Patent Publication Number: US-2007097452-A1

Title: Image forming apparatus for generating stable horizontal sync signal and method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 2005-0104843, filed Nov. 3, 2005 in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an image forming apparatus for generating a stable horizontal synchronization (hereinafter “sync”) signal and a method thereof. More particularly, the present invention relates to an image forming apparatus and method for minimizing a temporal variation occurring during a detection of a horizontal sync signal and generating a stable horizontal sync signal to improve the quality of a print job.  
      2. Description of the Related Art  
      An image forming apparatus, such as laser printer, focuses a laser beam output from a laser diode onto a photoconductive drum to form an electrostatic latent image. The image forming apparatus completes a printing operation by applying toner on the electrostatic latent image and transferring a toner image to printing paper.  
       FIG. 1  shows the structure of a laser scanning unit employed in a conventional image forming apparatus.  
      Referring to  FIG. 1 , the conventional image forming apparatus comprises a laser diode  10 , a collimating lens  11 , a cylinder lens  12 , a polygon mirror  13 , a motor  14 , an f theta (fθ) lens  15 , a reflection mirror  16 , a photoconductive drum  17 , a horizontal sync mirror  18 , and an optical sensor  19 .  
      The laser diode  10  is used as a light source and outputs a laser beam. The collimating lens  11  converts the laser beam output from the laser diode  10  into a parallel beam. The cylinder lens  12  focuses the parallel beam output from the collimating lens  11  on the polygon mirror  13 . The polygon mirror  13  is driven by the motor  14  to reflect the laser beam transmitted through the cylinder lens  12  at a predetermined angle.  
      The f theta (fθ) lens  15  may have a constant refraction ratio in accordance with an optical axis. The f theta (fθ) lens  15  corrects an aberration of the laser beam reflected by the polygon mirror  13  and focuses the laser beam on an accurate point of a scanning surface. The reflection mirror  16  reflects the laser beam passing through the f theta (fθ) lens  15  in a predetermined direction and makes it incident on a surface of the photoconductive drum  17  on which an image is formed.  
      The horizontal sync mirror  18  reflects the laser beam passing through the f theta (fθ) lens  15  in a horizontal direction and makes it incident on the optical sensor  19 . The optical sensor  19  receives the laser beam reflected from the horizontal sync mirror  18  and outputs an electric horizontal sync signal. A printer engine (not shown) receives the electric horizontal sync signal and uses it to correct an error occurring between scanning lines.  
       FIGS. 2A  to  2 C are views explaining a jitter occurring in the horizontal sync signal generated by the conventional image forming apparatus.  
      For example,  FIG. 2A  shows a video signal that forms a one-line image on the printing paper. The laser beam output from the laser diode  10  during a horizontal sync signal generation period is reflected by the horizontal sync mirror  18  and received by the optical sensor  19 . The laser beam output from the laser diode  10  during an image generation period is reflected by the reflection mirror  16  and is incident on the surface of the photoconductive drum  17 , thereby forming an electrostatic latent image.  
       FIG. 2B  is a graph showing a variation of the horizontal sync signal according to the intensity of the laser beam received at the optical sensor  19 .  FIG. 2C  is a graph showing the degree of the jitter occurring in the horizontal sync signal according to the intensity of the laser beam.  
      The optical sensor  19  outputs the horizontal sync signal when the intensity of the laser beam output from the laser diode  10  during the horizontal sync signal generation period reaches a certain threshold. The threshold may have a variable value (range C) rather than a fixed value. Therefore, the variation occurs at the time the horizontal sync signal is output.  
      For example, if the intensity of the laser beam is high (A), a temporal variation exists between the horizontal sync signal outputs. Additionally, if the intensity of the laser beam is low (B), a temporal variation exists between the horizontal sync signal outputs. When the intensity of the laser beam is low, the temporal variation is widened and the degree of the jitter occurring in the horizontal sync signal output from the optical sensor  19  increases.  
      When the temporal variation between the horizontal sync signal outputs is widened, an error occurring between scanning lines cannot be accurately corrected and a uniform image signal cannot be output. This causes the quality of the print job to deteriorate.  
      Light sources which use a low intensity of light are used more frequently as the types of light sources are verified and the light receiving efficiency of the photoconductive drum  17  increases. Therefore, the temporal variation between the horizontal sync signal outputs is widened. This causes the quality of a print job to deteriorate.  
      Accordingly, there is a need for an improved system and method for minimizing a temporal variation occurring during a detection of a horizontal sync signal and generating a stable horizontal sync signal to improve the quality of a print job.  
     SUMMARY OF THE INVENTION  
      An aspect of exemplary embodiments of the present invention is to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide an image forming apparatus to generate a stable horizontal sync signal. The stable horizontal sync signal is generated when an intensity of the laser beam generated during a horizontal sync signal generation period is increased and when a temporal variation between the horizontal sync signal outputs is minimized.  
      The above aspect is achieved when an image forming apparatus is provided. The image forming apparatus includes a switching unit to switch on an electric current according to a video signal, a light output unit to output an optical power corresponding to the electric current, which is switched on by the switching unit, an optical power controller to control an intensity of the electric current switched on by the switching unit, and a printer engine unit to output an optical power control voltage to amplify the intensity of the switched-on electric current during a horizontal sync signal generation period included in the video signal.  
      In an exemplary implementation, the printer engine unit outputs an optical power control voltage. The optical power control voltage is greater than an average voltage and is output during the horizontal sync signal generation period.  
      In another exemplary implementation, the light output unit outputs an optical power which is amplified based on the optical power control voltage during the horizontal sync signal generation period.  
      In a further exemplary implementation, the light output unit includes at least one of a laser diode and a vertical cavity surface emitting laser (VCSEL).  
      The above aspect is also achieved by providing a method for generating a horizontal sync signal. An electric current is switched on according to a video signal and an optical power control voltage is output to amplify an intensity of the switched-on electric current during a horizontal sync signal generation period included in the video signal. The intensity of the switched-on electric current is controlled based on the optical power control voltage and an optical power corresponding to the switched-on electric current is output.  
      When an optical power corresponding to the switched-on electric current is output, an optical power amplified during the horizontal sync signal generation section period based on the optical power control voltage is output.  
      An image forming apparatus is also provided to achieve the above aspect. The image forming apparatus includes a plurality of light output units, an optical sensor, a plurality of switching units, and a printer engine unit. The plurality of light output units is mounted according to a physical position difference to one another. The optical sensor detects lights output from the plurality of light output units. The plurality of switching units switch on the plurality of light output units in sequence. The printer engine unit controls the optical switching units so that the lights output from the plurality of light output units are detected by the optical sensor at the same time.  
      In another exemplary implementation, the printer engine controls the plurality of switching units to satisfy the following equation: 
 
