Abstract:
A method of media-to-image registration sets a top writing line margin on a print medium within an electrophotographic machine. Movement of a substantially stationary sheet of print medium along a print medium path within the electrophotographic machine is initiated. An image associated with the sheet of print medium is scanned onto a developing unit. The scanning step commences before the initiating step. Toner is adhered to the image on the developing unit. The toner is transferred from the developing unit to a moving toner transfer medium. The electrophotographic machine is provided with a toner transfer area in which the toner can be transferred to the moving sheet of print medium. A speed of the movement of the sheet of print medium is adjusted such that the toner is transferred from the moving toner transfer medium to a desired area on the sheet of print medium within the toner transfer area. The sheet of print medium moves substantially continuously between the initiating step and a time of the transfer of the toner to the sheet of print medium.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to imaging systems, and, more particularly, to a method of controlling paper position in a printing apparatus, such as an electrophotographic machine. 
     2. Description of the Related Art 
     In known electrophotographic machines, a sheet of paper is picked from an input device, such as a paper tray, prior to the start of the imaging process on a developing unit. As the paper moves through the paper path at a constant speed, the leading edge of the paper eventually trips a paper path sensor which signals the machine to commence imaging onto the developing unit. The image is laid down on a single rotating photoconductive drum in the developing unit. Eventually, the image on the drum is transferred to a transfer medium and then onto a sheet of paper which contacts the transfer medium at a precise point in time in order to establish a desired top writing line margin. This process is repeated for each individual sheet of paper, allowing the machine the luxury of waiting until each page is at a known location in the paper path before beginning the imaging process. Since the paper is picked prior to imaging, thereby allowing the electrophotographic process to wait for the paper to arrive at a certain location before commencing imaging, the margin above the top writing line on the paper can be accurately controlled. 
     It may be desirable or necessary to design an electrophotographic machine such that imaging is begun on a developing unit before the sheet of paper is even picked from the input device. This requirement may be due to size limitations on the printer which reduce the maximum length of the paper path. It may also be due to the use of multiple developing units, each transferring an image of a respective color onto a same location on the transfer medium. That is, after an image is first scanned onto a photoconductive drum for a first color, the image is transferred onto an intermediate transfer medium belt. The belt then moves over to receive an image of a second color from a second photoconductive drum. The second image is received on top of and overlaps the first image. This process repeats for each of the photoconductive drums, and the completed composite image eventually reaches the paper transfer nip where it is transferred from the intermediate transfer medium belt onto the paper. Since additional time is required for transferring a separate image from each developing unit sequentially, it may be necessary to begin imaging on at least a first of the developing units before the paper is picked from the input device. 
     A problem is that, after being picked, the paper can have several millimeters of error in the paper path. This error can be due to, for example, irregular pick times, paper stack height, and/or slippage. Thus, if the movement of the paper along the paper path continues at a constant speed, the paper will arrive at the toner transfer nip at an incorrect time. This results in a very unpredictable and inconsistent top writing line margin. 
     What is needed in the art is a method of media-to-image registration that eliminates errors in the size of a top writing line margin that are caused by commencing an imaging process onto an intermediate transfer medium before a sheet of paper is picked from an input device. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of accurately adjusting the position and speed of a sheet of paper in a paper path such that the paper arrives at a toner transfer nip at the same time as the image that the paper is to receive, and at an optimum speed for an accurate top writing line margin. 
     The invention comprises, in one form thereof, a method of setting a top writing line margin on a print medium within an electrophotographic machine. Movement of a substantially stationary sheet of print medium along a print medium path within the electrophotographic machine is initiated. An image associated with the sheet of print medium is scanned onto a developing unit. The scanning step commences before the initiating step. Toner is adhered to the image on the developing unit. The toner is transferred from the developing unit to a moving toner transfer medium. The electrophotographic machine is provided with a toner transfer area in which the toner can be transferred to the moving sheet of print medium. A speed of the movement of the sheet of print medium is adjusted such that the toner is transferred from the moving toner transfer medium to a desired area on the sheet of print medium within the toner transfer area. The sheet of print medium moves substantially continuously between the initiating step and a time of the transfer of the toner to the sheet of print medium. 
     An advantage of the present invention is that the top writing line margin can be accurately controlled even in cases where imaging begins before the paper is picked from the input device. 
