Abstract:
A method of aligning a laser printhead in an electrophotographic machine includes providing a fixture with a pair of fixed sensors. The fixed sensors are spaced apart along a length of a first fixture drum axis. Each of the fixed sensors is fixed relative to a second of the fixed drums. The first fixture drum is provided with a pair of floating sensors. The floating sensors are spaced apart along the length of the first fixture drum axis. Each of the floating sensors is fixed relative to the first fixture drum. A laser beam from the laser printhead is scanned across an outside surface of the first fixture drum, the fixed sensors and the floating sensors. An intersection of the scanned laser beam and the outside surface of the first drum defines a scan path. A first skew of the scan path relative to an axis of the first drum is measured using the floating sensors. A second skew of the scan path relative to an axis of the second drum is measured using the fixed sensors. The scan path of the laser beam is adjusted dependent upon each of the first skew and the second skew.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic machine, and, more particularly, to a method of aligning a laser printhead in an electrophotographic machine such as a tandem color laser printer. 
     2. Description of the Related Art 
     An electrophotographic machine such as a tandem color laser printer requires four individual laser scanner printheads to independently and simultaneously image a specific color on each of four respective photoconductive drums. In an in-line color electrophotographic imaging process, latent images are formed on the photosensitive drums, and the images are in turn developed using a predetermined color of toner. All four of these toner images are then transferred simultaneously onto a transfer medium, such as a belt. The developed images are then transferred to a sheet of media (such as paper) which travels past the transfer medium. The image in each color is created one line at a time, and the lines are oriented at right angles to the direction of travel of the sheet of media. The individually generated images combine to form a full-color image. In a typical multi-color laser printer, for example, the transfer medium passes through four color developing stations in series, with the colors being yellow, cyan, magenta and black. 
     It is recognized that in order for the multi-color laser printer to print accurately, the laser beams for all four colors must be in alignment, both in the scan direction (across the page) and the process direction (feed direction of the print medium). However, providing proper alignment of even a single laser printhead in relation to the sheet of media in the process direction can be difficult. This problem is compounded with the addition of each printhead, since the plurality of printheads must be in registration so that the individual images generated by each printhead can be superimposed correctly when combined. During printer assembly an attempt is made to optically align the laser printheads both individually and collectively, but the ability to provide precise alignment is limited by several factors, including component tolerances. 
     What is needed in the art is a method of quickly and accurately achieving skew adjustment and margin alignment in an electrophotographic machine. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fixture to achieve the skew adjustment and the margin alignment of all four color planes to the accuracy required for high quality color printing with minimum operator interaction in a production environment. 
     The invention comprises, in one form thereof, a method of aligning a laser printhead in an electrophotographic machine having a plurality of photoconductive drums. A first of the photoconductive drums is replaced with a drum fixture containing a pair of fixed sensors located where the image plane of the laser beam would intersect the drum. The fixed sensors are spaced apart along a length of the first photoconductive drum axis. Each of the fixed sensors is fixed relative to a second of the photoconductive drum axes. The first photoconductive drum fixture is also provided with a pair of floating sensors. The floating sensors are spaced apart along the length of the first drum fixture. Each of the floating sensors is fixed relative to the first photoconductive drum fixture, but this photoconductive drum fixture floats relative to the second photoconductive drum axis. A laser beam from the laser printhead is scanned across an outside surface of the first drum fixture, the fixed sensors and the floating sensors. An intersection of the scanned laser beam and the outside surface of the first drum defines a scan path. A first skew of the scan path relative to an axis of the first photoconductive drum is measured using the floating sensors. A second skew of the scan path relative to an axis of the second photoconductive drum is measured using the fixed sensors. The scan path of the laser beam is adjusted dependent upon each of the first skew and the second skew. 
     The invention comprises, in another form thereof, an electrophotographic machine including a plurality of photoconductive drums. A laser printhead scans a laser beam across a fixture in the location of a first of the photoconductive drums. A pair of fixed sensors are spaced apart along a length of the first photoconductive drum axis. The fixed sensors are fixed relative to a second of the photoconductive drum axes. The fixed sensors sense a skew of the laser beam relative to the second photoconductive drum axis. A pair of floating sensors are spaced apart along the length of the first photoconductive drum fixture. The floating sensors are fixed relative to the first photoconductive drum fixture. The floating sensors mounted to the first drum fixture sense a skew of the laser beam relative to the first photoconductive drum axis. A processing/feedback unit is in communication with the fixed sensors and with the floating sensors. The processing/feedback unit calculates a desired skew of the laser beam and provides an indication of a desired skew target. 
