Abstract:
A method of aligning a print image of an electrophotographic machine, the method including the steps of determining a power level of a laser beam, sensing a synch position of the laser beam associated with the scan line and varying a delay time before starting the scan line dependent upon the power level and the synch position.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of controlling the print registration of an electrophotographic machine, and, more particularly, to a method of altering the registration of a scan line in a laser printer dependent on a power level of a laser.  
         [0003]     2. Description of the Related Art  
         [0004]     In an inline color laser image printing process, the print medium typically passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow. In order for the multi-color laser to print, so that the colors are coordinated, the drum exposures must be coordinated for each of the four colors so that each will be properly registered on the printed paper.  
         [0005]     Any misalignment in the color planes results in print quality problems, which may result in incorrect tints and hues of a color. Changes in the environment, changes in cartridges and components can result in variations in the color intensity of each of the color planes. A typical adjustment to alter the color intensity is to adjust the laser power applicable to that color. An increase in laser power typically increases the light output of the laser diode. More light from the laser diode makes the intensity of the color associated with that particular laser to be more intense or darker.  
         [0006]     It is typical in a laser printer to have a scanning laser beam for each of the four color developing stations. Each color is associated with a color toner and the laser impinges multiple scan lines on a photoconductive drum, thereby altering a characteristic on the drum. The toner is attracted to portions of the photoconductive drum and it is then transferred to a transfer belt. The transfer belt is rotated to a second photoconductive drum, where toner associated with the scan line on the transfer belt from the first photoconductive drum needs to align with toner associated with the corresponding scan line to be transferred from the second photoconductive drum. This continues for the third and fourth color developing stations as well.  
         [0007]     What is needed in the art is a method of registering scan lines of multiple photoconducting drums in order to avoid undesirable shifts in registration.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a method of precisely controlling the registration between multiple laser scanners so that color planes can be accurately registered relative to each other.  
         [0009]     The invention comprises, in one form thereof, a method of aligning a print image of an electrophotographic machine on a print medium, the method including the steps of determining the power level of a laser providing at least one photosensitive device, illuminating a light sensor with light from the laser. A signal is generated from the light sensor depending upon the illuminating step, altering the signal dependent upon the power level and starting a scan line from the laser onto at least one development device dependent upon the altered signal.  
         [0010]     The invention comprises in another form thereof a method of adjusting the registration of a scan line in an electrophotographic machine, the method including the steps of determining the power level of a laser beam, sensing a sync position of the laser beam associated with the scan line and varying a delay time before starting the scan line dependent upon the power level and the sync position.  
         [0011]     An advantage of the present invention is that the laser beams of a multi-color electrophotographic machine can be registered to start each scan line at an appropriate place on each photoconductive drum.  
         [0012]     Another advantage is that a variation of laser power is compensated for in order to properly delay the start time of the scan line thereby registering each of the four scan lines in a coordinated manner. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0014]      FIG. 1  is a side, sectional view of one embodiment of a multi color laser printer in which the method of the present invention may be used;  
         [0015]      FIG. 2  is a cross sectional view of one of the polygon mirrors of  FIG. 1  reflecting a laser beam;  
         [0016]      FIG. 3  is a fragmentary top view of one of the photoconductive drums of  FIG. 1 ;  
         [0017]      FIG. 4  is a schematic side view of one of the laser printheads of  FIG. 1  and a corresponding photoconductive drum;  
         [0018]      FIG. 5  is a schematic view of a laser beam reflecting upon the polygon mirror of  FIG. 2  and illustrating the start points of scan lines;  
         [0019]      FIG. 6  is a timing diagram illustrating the relationship between a synchronization signal and a signal for the start of a scan line of  FIG. 5  of the laser printer illustrated in  FIG. 1 ; and  
         [0020]      FIG. 7  is a block diagram of the operation of one embodiment of the method of the present invention utilized in the printer of  FIG. 1 . 
     
    
       [0021]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Referring now to the drawings and, more particularly, to  FIG. 1 , there is shown one embodiment of a multi-color laser printer  10  including laser printheads  12 ,  14 ,  16  and  18 , a black toner cartridge  20 , a magenta toner cartridge  22 , a cyan toner cartridge  24 , a yellow toner cartridge  26 , photoconductive drums  28 ,  30 ,  32  and  34 , an intermediate transfer member belt  36  and an engine controller  37 .  
