Abstract:
A system and method for reducing scan line jitter caused by facet cut variation in scan systems employing a plurality of laser sources and a sensor for generating the timing for the laser sources. The system includes a controller for determining a unique time delay for each facet of the rotating mirror, and controlling the laser sources so that video provided by each laser source is delayed in a scan line by the unique time delay corresponding to the facet of the rotating mirror used in creating the scan line.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    Pursuant to 35 U.S.C. §119, this application claims the benefit of the earlier filing date of Provisional Application Ser. No. 61/483,635, filed May 6, 2011, entitled “Laser Scan Unit for an Imaging Device,” the content of which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present application relates generally to electronic devices having a laser scan unit (LSU), and particularly to improved electrophotographic devices having reduced jitter and scan line variability for on-axis based LSU systems. 
         [0004]    2. Description of the Related Art 
         [0005]    In an LSU of an electrophotographic color imaging device, it is typical for each imaging channel to have its own optical sensor, called an “hsync sensor,” to detect its laser beam having been deflected from a polygonal mirror and to create a beam detect signal for use in triggering video data being included in the channel&#39;s laser beam for impinging on the channel&#39;s corresponding photoconductive drum. In more recent LSU design architectures, two beams share a single hsync sensor with one of the channels creating the start of scan (SOS) signal and the other channel using a delayed version of that SOS signal. Because one channel is imaging off of a facet of the rotating polygonal mirror that is not associated with the optical sensor generating the SOS signal, scan jitter can be induced into that channel. With such LSUs generating laser beams on-axis relative to the facets of the rotating polygonal mirror, the laser beams impinge on the polygon mirror such that only the variation in one or more facet cuts of the mirror is seen to induce scan jitter. 
         [0006]    What is needed, then, is an improved LSU system which reduces or substantially eliminates scan jitter induced by facet cut variation of the polygonal mirror of an LSU. 
       SUMMARY 
       [0007]    Example embodiments overcome the shortcomings of prior systems and thereby satisfy a significant need for a scanning system having reduced jitter for channels which do not generate synchronization signals for controlling the channels. In accordance with an example embodiment, a scan system includes a rotating mirror having a plurality of facets; a plurality of laser sources, each laser source positioned in proximity to the rotating mirror for generating a laser beam directed thereat; and an optical sensor for receiving one of the laser beams reflected by the facets of the rotating mirror and for generating a horizontal synchronization signal in response to the reception. The system further includes a controller operably coupled to the rotating mirror, the laser sources and the optical sensor, for determining a unique time delay for each facet of the rotating mirror, and controlling the laser sources so that video provided by each laser source is delayed in a scan line by the unique time delay corresponding to the facet of the rotating mirror used in creating the scan line. 
         [0008]    Further, the system may measure and accumulate timing information for each facet and average same. From the averaged facet timing information, the system may generate a signature value for each facet and the unique time delay by integrating or combining the time differences of those facets from the detecting facet to the imaging facet, scaling the result by a predetermined value, and adding thereto a predetermined value to ensure that all unique time delays are positive. The unique time delay for each facet is then incorporated into each channel to affect the timing for providing video therein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above-mentioned and other features and advantages of the various embodiments, and the manner of attaining them, will become more apparent and will be better understood by reference to the accompanying drawings, wherein: 
           [0010]      FIG. 1  illustrates a portion of an LSU system in which facet cut variation exists; 
           [0011]      FIG. 2  illustrates mirror facet delay and scan jitter resulting from the variation of  FIG. 1 ; 
           [0012]      FIG. 3  is a block diagram of circuitry for measuring and tracking hsync-to-hsync delays according to an example embodiment; 
           [0013]      FIG. 4  is a flowchart and corresponding graphs for determining facet offset values according to an example embodiment; 
           [0014]      FIG. 5  is a block diagram of circuitry for utilizing the determined facet offset values for reducing scan jitter according to an example embodiment; 
           [0015]      FIG. 6  illustrates the results of utilizing the determined facet offset values of  FIG. 4 ; 
           [0016]      FIG. 7  is a side view of an electrophotographic imaging device incorporating the circuitry and algorithms of the example embodiments; and 
           [0017]      FIG. 8  is a top view of a rotating, polygonal mirror illustrating the use of the sum of time variations between a facet used in detecting a laser beam and a facet used in creating a scan line in an imaging operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following description and drawings illustrate embodiments sufficiently to enable those skilled in the art to practice it. It is to be understood that the subject matter of this application is not limited to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The subject matter is capable of other embodiments and of being practiced or of being carried out in various ways. For example, other embodiments may incorporate structural, chronological, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of the application encompasses the appended claims and all available equivalents. The following description is, therefore, not to be taken in a limited sense, and the scope of the present application as defined by the appended claims. 