scanning line velocity×Δt D   ≦D  
 
      wherein Δt D  denotes a temporal difference between two lights which are activated and D denotes a space between two lights.  
      In a further exemplary implementation, the printer engine unit controls the plurality of switching units to satisfy the following equation: 
 
scanning line velocity×Δt OVERLAP   ≦W  
 
      wherein Δt OVERLAP  denotes a time when four lights are commonly received at the optical sensor and W denotes a light receiving width of the optical sensor.  
      Other objects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a view illustrating a laser scanning unit employed in a conventional image forming apparatus;  
       FIGS. 2A  to  2 C are views explaining a jitter occurring in a horizontal sync signal generated in the conventional image forming apparatus;  
       FIG. 3  is a block diagram illustrating an image forming apparatus according to an exemplary embodiment of the present invention;  
       FIG. 4  is a view explaining a method for stabilizing a horizontal sync signal generated by the image forming apparatus according to an exemplary embodiment of the present invention;  
       FIG. 5  is a flowchart illustrating the operation of the image forming apparatus according to an exemplary embodiment of the present invention;  
       FIG. 6  is a block diagram schematically illustrating an image forming apparatus according to another exemplary embodiment of the present invention; and  
       FIG. 7  is a view explaining a method for stabilizing a horizontal sync signal generated by the image forming apparatus according to another exemplary embodiment of the present invention. 
    