     Another advantage is that the paper does not have to be gated or staged in the paper path. 
    
    
     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 partial, schematic, side view of one embodiment of a laser printer in which the method of the present invention may be used; 
     FIG. 2 a  is a plot of the speed of the paper transport motor of FIG. 1 versus time in an implementation of one embodiment of the method of the present invention; 
     FIG. 2 b  is a plot of the paper position error versus time which may result from the paper transport motor plot of FIG. 2 a,    
     FIG. 3 a  is another plot of the speed of the paper transport motor of FIG. 1 versus time in an implementation of one embodiment of the method of the present invention; and 
     FIG. 3 b  is a plot of the paper position error versus time which may result from the paper transport motor plot of FIG. 3 a.    
     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 toner cartridges  12 ,  14 ,  16 ,  18 , photoconductive drums  20 ,  22 ,  24 ,  26 , a drum motor  28 , an intermediate transfer member belt  30 , a belt motor  32 , an input paper tray  34 , a paper path motor  36 , paper path sensors S 1 , S 2 , and a microcontroller  38 . 
     Each of four laser print heads (not shown) scans a respective laser beam in a scan direction, perpendicular to the plane of FIG. 1, across a respective one of photoconductive drums  20 ,  22 ,  24  and  26 . Each of photoconductive drums  20 ,  22 ,  24  and  26  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 the laser beams. During each scan of a laser beam across a photoconductive drum, each of photoconductive drums  20 ,  22 ,  24  and  26  is continuously rotated, clockwise in the embodiment shown, in a process or “cross-scan” direction indicated by direction arrow  40 . The scanning of the laser beams 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  12 ,  14 ,  16  and  18  is of a separate, respective color, such as cyan, magenta, yellow and black. Thus, each of the four laser print heads controls printing in a respective color, such as cyan, magenta, yellow or black. Further, the toner in each of toner cartridges  12 ,  14 ,  16  and  18  is negatively charged to approximately −600 volts. Thus, when the toner from cartridges  12 ,  14 ,  16  and  18  is brought into contact with a respective one of photoconductive drums  20 ,  22 ,  24  and  26 , 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  30  rotates in the direction indicated by arrow  42 , the toner from each of drums  20 ,  22 ,  24  and  26  is transferred to the outside surface of belt  30  in a respective drum transfer nip  44 . As a print medium, such as paper  46 , travels along a paper path  48  in the direction indicated by arrow  49 , the toner is transferred from belt  30  to the surface of the paper  46  in a paper transfer nip  50  between opposing rollers  52  and  54 . Paper transfer nip  50  is also referred to herein as a “toner transfer nip”. 
     Imaging begins, at least on first photoconductive drum  20 , before a first sheet of paper  46  is picked from input tray  34 . The image begins to be transferred onto transfer belt  30 , and when the image on belt  30  reaches a point that is a certain distance away from nip  50 , tray  34  receives a pick command from microcontroller  38 . 
     By monitoring the printhead scan data, microcontroller  38  determines when the electrophotographic system begins to image on photoconductive drum  20 . Microcontroller  38  then determines at what point in time the first line of the image is placed onto transfer belt  30  by monitoring, in addition to the scan data, the number of revolutions and rotational position of drum motor  28 . Drum motor  28  drives photoconductive drum  20 . Drum motor  28  may or may not also drive drums  22 ,  24  and  26 . The number of revolutions and rotational position of drum motor  28  is ascertained by an encoder  56 , as is well known in the art. 
     As the first writing line is transferred onto transfer belt  30 , microcontroller  38  begins to track incrementally the position of the image on belt  30  by monitoring the number of revolutions and rotational position of belt motor  32 . Similarly to drum motor  28 , the number of revolutions and rotational position of belt motor  32  can be ascertained by another encoder  58 . From the number of rotations and rotational position of belt motor  32 , the linear movement of belt  30  and the image carried thereby can be directly calculated. Since both the location of the image on transfer belt  30  and the length of belt  30  between the first drum transfer nip  44  and paper transfer nip  50  is known, the distance remaining for the image to travel before reaching paper transfer nip  50  can also be calculated. 