     The assembly and alignment of a tandem color laser printer requires that the line created by the intersection of the plane of the laser scan and the surface of the photoconductive drum cause the image when transferred to the transfer medium to be parallel to the image of the reference photoconductive drum (usually the black drum). This reference photoconductive drum (black) datum is chosen as the datum for parallel transferred images. Thus, the black laser scan is aligned parallel to this black reference photoconductive drum datum axis just as is done in a single color (mono) laser printer. The other laser scans are aligned skewed with their respective photoconductive drum datum axis in an amount equal to the skew between their respective photoconductive drum datum axis and the black reference photoconductive drum datum axis. Thus, each of the non-black color laser scans will be adjusted to have twice the skew relative to the black laser scan (or to the black reference photoconductive drum datum axis) that its respective color photoconductive drum datum axis has compared to the black reference photoconductive drum datum axis. This implies that the skew in each photoconductive drum datum axis relative to the black reference photoconductive drum datum axis must be measured with the adjustment fixture of the present invention. The adjustment fixture must also measure the skew in the respective laser scan relative to its respective photoconductive drum datum axis. 
     It is desired to adjust the skew of the image generated by each printhead relative to the black reference photoconductive drum datum axis to within 0.015 mm over the 215.9 mm (8.5 inch) writing line length. This is achieved by adjusting the mechanical position of the printhead relative to its respective photoconductive drum datum axis during the assembly process. The printhead is designed with a coarse adjustment and a fine adjustment feature that can allow this precise skew adjustment to be achieved. The assembly operator receives easy to use feedback which provides the required target to achieve the desired skew adjustment and the instantaneous status of the laser scan relative to this desired target. 
     The assembly and alignment of a tandem color laser printer also requires aligning other registration characteristics of the four transferred images. The adjustment fixture of the present invention provides data to the printer that allows the printer to electronically adjust the left-right margin locations for all four colors to an equal nominal location, adjust the line lengths of all four colors to be equal, and adjust the relative timing of imaging each color to correct for the process direction spacing of each photoconductive drum datum axis relative to the black photoconductive drum datum axis. These three settings are then stored in the printer NVRAM ready for customer use. 
     An advantage of the present invention is that, in a production environment, an assembly operator can quickly achieve the skew adjustment and margin alignment of all four color planes that is required for high quality color printing. 
    
    
     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 block diagram of one embodiment of an alignment fixture of the present invention interacting with a laser printer; 
     FIG. 2 is a schematic diagram of the fixture drums and the laser printer of FIG. 1; 
     FIG. 3 is a partial top view of the alignment fixture of FIG. 1; and 
     FIG. 4 is a schematic diagram of sensors of the alignment fixture of FIG.  1 . 
    
    
     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 an adjustment fixture  10  of the present invention interacting with a multicolor laser printer  12 . 
     Printer  12  includes a black toner cartridge  14  (FIG.  2 ), a magenta toner cartridge  16 , a cyan toner cartridge  18  and a yellow toner cartridge  20  with corresponding photoconductive drums  22 ,  24 ,  26  and  28 . Each drum  22 ,  24 ,  26  and  28  engages a transfer belt  30 . Each of printheads  32 ,  34 ,  36  and  38  scans a respective image onto a respective one of photoconductive drums  22 ,  24 ,  26  and  28 . A paper drive motor  39  moves paper into engagement with transfer belt  30 . 
     Adjustment fixture  10  includes a sensing unit  40  and a processing/feedback unit  42  in bi-directional communication with control electronics  44  of printer  12 . Sensing unit  40  is placed inside laser printer  12  in the space that is normally occupied by the four printer cartridges  14 ,  16 ,  18  and  20 . Sensing unit  40  registers itself to each corresponding datum axis of photoconductive drums  22 ,  24 ,  26  and  28 . Sensing unit  40  then detects the skew between each datum axis of color photoconductive drums  24 ,  26  and  28  relative to the reference datum axis of black photoconductive drum  22  and feeds this information to processing/feedback unit  42 . Sensing unit  40  also detects the location in both the scan and the process direction of a particular energized spot at each end of the writing line (215.9 mm apart) and feeds this information to processing/feedback unit  42 . 
     Processing/feedback unit  42  is located on the assembly operator&#39;s bench external to printer  12 . Processing/feedback unit  42  receives data from sensing unit  40 , processes the data, and provides the assembly operator with a visual indication of the desired skew target for the particular printhead being mechanically adjusted and an interactive visual indication of the orientation of the scan line relative to this target. 
     Processing/feedback unit  42  also communicates with printer controls  44  to trigger printer  12  to generate the necessary laser patterns to enable the skew adjustment by the operator and also the necessary patterns to achieve the electronic alignments needed. Once printer  12  generates the desired patterns for the electronic alignment process, then sensing unit  40  gathers the data necessary to allow printer  12  to perform the desired electronic adjustments. Processing/feedback unit  42  then receives one or more sets of data from sensing unit  40  in order to confirm that the electronic adjustments made by printer  12  have indeed achieved the desired alignment results. This confirmation is communicated to printer controls  44  for final storage of the results in printer memory  46 , which may be in the form of non-volatile random access memory. 