         [0019]    Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
         [0020]    Example embodiments may be implemented in hardware in an integrated circuit, such as an Application Specific Integrated Circuit (“ASIC”). It is understood, however, that example embodiments may be at least partly implemented by a general purpose processor or microcontroller. 
         [0021]    Referring now to the drawings and particularly to  FIG. 7 , there is shown an electrophotographic image forming apparatus  700 , in this case a color laser printer. An image to be printed is electronically transmitted to a print engine processor or controller  702  by an external device (not shown) or may comprise an image stored in a memory of the controller  702 . The controller  702  includes system memory, one or more processors, and other logic necessary to control the functions of electrophotographic imaging. 
         [0022]    In performing a print operation, the controller  702  initiates an imaging operation where a top substrate of a stack of media is picked up from a media or storage tray  704  by a pick mechanism  706  and is delivered to a substrate transport apparatus formed by a pair of aligning rollers  708  and a substrate transport belt  710  in the illustrated embodiment. The substrate transport belt  710  carries the picked substrate along a substrate path past each of four image forming stations  712  which apply toner to the substrate. The image forming station  712 K includes a photoconductive drum that delivers yellow toner to the substrate in a pattern corresponding to a black (K) image plane of the image being printed. The image forming station  712 M includes a photoconductive drum that delivers magenta toner to the substrate in a pattern corresponding to the magenta (M) image plane of the image being printed. The image forming station  712 C includes a photoconductive drum that delivers cyan toner to the substrate in a pattern corresponding to the cyan (C) image plane of the image being printed. The image forming station  712 Y includes a photoconductive drum that delivers yellow toner to the substrate in a pattern corresponding to the yellow image plane of the image being printed. The controller  702  regulates the speed of the substrate transport belt  710 , substrate pick timing, and the timing of the image forming stations  712  to effect proper registration and alignment of the different image planes to the substrate. 
         [0023]    To effect the imaging operation, the controller  702  manipulates and converts data defining each of the KMCY image planes into separate corresponding laser pulse video signals, and the video signals are then communicated to a printhead  714 . The printhead  714  may include four laser light sources  716  (only two illustrated for reasons of clarity) and at least one polygonal mirror  718  supported for rotation about a rotational axis, and post-scan optical systems receiving the light beams emitted from the laser light sources  716 . Each laser of the laser light sources  716  emits a respective laser beam which is reflected off the rotating polygonal mirror  718  and is directed towards a photoconductive drum of a corresponding image forming station  712  by select lenses and mirrors in the post-scan optical systems of printhead  714 . Following impingement of laser beams across the photoconductive drums, toner is collected onto the impinged regions which is then transferred to the substrate sheet, after which the transferred toner is fused onto the sheet as it passes through fuser  178 , which fuses the toner by application of heat and pressure. 
         [0024]      FIG. 1  illustrates an on-axis LSU architecture in which two channels, in this case the channels for colors yellow and cyan, share a single hsync sensor  102 . Rotating polygonal mirror  104  is depicted at different times of a single rotation during a laser scan operation. A laser source  106 , associated with the imaging of yellow toner, creates a laser beam LY that sweeps across the hsync sensor  102 . Hsync sensor  102  generates the SOS signal responsive to detection of beam LY. Yellow video data is triggered by the detection of beam LY, reflecting off mirror  104  and impinging hsync sensor  102 . By delaying the beam-detecting hsync event for a time corresponding to the rotation of facets of mirror  104 , a laser source  108 , corresponding to the imaging of cyan toner, will image off of the same facet of mirror  104  that created the hsync event. In other words, delaying the hsync event for the cyan channel by an amount corresponding to rotating a facet of mirror  104  from being impinged by laser LY to being impinged by laser LC, results in both the yellow and cyan channels imaging off of the same mirror facet. However, due to velocity variations of the motor which rotates mirror  104 , the cyan scan line can experience location variation. To compensate for this location variation, the delay from the hsync signal generated by hsync sensor  102  can be reduced to less than one facet. Unfortunately, this forces the cyan channel to image off of a facet of mirror  104  that is different from the facet that created the hsync signal. Depending on the amount of facet cut variation of mirror  104 , such an on-axis LSU system induces scan jitter in the cyan channel. 