     At some designated time, input tray  34  receives a command from microcontroller  38  to pick a sheet of paper. The sheet of paper moves through paper path  48  at a constant speed and eventually trips a paper path sensor S 1 . Microcontroller  38  immediately begins tracking incrementally the position of the paper by monitoring the feedback of yet another encoder  60 , this one being associated with paper path motor  36 . From the tracked distance traveled by the sheet of paper after tripping paper path sensor S 1 , and the known distance between S 1  and paper transfer nip  50 , the distance remaining for the sheet of paper to travel before reaching paper transfer nip  50  can be calculated. 
     The inter-sheet gap is small in comparison to the paper path length and the image path length. Thus, no adjustment to paper position can begin until the preceding sheet has been completely cleared from paper path  48 . This is determined by monitoring another paper path sensor S 2 . By monitoring when the trailing edge of the preceding sheet has cleared sensor S 2 , microcontroller  38  can determine when the preceding sheet has exited the last driven rolls of motor  36 , thereby allowing the correction of the position error of the current sheet to begin. 
     From the two calculated distances, i.e., from the image on belt  30  to nip  50  and from the current sheet of paper to nip  50 , microcontroller  38  calculates a correction needed to remove position error of the paper relative to its image so as to enable the paper to arrive at transfer nip  50  at the time and speed required to produce an accurate top writing line margin with acceptable tolerance. The correction is accomplished by incrementally adjusting the linear speed of the paper through paper path  48  by incrementally changing the velocity of paper path motor  36 . The speed of paper path motor  36  is increased or decreased depending upon whether the current sheet of paper is behind or ahead of a desired, target position. 
     The feedback from encoder  58  is used as an interrupt into microcontroller  38 . The timing of this interrupt is chosen such that minimum bandwidth is required, e.g., approximately 1 ms between interrupts. Fixed time interrupts could also be used. Each time microcontroller  38  receives this interrupt, the error in the relationship between the image position and the paper position is determined. This position error is multiplied by a gain factor in a control equation that produces a new desired velocity for motor  36 . If the error is zero, the velocity of paper path motor  36  is not changed from the nominal. If the sheet of paper is ahead of or behind the desired position, a new paper path motor speed that would reduce the error for that sampling point is calculated and then implemented by microcontroller  38 . As the position error decreases, the amount of velocity change also decreases to the point that zero error produces the nominal velocity. The new, changed speed of paper path motor  36  is limited by minimum and maximum values in order to allow for reasonable power requirements, acceptable acoustics, and a stable control system. This also bounds the error that can be removed. 
     The new desired speed is then fed into a motor speed control algorithm within microcontroller  38 . Feedback from encoder  60  is used to maintain the speed of motor  36  at the new desired speed, which may be unchanged if the sheet of paper is already “on schedule”, i.e., at a proper point along paper path  48  in order to arrive at paper transfer nip  50  at the desired point in time and at the desired speed. A signal from encoder  60  is used as another interrupt into microcontroller  38 . Again, the timing of the interrupt is chosen such that minimum bandwidth is required, e.g., approximately 1 ms between interrupts. The control code within microcontroller  38  sets the gain, encoder divide-by values (which sets interrupt timing), and other control parameters based upon the newly requested speed from the error correction routine. Each time microcontroller  38  receives this interrupt, microcontroller  38  uses the parameters to adjust the voltage applied to paper path motor  36  in order to maintain the desired speed. 
     The desired speed of paper path motor  36  is updated on every feedback pulse from belt motor encoder  58 . These updates continue until the remaining distance between sheet of paper and paper transfer nip  50  is substantially zero. At this time, the speed of the sheet of paper is held constant, and the image is transferred onto the sheet of paper with a correct top writing line margin that is within the acceptable tolerance. 
     If the top writing line margin is not within the acceptable tolerance, it is possible that the error has been introduced by variances in the predetermined number of motor revolutions required to transport the paper from sensor S 1  to paper transfer nip  50 , and/or the predetermined number of motor revolutions required for the image to travel between photoconductive drum  20  and paper transfer nip  50 . This new error is repeatable, and can be eliminated by allowing these values to be “tweaked” and stored in a nonvolatile memory  62  connected to microcontroller  38 . For example, if a test print page shows that the top margin is out of specifications, then an operator can use a software utility to adjust the stored values that microcontroller  38  uses to calculate position errors. These stored values represent the distance between paper sensor S 1  and paper transfer nip  50 , and/or the length of transfer belt  30  between photoconductive drum  20  and paper transfer nip  50 . By adjusting the stored values, the image can be moved up or down on the page by the adjusted amount. These values are stored and used for any and all future print pages. Thus, the adverse effects of the manufacturing tolerances of paper path  48  and transfer belt  30  can be eliminated. 