         [0023]     Each of laser printheads  12 ,  14 ,  16  and  18  project a respective laser beam  38 ,  40 ,  42  and  44  off of a respective polygon mirror  46 ,  48 ,  50  or  52 . As each of polygon mirrors  46 ,  48 ,  50  and  52  rotates, it scans a respective one of reflected laser beams  38 ,  40 ,  42  and  44  in a scan direction, perpendicular to the plane of  FIG. 1 , across a respective photoconductive drum  28 ,  30 ,  32  and  34 . Each of photoconductive drums  28 ,  30 ,  32  and  34 , also known as photosensitive development device  28 ,  30 ,  32  and  34 , is negatively charged to approximately −950 volts and is subsequently discharged to a level of approximately −250 volts in the area of its peripheral surface that is impinged by a respective one of laser beams  38 ,  40 ,  42  and  44 . During each scan of a laser beam across a photoconductive drum, each of photoconductive drums  29 ,  30 ,  32  and  34  is continuously rotated, clockwise in the embodiment shown, in a process direction indicated by direction arrow  54 . The scanning of laser beam  38 ,  40 ,  42  and  44  across the peripheral surfaces of the respective photoconductive drums is cyclically repeated, thereby discharging the areas of the peripheral surfaces on which the laser beams impinge.  
         [0024]     The toner in each toner cartridge  20 ,  22 ,  24  and  26  is negatively charged and is transported as a thin, uniform layer on the surface of a developer roll with a core potential of approximately −600 volts. When the toner from cartridges  20 ,  22 ,  24  and  26  is brought into contact with a respective photoconductive drum  28 ,  30 ,  32  and  34 , the toner is attracted to and adheres to the portions of the peripheral surfaces of the drums that have been discharged to −250 volts by the laser beams. As belt  36  rotates, in the direction indicated by arrow  56 , the toner from each of drums  28 ,  30 ,  32  and  34  is transferred to the outside surface of belt  36 . As a print medium, such as paper, travels along path  58  or duplexing path  60 , the toner is transferred to the surface of the print medium in nip  62 .  
         [0025]     Printheads  12 ,  14 ,  16  and  18  are structurally substantially identical. Accordingly, to simplify the discussion and for ease of understanding the invention, only the structure of printhead  12  will be described in detail below in relation to  FIGS. 2-4 . However, it is to be understood that the discussion that follows with respect to printhead  12  also applies to each of print heads  14 ,  16  and  18 .  
         [0026]     Now, additionally referring to  FIGS. 2-4 , an embodiment of a polygon mirror  46  is shown, as viewed in the direction of arrow  64  in  FIG. 1 . Polygon mirror  46  is shaped as an octagon with eight reflective sides or facets  66 . As polygon mirror  46  rotates in the direction indicated by arrow  68 , laser beam  38  reflects off of facets  66  between points  70  and  72  toward photoconductive drum  28 . Thus as polygon mirror  46  rotates in direction  68  the reflected laser beam  38  is caused to scan across the peripheral surface of photoconductive drum  28  in a scan direction  74 . Scan lines  76  have a separation distance  78  that is dependent on the rotational speed of polygon mirror  46  and the rotational speed of photoconductive drum  28 .  
         [0027]     Now, additionally referring to  FIG. 5 , there is shown a laser  80  producing laser beam  38 , which is reflected from a facet  66  on polygon mirror  46 . As polygon mirror  46  rotates in direction  68  reflected laser beam  38  crosses a light sensor  82 , which is also known as a horizontal synchronous sensor  82  abbreviated as “HSYNC” sensor  82 . Laser beam  38  having crossed HSYNC sensor  82  an electronic signal is generated by HSYNC sensor  82  to report the position of laser beam  38 , which is used to initiate the beginning of a delay time until the start of a scan line  76 . HSYNC sensor  82  is sensitive to light, as such a higher intensity laser beam  38  causes HSYNC sensor  82  to trigger earlier than if laser beam  38  is of a lower intensity. As can be seen in  FIG. 5 a  scan line  76 A produced by a low laser power will be delayed by a greater distance than if a high laser power is utilized as in scan line  76 B. This effect exists because HSYNC sensor  82  indicates the beginning of a delay time that then results in a position that scan line  76  begins impinging upon photoconductive drum  28 . A variation in the timing of the signal from HSYNC sensor  82  causes the start of a scan line  76  to vary. Assuming that the velocity of rotation of polygon mirror  46  is constant, a shift in the timing of the signal from HSYNC sensor  82  leads to a shift in when the scan line delay time is expired resulting in a positional shift of the scan line.  
