit  232  outputs the transfer voltage with reference to the transfer table  242 . If the type of paper is determined in the paper type determining unit  231 , the transfer voltage outputting unit  232  outputs a transfer voltage corresponding to the transfer roller resistance with reference to the transfer table for the determined type of paper. If the type of the paper is not determined by the paper type determining unit  231 , the transfer voltage outputting unit  232  outputs a transfer voltage with reference to a general transfer table.  
         [0033]     The paper resistance recognition table  241  is calculated using a variation in transfer roller resistance due to environment conditions and a life span of the transfer roller and the variation in paper resistance  
         [0034]      FIG. 3A  is a graph illustrating the variation in transfer roller resistance and  FIGS. 3B and 3C  are graphs illustrating variations in paper resistance obtained by experiments. The variation in transfer roller resistance of  FIG. 3A  and the variations in paper resistance of  FIGS. 3B and 3C  with respect to the life span of the developing unit were experimentally obtained. The values of transfer roller resistance and paper resistance shown in  FIGS. 3A through 3C  are given in units of current. Since current is generally inversely related to electrical resistance, lower current values of current shown in  FIGS. 3A-3C  represent higher resistance values and higher current values represent lower resistance values. Thus, the transfer roller resistance increases during the life span (LIFETIME (PV) of the transfer roller.  
         [0035]     The result of calculating a paper resistance recognition table for A-sized paper based on the results in  FIGS. 3A and 3B  is shown in  FIG. 3D , and the result of calculating a paper resistance recognition table for B-sized paper based on the results in  FIGS. 3A and 3C  is shown in  FIG. 3E . In  FIGS. 3D and 3E , the relationship between the transfer roller resistance and the paper resistance is linear.  
         [0036]     The relationship between the type of paper and the paper resistance recognition table  241  may be expressed by equation (1): 
 
 Yi=eX+ƒ±α   (1)
 
 where Yi denotes the paper resistance (current), X denotes the transfer roller resistance (current), α denotes an error range, and e and f denote constants obtained from the variation in transfer roller resistance and the variation in paper resistance. 
 
         [0037]     When the paper type determining unit  231  determines the type of paper using the transfer roller resistance and the paper resistance, the transfer roller resistance and the paper resistance may not accurately match values in the sheet resistance recognition table  241 . However, if the transfer roller resistance and the paper resistance fall within a range α of an entry in the paper resistance recognition table for a type of paper, the paper is regarded as the type of paper corresponding to the entry. If the transfer roller resistance and the paper resistance do not lie in a range α of an entry in the paper resistance recognition table  241  for a type of paper, the paper type determining unit  231  does not determine the type of paper.  
         [0038]     A transfer voltage representing a good printing state for each type of paper is experimentally obtained and stored in the transfer table  242 .  FIGS. 3F and 3G  are graphs illustrating a transfer table representing the transfer voltage corresponding to a transfer roller resistance for paper types 1, 2, 3 and 4. The paper types are identified more specifically in Table 1 below with reference to commercially available paper.  
                   TABLE 1                       Paper Type   Identification                   1   hoopoe (sim)       2   hoopoe (Dup)       3   Sanyi7, 80 (sim)       4   Sanyi7, 80 (Dup)                  
 
         [0039]      FIG. 4  is a flowchart of a method of controlling a transfer voltage in an image forming apparatus according to an embodiment of the present invention.  
         [0040]     Referring to  FIGS. 1, 2 , and  4 , in operation S 400 , the HVPS  160  applies a constant voltage to the transfer roller  120  in a state where the photosensitive drum  110  and the transfer roller  120  form the transfer nip N, and the transfer roller resistance detecting unit  210  detects the transfer roller resistance as the combined resistance of the transfer roller  20  and the photosensitive drum  110 .  
         [0041]     In operation S 410 , the HVPS  160  applies a voltage to the transfer roller  160  that is equal to the voltage applied when detecting the transfer roller resistance while a paper P approaches the transfer nip N, and the paper resistance detecting unit  220  detects the paper resistance as the combined resistance of the transfer roller  120 , the paper P, and the photosensitive drum  110 .  
         [0042]     In operation S 420 , the transfer roller resistance and the paper resistance are applied to the paper resistance recognition table  241  to determine a type of the paper P.  
         [0043]     In operation S 430 , it is determined whether the type of paper can be determined. If the type of paper can be determined, a transfer voltage corresponding to the transfer roller resistance is applied to the transfer roller  120  with reference to the transfer table  242  for the determined type of paper, in operation S 440 . Otherwise, another transfer voltage corresponding to the transfer roller resistance is applied to the transfer roller  120  with reference to a general transfer table.  
         [0044]     According to aspects of the present invention, a proper transfer voltage is applied in consideration of environmental changes of the transfer roller and any change caused by long term usage, so that it is possible to prevent a transfer failure caused by the application of an improper transfer voltage or an image distortion caused by the application of an overvoltage.  
         [0045]     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 this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.