     A comparison of FIGS. 2 a  and  2   b  illustrates the operation of one embodiment of the method of the present invention. At time =0.1 second, the preceding sheet of paper clears sensor S 2 , and, as can be seen in FIG. 2 b,  the current sheet is one inch (25.4 mm) behind its desired location along paper path  48  at that point in time. Upon sensing that the current sheet is one inch behind, microcontroller  38  increases the speed of paper transport motor  36  from a steady-state speed of 1000 rpm to a higher steady-state speed of approximately 1475 rpm, as shown in FIG. 2 a.  Microcontroller  38  monitors the ensuing reduction in the paper position error, as illustrated in FIG. 2 b . At time =0.48 second, when the paper position error is approximately 5 mm, microcontroller  38  begins to command speeds of motor  36  less than the maximum. The system continues to measure error and adjust the speed of motor  36 . Eventually, the speed of paper transport motor  36  decays back down to the desired steady-state speed of 1000 rpm such that the sheet of paper enters nip  50  coincident with its image, which is at time of 1.0 second in this example. Thus, the paper position error is removed and motor  36  is returned to the nominal, steady-state paper path speed of 1000 rpm before the sheet of paper enters toner transfer nip  50 . The speed of 1475 rpm is the maximum allowable desired speed in this embodiment. 
     A comparison of FIGS. 3 a  and  3   b  illustrates a second scenario wherein the sheet of paper is 0.5 inch (12.7 mm) ahead of its desired location along paper path  48  at time =0.1 second when the preceding sheet of paper clears sensor S 2 . Microcontroller  38  then decreases the speed of paper transport motor  36  from the steady-state speed of 1000 rpm to a lower steady-state speed of approximately 750 rpm, as shown in FIG. 3 a.  The speed of 750 rpm is the minimum allowable desired speed in this embodiment. Microcontroller  38  monitors the ensuing reduction in the paper position error, as illustrated in FIG. 3 b.  At time =0.43 second, when the paper position error is approximately −3 mm, microcontroller  38  begins to command speeds greater than the minimum. The system continues to measure error and adjust the speed of motor  36 . Eventually the speed of paper transport motor  36  increases back up to the desired steady-state speed of 1000 rpm such that the sheet of paper enters nip  50  coincident with its image, which is at time of 1.0 second in this example. Thus, in this second scenario too, the paper position error is removed and motor  36  is returned to the nominal, steady-state speed of 1000 rpm before the paper enters toner transfer nip  50 . 
     The present invention incorporates multiple feedback loops in a control system such that paper position errors introduced into the paper path can be effectively reduced in order to provide an accurate top writing line margin, and to allow the paper to be “handed-off” at an optimal speed into toner transfer nip  50 . Feedback from the printhead scan data, drum motor  28 , and toner transfer belt motor  32  are used to determine and track the image position on belt  30 . Feedback from paper path sensors S 1  and S 2  and the signals from paper path motor encoder  60  are used to determine and track paper position. Position error can then be calculated between paper and image. The speed of paper path motor  36  is adjusted by microcontroller  38  to effectively remove the position error such that the paper arrives at transfer nip  50  coincident with the image and at the desired speed. Care is taken to keep the bandwidth demands on microcontroller  38  to a minimum by using a signal from belt motor encoder  58  to set the frequency of changes to the speed of paper transport motor  36 , and by using a signal from paper transport motor encoder  60  to actually control the speed of paper transport. The speed of paper transport motor  36  is also carefully controlled to provide acceptable power levels, acoustics and stability. 
     All three encoders generate a predetermined number of pulses per revolution of the corresponding motor. Of course, it would be possible to use any or all of the known types of encoders, or another type of device that senses the rotation of a motor, or by which motor shaft position could be deduced. 
     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 adaptions of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within know or customary practice in the art to which this invention pertains and which falls within the limits of the appended claims.