    
      Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
       FIG. 3  is a block diagram illustrating an image forming apparatus according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 3 , the image forming apparatus comprises a printer engine unit  100 , a switching unit  110 , an optical power controller  120 , a light output unit  140 , an optical sensor  160 , and a voltage detector  180 .  
      The printer engine unit  100  comprises a video controller (not shown) to output a video signal and an optical power control voltage. The video signal corresponds to an image to be printed on printing paper and the optical power control voltage adjusts the intensity of light output from the light output unit  140 . The optical power control voltage typically ranges from 0.6V to 2.0V and may have a constant average voltage of 1.5V in the related art. According to an exemplary embodiment of the present invention, the printer engine unit  100  outputs a voltage grater than the average voltage during a horizontal sync signal generation period.  
      The switching unit  110  switches an electric current on or off according to the video signal output from the printer engine unit  100 . For example, the switching unit  110  is switched on during the horizontal sync signal generation period and a predetermined level of electric current is applied to the power output unit  140 . Also, the switching unit  110  is switched on or off according to the video signal during an image generation period and applies the electric current to the light output unit  140  or stops the electric current supply.  
      The optical power controller  120  controls the level of electric current, which is switched on by the switching unit  110 , according to the optical power control voltage output from the printer engine unit  100 . That is, the optical power controller  120  amplifies the level of electric current switched on by the switching unit  110  according to the voltage which is output from the printer engine unit  100  during the horizontal sync signal generation period and may have a greater value than the average voltage.  
      The light output unit  140  generates a light according to the electric current switched on by the switching unit  110  and outputs the light. The light output unit  140  uses a laser diode or a vertical cavity surface emitting laser (VCSEL).  
      The optical sensor  160  receives the light output from the light output unit  140  and outputs an electric signal. That is, the optical sensor  160  receives the light output from the light output unit  140  during the horizontal sync signal generation period, and outputs the electrical signal until the intensity of the received light reaches a certain threshold and decreases beyond the threshold. This electrical signal is referred to as a horizontal sync signal.  
      The voltage detector  180  detects the horizontal sync signal output from the optical sensor  160  and transmits the horizontal sync signal to the printer engine unit  100 . The printer engine unit  100  starts to output the video signal based on the detected horizontal sync signal to form an image during the image generation period.  
       FIG. 4  is a view explaining a method for stabilizing the horizontal sync signal generated by the image forming apparatus according to an exemplary embodiment of the present invention.  
      The video signal is output from the video controller (not shown) included in the printer engine unit  100  and includes the horizontal sync signal generation period and the image generation period.  
      The optical power control voltage is also output from the printer engine unit  100 , and is output in the level of {circle around (1)} during the horizontal sync signal generation period and output in the level of {circle around (2)} during the other remaining period.  
      The optical power is output from the switching unit  110  in the level of {circle around (1)}′ during the horizontal sync signal generation period and output in the level of {circle around (2)}′ during the image generation period.  
       FIG. 5  is a flowchart illustrating the operation of the image forming apparatus according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 5 , the printer engine unit  100  outputs a video signal corresponding to print data at step S 200 . The output video signal includes a horizontal sync signal generation period and an image generation period.  
      When the horizontal sync signal generation period is input to the switching unit  110  at step S 220 , the optical power controller  120  controls the switching unit  110  to amplify the intensity of electric current according to an optical power control voltage. That is, the optical power controller  120  amplifies the intensity of electric current, which is switched on by the switching unit  110 , according to the optical power control voltage output from the printer engine unit  100  during the horizontal sync signal generation period at step S 240 .  
      The light output unit  140  outputs the optical power amplified by the electric current output from the switching unit  110  at step S 240 . Through the above-described process, a stable horizontal sync signal is generated.  
       FIG. 6  is a block diagram illustrating an image forming apparatus according to another exemplary embodiment of the present invention.  
      The image forming apparatus illustrated in  FIG. 6  comprises a plurality of first through N-th light output units  340 - 1 ,  340 - 2 , . . . ,  340 - n  and a plurality of first through N-th switching units  310 - 1 ,  310 - 2 , . . . ,  310 - n.  The first through N-th switching units  310 - 1 ,  310 - 2 , . . . ,  310 - n  switch an electric current to be applied to the first through the N-th light output units  340 - 1 ,  340 - 2 , . . .  340 - n.    
      A printer engine unit  300  outputs a constant level of optical power control voltage unlike the embodiment shown in  FIG. 3 . As shown in  FIG. 4 , the printer engine unit  300  outputs an optical power control voltage in the level of {circle around (2)} during both the horizontal sync signal generation period and the other remaining period. The printer engine unit  300  outputs a video signal so that a first video signal output through the first light output unit  340 - 1  and an N-th video signal output through the N-th light output unit  340 - n  are overlapped during the horizontal sync signal generation period.  
      The first through the N-th switching units  310 - 1 ,  310 - 2 , . . . ,  310 - n  switch on electric currents according to the output video signal and supply the electric currents to the first through the N-th light output unit  340 - 1 ,  340 - 2 , . . .  340 - n.  An optical power controller  320  controls the first through the N-th switching units  310 - 1 ,  310 - 2 , . . . ,  310 - n  to make the switched-on electric currents equal.  
      The first through the N-th light output units  340 - 1 ,  340 - 2 , . . .  340 - n  output the same level of optical power according to the electric currents switched on by the first through the N-th switching units  310 - 1 ,  310 - 2 , . . . ,  310 - n.  The output lights are received at the optical sensor  360 . The optical sensor  360  receives all the lights output from the first through the N-th light output units  340 - 1 ,  340 - 2 , . . . ,  340 - n,  and accordingly, the intensity of optical power received at the optical sensor  360  increases in proportion to the number of received lights.  
       FIG. 7  is a view explaining a method for stabilizing the horizontal sync signal generated by the image forming apparatus according to another exemplary embodiment of the present invention.  
       FIG. 7  illustrates 4 light output units, in which the first through fourth lights L 1 , L 2 , L 3 , L 4  received at the optical sensor  360  have the same level of optical power. The printer engine unit  300  outputs the first through fourth video signals to satisfy the following equation 1 or 2 to activate all the first through the fourth lights L 1 , L 2 , L 3 , L 4  when they pass through the optical sensor  360 : 
 scanning line velocity×Δt D   ≦D    [Equation 1] 
      wherein the scanning line velocity is a rotation velocity of a polygon mirror (not shown), Δt D  denotes a temporal difference between two lights which are activated, and D denotes a space between two lights. 
 
scanning line velocity×Δt OVERLAP   ≧W    [Equation 2]
 
      wherein Δt OVERLAP  denotes a time that the four lights are commonly activated and received at the optical sensor  360 , and W denotes a light receiving width of the optical sensor  360 .  
      According to the above condition, the intensity of optical power received at the optical sensor  360  is amplified so that a stable horizontal sync signal is output.  
      According to an exemplary embodiment of the present invention as described above, the temporal variation between the horizontal sync signal outputs is minimized by increasing the intensity of optical power generated during the horizontal sync signal generation period, so that the stable horizontal sync signal can be generated and the deterioration of the quality of the print job can be prevented.  
      While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.