Abstract:
A method of determining a position of a laser beam in an electrophotographic machine includes providing a sensor device having a laser beam receiving surface with a first edge and a second edge. The second edge is nonparallel to the first edge. The laser beam is scanned across the receiving surface in a scan direction perpendicular to a process direction. The laser beam intersects each of the first edge and the second edge of the receiving surface during the scanning. A time period between when the laser beam intersects the first edge and when the laser beam intersects the second edge of the receiving surface is measured. A process position of the laser beam along the process direction is calculated based upon the measured time period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic machine, and, more particularly, to determining a position of a laser beam in an electrophotographic machine, such as a laser printer. 
     2. Description of the Related Art 
     In an in-line 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 printer to print at the same speed as a monochrome laser printer, photoconductive drum exposures must occur for all four colors simultaneously. Thus, alignment of the four color developing stations in both the process direction (feed direction of the print medium) and scan direction (across the page) is critical. 
     The process location of each scanning laser beam must overlap to prevent color mis-registration in the process direction. Each color must have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Although the laser beams can be aligned when the laser printer is first assembled, thermal changes occurring during operation of the laser printer can cause subsequent misalignment of the laser beams. 
     It is known to use a horizontal synchronization (HSYNC) sensor to determine the location of the start of the scan of the laser beam across the photoconductive drum. The HSYNC sensor has a rectangular photosensitive surface which is placed somewhere in the laser print head near the start of a scan line. When the laser beam strikes the HSYNC sensor surface at the start of a scan line, the photodiode sensor detects the presence of the laser beam and thereby identifies the location of the laser beam. Such rectangular sensors, however, cannot be used to determine a location of the laser beam in the process direction. 
     What is needed in the art is a low-cost method of aligning multiple laser beams in a process direction of a laser printer such that all of the laser beams can operate simultaneously to thereby achieve the same printing speed as that of a monochrome laser printer. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of real time detection of process direction location of multiple scan lines of a multicolor electrophotographic machine, such as a laser printer. 
     The invention comprises, in one form thereof, a method of determining a position of a laser beam in an electrophotographic machine. A sensor device having a laser beam receiving surface with a first edge and a second edge is provided. The second edge is nonparallel to the first edge. The laser beam is scanned across the receiving surface in a scan direction perpendicular to a process direction. The laser beam intersects each of the first edge and the second edge of the receiving surface during the scanning. A time period between when the laser beam intersects the first edge and when the laser beam intersects the second edge of the receiving surface is measured. A process position of the laser beam along the process direction is calculated based upon the measured time period. 
     An advantage of the present invention is that the laser beams of a multicolor electrophotographic machine can be aligned in the process direction in real time while the machine is operating. 
     Another advantage is that only one sensor is required to detect a laser beam location in both the scan and process directions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a side, sectional view of one embodiment of a multicolor laser printer in which the present invention may be used; 
     FIG. 2 is a schematic view of one embodiment of a sensor device of the present invention; 
     FIG. 3 is a plot of an output voltage produced by the sensor device of FIG. 2 when a laser beam is scanned across it; 
     FIG. 4 is a schematic view of another embodiment of the sensor device of the present invention; and 
     FIG. 5 is a plot of the output voltage of the sensor device of FIG. 4 when a laser beam is scanned across it. 
    