         [0028]     Now, additionally referring to  FIG. 6 , there is illustrated the relationship between the HSYNC n  and the Video n  signals. Subscript ‘n’ is representative of the color of each printheads  12 ,  14 ,  16  and  18 . The HSYNC n  signal is driven low when laser beam  38  crosses HSYNC sensor  82 . This triggers the beginning of a delay time indicated as a detect-to-print delay. Video n  is an enabling signal for laser diode  80 . When Video n  is low, laser diode  80  is enabled to illuminate. When Video n  is high, laser diode  80  is unenabled. Once the signal from HSYNC n  is detected, laser diode  80  is unenabled as shown in the Video n  signal of  FIG. 6 . Laser diode  80  remains off or unenabled until the detect-to-print delay time has expired as illustrated in  FIG. 6 . After the detect-to-print delay has expired, laser diode  80  may be illuminated or not illuminated depending upon the need to place image information, in the form of pels, (a pel is defined as the smallest discrete component of an image), upon photoconductive drum  28  along scan line  76 .  
         [0029]     It is the variation of the detect-to-print delay time based on the known power output of laser diode  80  that is central to an embodiment of a method of the present invention. The method being illustrated in  FIG. 7  as method  100 , which starts at step  102  where method  100  is initiated.  
         [0030]     As step  104 , laser beam  38  is detected by HSYNC sensor  82 . As polygon mirror  46  rotates, in direction  68 , laser beam  38  is unenabled at step  106 . HSYNC sensor  82  generates a signal to controller  37  to indicate the detection of light from laser  80 .  
         [0031]     At step  108 , the laser power level of laser diode  80  is obtained from a memory location. The laser power level has been predetermined and stored in memory by either an automatic system that measures the power intensity of laser diode  80  or by an empirical measurement method from an operator making measurements of a physical media. Additionally, laser power of laser diode  80  may be affected by other adjusting algorithms in engine controller  37  or altered based upon a replacement component within electrophotographic printer  10 .  
         [0032]     At step  110 , the detect-to-print delay time is adjusted based upon the known power level of laser diode  80 . The adjustment of the detect-to-print delay time may be a result of an algorithm operating with the known power level of laser diode  80 . Alternatively, the adjustment to the detect-to-print delay time is obtained from a lookup table having predetermined delay times relative to specific power level ranges of laser diode  80 .  
         [0033]     At step  112 , method  100  determines whether the detect-to-print delay time has expired. If the detect-to-print delay time has not expired, step  112  returns to itself. If the detect-to-print delay time has expired, then step  112  proceeds to step  114 . The detect-to-print delay time that is utilized in step  112  has been altered to properly position the start of scan line  76  as it runs in direction  74 .  
         [0034]     At step  114 , laser diode  80  is enabled. At step  116 , the information in the form of a series of pels related to a scan line, from engine controller  37  is transferred by varying the intensity of laser beam  38  as it impinges upon photoconductive drum  28 . Method  100  ends at step  118  where engine controller  37  or a control circuit associated with laser printhead  12  performs other functions before reinitiating method  100  for the next scan line  76 .  
         [0035]     Alternatively, a detect-to-print delay time may be assigned to each of laser printheads  12 ,  14 ,  16  and  18  based on measured line registrations of each of the four color cartridges. Laser printheads  14 ,  16  and  18  are aligned to laser printhead  12  in the present invention.  
         [0036]     Advantageously, the variation in a detect-to-print delay time allows for sub-pel size movements along scan line  76 . This type of adjustment allows for accurate registration of printheads  14 ,  16  and  18  relative to black printhead  12 . Although the registration of color printheads  14 ,  16  and  18  to black printhead  12  is completely arbitrary it is a convention utilized in the present invention to describe method  100  herein.  
         [0037]     It should be noted that in the present invention the lengths of scan line  76  are unchanged and it is the adjustment of the position of the start of the scan lines that is affected by the present invention. Printer  10  includes a system to measure colors and make adjustments to vary the color quality, this involves the adjustment of laser power. Advantageously, the present invention alleviates or at least minimizes the shifting of scan line  76 , caused by an alteration in the laser power output of laser diode  80 . The laser power of each color and the black printhead is stored in memory as a result of the color evaluation process. If the laser power is adjusted in this process, then the detect-to-print delay time is adjusted by the present invention so that the start point of the scan line does not vary. If laser power increases, relative to the value stored, then the detect-to-print delay time is increased. Conversely, if laser power decreases, relative to a value stored during alignment, then the detect-to-print delay time is decreased. For example, the laser power may be stored and represented as an 8-bit number varying from 0-255. The relative number stored represents the laser power of laser diode  80 . A table stores the values which is use by the printer firmware, to determine the detect-to-print delay that is used for each of the four colors. It is also contemplated that a unique table may be utilized for each of the printheads.  
         [0038]     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.