         [0025]    Facet cut variation in mirror  104  also induces variation associated with the hsync-to-hsync timing. In  FIG. 1 , mirror  104  is illustrated as having a facet cut variation associated with adjacent facets 0 and 1, shown in dashed lines. This particular facet cut can be seen to change from the ideal the assertion of the SOS signal corresponding to facets 0 and 1.  FIG. 2  illustrates variation of the average delay from ideal the assertion of the SOS signal for each facet of mirror  104 , with facet 0 providing one timing (delayed) and facet 1 providing a second timing (faster) than the ideal timing for a perfect cut mirror  104 . The facet cut variation with respect to facets 0 and 1 causes scan jitter for the cyan channel associated with such facets. A correction factor, applied with respect to the timing of video associated with facets 0 and 1 of mirror  104  for this particular example, substantially reduces scan jitter in the cyan channel. The technique for generating the correction factor is described in detail below. 
         [0026]      FIG. 3  is a block diagram of circuitry  300  utilized for determining hsync-to-hsync delays, i.e., the time delays associated with each facet of mirror  104 . Circuitry  300  captures and accumulates the time between hsync signals on a per facet basis for a specified number of scanner rotations. In particular, circuitry  300  receives the SOS signal generated by hsync sensor  102 . Hsync sensor  102  asserts its SOS signal each time laser beam LY impinges the sensor. A filter  302  receives the SOS signal and generates a filtered SOS at its output. A facet tracking block  304  receives the filtered SOS signal and tracks the particular facet of mirror  104  used in generating the recent assertion of the SOS signal. The output of facet tracking block  304  is at least one signal which is used to select, via multiplexer circuit  306 , previously recorded delay data for the selected mirror facet. 
         [0027]    Circuitry  300  further includes control circuitry  308  which receives the filtered SOS signal from filter  302  and a clock signal. The clock signal may be a multiple of the pel clock signal used in delivering video data for each channel of the LSU. Control circuitry  308  includes timer circuitry for generating an output signal of the delay, measured in cycles of the input clock signal, between assertions of the SOS signal. An adder or accumulator  310  receives the SOS assertion delay and the previously recorded facet timing data and generates a sum thereof which is placed at the output of adder  310 . A demultiplexer  312  receives the delay sum output of adder  310  and provides same to an output of demultiplexer  312  as selected by the output of facet tracking block  304 . Storage  314 , which may be implemented as volatile or nonvolatile memory, registers, latches or the like, maintains the delay sum information for each facet of mirror  104 . 
         [0028]    The operation of circuitry  300  is as follows. Storage  314  maintains previously determined facet delay information for each facet of mirror  104 . Hsync sensor  102  asserts the SOS signal each time a facet of mirror  104  reflects laser beam LY onto the sensor. The SOS signal is received and filtered by filter  302 . Facet tracking block  304  tracks the particular facet of mirror  104  which deflected laser beam LY and generates a selection signal indicating the particular facet. The selection signal selects the previously determined facet delay information for the particular facet and provides same to adder  310 . Meanwhile, control block  308  counts the amount of delay between successive assertions of the SOS signal and provides the delay amount to adder  310 , which adds the previously determined facet delay information for the particular mirror facet and the delay between successive SOS signal assertions to obtain a delay sum signal. The delay sum signal is then provided to storage  314  for the particular mirror facet selected. In the example embodiment, the newly generated delay sum signal may replace the previously determined facet delay information for the selected mirror facet identified by facet tracking block  304 . This procedure then repeats for each mirror facet for a predetermined number of revolutions of mirror  104 . At the end of the predetermined number of revolutions, each location of storage  314  includes the sum of the accumulated delay times for each facet of mirror  104 . At the completion of the mirror facet delay measurements, the accumulated delay times maintained in storage  314  may be placed in a buffer (not shown) in which a number of sets of previously measured accumulated delay times may be maintained. The buffer may discard the oldest accumulated delay times when a new set thereof is provided to the buffer. This operation may be performed at the start of each print operation, for example. 