    
     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 
     Referring now to the drawings and, more particularly, to FIG. 1, there is shown one embodiment of a multicolor laser printer  10  including laser print heads  12 ,  14 ,  16 ,  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 ,  34 , and an intermediate transfer member belt  36 . 
     Each of laser print heads  12 ,  14 ,  16  and  18  scans a respective laser beam  38 ,  40 ,  42 ,  44  in a scan direction, perpendicular to the plane of FIG. 1, across a respective one of photoconductive drums  28 ,  30 ,  32  and  34 . Each of photoconductive drums  28 ,  30 ,  32  and  34  is negatively charged to approximately −900 volts and is subsequently discharged to a level of approximately −200 volts in the areas of its peripheral surface that are 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  28 ,  30 ,  32  and  34  is continuously rotated, clockwise in the embodiment shown, in a process direction indicated by direction arrow  46 . The scanning of laser beams  38 ,  40 ,  42  and  44  across the peripheral surfaces of the photoconductive drums is cyclically repeated, thereby discharging the areas of the peripheral surfaces on which the laser beams impinge. 
     The toner in each of toner cartridges  20 ,  22 ,  24  and  26  is negatively charged to approximately −600 volts. Thus, when the toner from cartridges  20 ,  22 ,  24  and  26  is brought into contact with a respective one of photoconductive drums  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 −200 volts by the laser beams. As belt  36  rotates in the direction indicated by arrow  48 , 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  50 , the toner is transferred to the surface of the print medium in nip  54 . The laser beam of each of print heads  12 ,  14 ,  16  and  18  impinges upon a respective one of sensor devices  56 ,  58 ,  60  and  62 , each of which is placed near the end of a scan line of the associated laser beam. 
     One embodiment of a sensor device  56  is shown in FIG. 2 as viewed in the direction of laser beam  38 . Sensor device  56  includes a receiving surface  64  which, while being impinged upon by laser beam  38  moving in scanning direction  66 , transmits a voltage signal (FIG.3) to a microcontroller  68  on a sign path  69 . A leading edge  70  of the voltage signal is caused by laser beam  38  intersecting a leading edge  72  of receiving surface  64 . Similarly, trailing edge  74  of the voltage signal is caused by laser beam  38  intersecting a trailing edge  76  of surface  64 . Sensor  56  is provided on a rigid frame  77 , illustrated schematically in FIG.  1 . Frame  77  supports at least one of photoconductive drums  28 ,  30 ,  32 ,  43 , and at least one of printheads  12 ,  14 ,  16 ,  18 . 
     A time duration t d  between the leading and trailing edges of the voltage signal which is sent from circuits  93  and  100  to microcontroller  68  varies with the position of laser beam  38  along process direction  46 . As is evident from FIG. 2, the width of receiving surface  64  increases along process direction  46 . Thus, given a constant speed of the impingement point of laser beam  38  across receiving surface  64  in scanning direction  66 , laser beam  38  will take a longer period of time to traverse the width of receiving surface  64  the further laser beam  38  is along process direction  46 . By measuring time duration td of the voltage signal between leading edge  70  and trailing edge  74 , microcontroller  68  can determine the position of laser beam  38  along process direction  46 . Of course, the position of laser beam  38  along scanning direction  66  at the location of this sensor relative to the HSYNC sensor is also determined when laser beam  38  intersects leading edge  72  of surface  64 , the position of which is fixed. 
     Instead of calculating a position of laser beam  38  along process direction  46  for each time duration t d  of the voltage signal, microcontroller  68  may use a look up table which describes the position of laser beam  38  along process direction  46  for selected values of time duration t d . Microcontroller  68  can then interpolate between the values of the look up table in order to calculate the exact position of laser beam  38  along process direction  46 . Of course, a separate look up table must be used for each possible scanning speed of laser beam  38 . 
     Due to saturation of sensor device  56  by amplification of the laser beam, the time duration t d  of the voltage signal may also be a function of the optical power of the laser beam. Thus, the optical power of the laser beam must be held constant during a scan along direction  66  and between scans. Else, the optical power of laser beam  38  has to be factored into the calculation of the process location of laser beam  38 . 
     Another embodiment of a sensor device  78  (FIG. 4) includes two separate photosensitive sensor receiving surface portions  80  and  82 . Sensor portion  80  has a standard HSYNC configuration, with both leading edge  84  and trailing edge  86  being perpendicular to scanning direction  66 . Portion  82 , however, is oriented at an angle such that leading edge  88  and trailing edge  90  are parallel to each other, but are nonparallel to leading edge  84  and trailing edge  86  of portion  80 . With laser beam  38  scanning in direction  66  and intersecting leading edge  84  at time t 1,  a time at which laser beam  38  intersects leading edge  88  varies between t 2  and t 3  depending upon a process position of laser beam  38  along process direction  46 . 
     Receiving surface portions  80  and  82  produce respective photocurrent signals on signal paths  81  and  83  that are converted to respective voltage signals  85  and  87  by transimpedance amplifiers  93 . These voltage signals  85  and  87  are merged on common signal path  91  by connecting the open collector outputs of buffer comparators  100 . The common signal path  91  (known in the art as a hard-wired logic gate) is connected to microcontroller  68 . Microcontroller  68  can then measure a time duration between laser beam  38  intersecting either of leading edge  84  and trailing edge  86  and laser beam  38  intersecting either of leading edge  88  or trailing edge  90 . That is, microcontroller  68  can measure a time duration between falling edge  92  and either of falling edge  94  or rising edge  96 . Alternatively, microcontroller  68  can measure a time duration between rising edge  98  and either of falling edge  94  or rising edge  96 . This embodiment has the advantage that the more stable leading edge may be used for each sensor to determine the time interval related to beam process location. 
     With each of sensor portions  80  and  82  having a fixed width in scanning direction  66 , microcontroller  68  can calculate a speed of laser beam  38  based upon the time duration of either of the two pulses in the voltage signal. Thus, only one look up table, applicable for each possible scanning speed of laser beam  38 , is needed to calculate the process position of laser beam  38  in direction  46 . Such a look up table could, for example, provide a list of values of the process distance as a function of the change in the time duration between falling edges  92  and  94 . Alternatively, such a look up table could provide a list of values of the time duration between falling edges  92  and  94  as a percentage of the time duration of the first pulse between falling edge  92  and rising edge  98 . 
     At initial factory set up, an initial time difference t d0  equal to the time duration between falling edges  92  and  94  is stored in the printer non-volatile random access memory. If, during operation, laser beam  38  drifts to a new process location, this change in location Δy can be determined using the new time duration td, either from this look up table or from the following equation: 
     
       
         Δ y=K ( t   d   −t   d0 ), 
       
     
     wherein K is the gain describing the characteristics of the relationship between the process direction location and the time t d  between the leading edges  92  and  94 . For example, if the angle 99 (FIG. 4) is a constant 45°, then 
     
       
           K =tan 45°. 
       
     
     If this angle  99  were to vary over the length of edge  88 , then K would become a function of time duration t d . 
     The above-described process can be repeated for each of the other laser beams  40 ,  42  and  44  in order to determine drift in their respective positions in process direction  46 . The process positions of one or more of laser beams  38 ,  40 ,  42  and  44  can then be adjusted such that each of the colors of laser printer  10  can be printed in alignment along process direction  46 . 
     The sensors are shown as producing signals with “negative logic.” However, it is to be understood that the voltage signals can also be “positive logic.” That is, the signals may be positive pulses with rising edges in place of falling edges  70 ,  92 ,  94 , and with falling edges in place of rising edges  74 ,  96 ,  98 . 
     In the embodiments shown, each of the leading and trailing edges of the sensor device are linear. However, it is to be understood that it is also possible for the leading and trailing edges of the sensor device to be non-linear, such as curved. Microcontroller  68  must factor in the particular geometry of the sensor device when calculating the process position of the laser beam. 
     The present invention has been described herein as being used in conjunction with a laser printer. However, it is to be understood that it is possible for the present invention to be used in conjunction with any type of electrophotographic printer. 
     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 falls within the limits of the appended claims.