         [0029]    With the above-mentioned buffer containing accumulated delay times for each facet of mirror  104 , the controller  702  associated with the LSU determines an offset value to use in triggering the application of video data for use with each facet of mirror  104 . With reference to  FIG. 4 , for each facet of mirror  104 , the controller computes an average delay time value at  410 . For each mirror facet, this may be the average of the delay times maintained in the buffer. Next, at  415  the controller determines minimum and maximum cutoff values based at least in part on the average delay time computed at  410 . Using the cutoff values, the controller  702  at  420  computes an average facet value (AFV) for each facet of mirror  104  by discarding values falling outside of the region bounded by the minimum and maximum cutoff values. The AFV for each mirror facet may be maintained in memory. 
         [0030]    Next, the controller  702  determines a valid signature VS for each mirror facet at  425  by computing an average of the AFVs of the mirror facets and subtracting the average from each AFV. The valid signatures VS of the mirror facets are signed values indicating facet time variation. The facet time variation is used in the computation of the image start variation values. The image start variation value for a channel is the sum of the facet time variations from the facet detecting the laser beam to the facet used in imaging and is therefore dependent on characteristics such as mechanical layout, facet count, and polygon mirror rotation. The sum of the facet time variation has the units of time and with knowledge of the optical system, the sum of the facet time variation is scaled to convert from time to distance variation at the photoconductive drum to generate at  430  a facet offset FO value for each facet of mirror  718 .  FIG. 8  depicts an eight faceted polygon mirror  800  with the detection facet  802  three facets away from the imaging facet  804 . The facet offset FO of a mirror facet is used in the LSU system in triggering the inclusion of video data in the laser beam when reflecting from the mirror facet. 
         [0031]    With reference to  FIG. 5 , there is shown a block diagram of the facet offset circuitry  500  for using the facet offsets FOs for triggering video data in the laser beams for impingement on the photoconductive drums. In one example embodiment, blocks of circuitry  300  ( FIG. 3 ) that are common may be utilized in facet offset circuitry  500  in order to reduce circuit (chip and/or board) size. In another example embodiment, each of the blocks of facet offset circuitry  500  may be separate from the blocks used in circuitry  300 . The description of facet offset circuitry  500  will be described following the latter example embodiment. A SOS filter  502  may receive the SOS signal from the hsync sensor  102  and generate a filtered version thereof at its output. A facet tracking block  504  receives the filtered SOS signal and tracks the particular facet used in generating the SOS signal. The output of facet tracking block  504  is at least one signal which selects the facet offset FO value for the particular facet tracked. In particular, the output of facet tracking block  504  is the selection input of multiplexer circuitry  506 , which includes a data input for each facet offset FO maintained in storage  508 . In this way, facet tracking block  504  tracks the current facet used in generating the most recent assertion of the SOS signal and selects the facet offset FO value corresponding to the current facet. 
         [0032]    An adder or accumulator  510  receives a predetermined image delay value at a first input and the output of multiplexer circuitry  506  at a second input, and generates a sum of thereof at the output of adder  510 . In an example embodiment, the predetermined image delay value may be a constant. Thus the sum output of adder  510  is an image delay value for the current mirror facet that is based in part upon the facet offset FO value corresponding thereto. This image delay value is amount of delay following the assertion of the SOS signal before video data is included in the laser signal.  FIG. 6  illustrates the use of the image delay value of each facet of mirror  104  used by each channel. As can be seen, the video to be reflected from a particular facet of mirror  104  is delayed by the image delay value corresponding to the particular facet. By providing a correction factor, in this case a unique facet offset FO for each mirror facet, to the channels that are not involved in the creation of the hsync signal, scan jitter is substantially reduced in such channels. 
         [0033]    The foregoing description of several methods and an embodiment of the invention have been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.