     Sensing unit  40  includes a very rigid plate  48  (FIG. 3) that has a drum fixture  76  rigidly attached thereto. This rigid plate  48  includes a fixed sensor  50  at each end of each nominal photoconductive drum  22 ,  24 ,  26 ,  28  axis. These fixed sensors  50  detect in both the scan direction, indicated by double arrow  52 , and the process direction, indicated by double arrow  54 . Sensors  50  are rigidly fixed such that a line between a pair of sensors  50  that correspond to a same one of photoconductive drums  22 ,  24 ,  26 ,  28  is parallel to a V-block reference datum  56  (FIG. 4) of black photoconductive drum  22 . That is, there are a total of eight fixed sensors  50 , with each imaginary line that joins a corresponding one of the four pairs of fixed sensors  50  being parallel to black photoconductive drum datum  56 . Sensors  50  are fixed relative to black photoconductive drum datum  56 , and each is located as accurately as possible to either a 0 mm location or a 215.9 mm location at an end of a corresponding one of photoconductive drums  22 ,  24 ,  26 ,  28 . 
     Sensing unit  40  also includes six floating sensors  58 , each disposed at a corresponding end of a corresponding one of the non-black drum fixtures  78 ,  80  and  82 . Sensors  58  are rigidly mounted to each one of the non-black drum fixtures  78 ,  80 ,  82  which float such that they each align with a corresponding V-block  60  of photoconductive drums  24 ,  26  and  28  when sensing unit  40  is lowered into place. Floating sensors  58  are located on the outboard side of, and as close as possible to, fixed sensors  50 , which are fixed parallel to the black V-block reference datum  56 . Each sensor  50 ,  58  can be in the form of a charged coupled device (CCD) array or a dual axis position sensitive diode, for example. The floating sensors  58  may also be located on the inboard side of fixed sensors  50 . 
     The laser scan for each color is used as the light source that is measured by sensors  50 ,  58  on each end of the non-black drum fixtures  78 ,  80 ,  82 . The difference in the process direction  54  values obtained between the two readings on one end and the two readings on the other end characterizes the skew of the photoconductive drum V-blocks  60  for that color relative to the black reference photoconductive drum datum  56 . This information is then used to provide the operator with a target that is used to align the printhead for that non-black color. In the case of the black image printhead, only the fixed sensors  50  are used to align the black printhead to the black reference photoconductive drum axis. 
     After each printhead skew has been set, fixed sensors  50  are used to measure the location of the first and last dots or picture elements (PEL&#39;s) that form the scan line. Sensors  50  feed this location information to printer controller  44 . The locations of the first and last PEL&#39;s are determined by imaging a line of nominal length (215.9 mm) across drum fixture  76  for example. At the start-of-scan end, if no beam is detected on sensor  50 , then the scan line must be started earlier in order to pull the first PEL back onto sensor  50 . More particularly, the number of counts that occur after the horizontal synchronization signal and before the start-of-scan must be reduced. On the other hand, if the scan line extends completely across sensor  50 , then the scan line must be started later in order to push the first PEL onto sensor  50 . More particularly, the number of counts that occur after the horizontal synchronization signal and before the start-of-scan must be increased. Once the start of the print line is on sensor  50 , the location of this starting point is determined by the output of sensor  50 . 
     A technique similar to the above technique of locating the start-of-scan PEL is used to detect the location of the end-of-scan PEL. If no scan line is detected on the sensor  50  that is disposed at the end-of-scan, then PEL slices are added in order to lengthen the scan line. On the other hand, if the scan line extends completely across the sensor  50  that is disposed at the end-of-scan, then PEL slices are removed in order to shorten the scan line. 
     Calibration is accomplished by mounting sensing unit  40  onto a very rigid calibration plate  62  which has the skew between photoconductive drum V-blocks  60  very accurately minimized. Thus, sensing unit  40  now has the ideal “no skew” condition set up between the photoconductive drum V-blocks  60 . Next, a glass standard  74  is placed between a collimated light source and the sensors. Glass standard  74  has etched apertures  64  extending therethrough which are accurately aligned parallel to the black reference photoconductive drum V-blocks  60 . Thus, the six sensors  58  that float with the V-blocks  60  for each of the non-black colors can be calibrated to the ideal “no-skew” values provided by the calibration fixture. Likewise, the eight sensors  50  that are fixed relative to the black photoconductive drum datum  56  are calibrated for the “no-skew” location parallel to the black photoconductive drum datum  56 . In addition, the eight fixed sensors  50  use the etched apertures  64  on the glass reference standard  74  to establish the ideal scan direction  52  and process direction  54  locations for each color. 
     The test sequence is controlled by the alignment fixture processing/feedback unit  42 . Processing feedback unit  42 , when initialized by the operator mounting sensing unit  40  into printer  12  and connecting printer  12  to processing/feedback unit  42 , communicates to a raster image processor (RIP)  66  in printer control electronics  44  to initiate the alignment sequence. Raster image processor  66  generates a special page to turn on a constant laser scan line (100% duty cycle) which scans the entire tip-to-tip distance. Raster image processor  66  then sends a request to print a diagnostic page to a print engine  68 . Print engine  68  starts the page under the normal print sequence, but does not drive transfer belt  30 , cartridges  14 ,  16 ,  18   20 , the voltages on drums  22 ,  24 ,  26 ,  28 , or paper drive motor  39 . 
     Dots  70  in FIG. 4 indicate the locations of the spots on sensors  50 ,  58  that were located through apertures  64  of glass reference standard  74 . The path of the scan line, also referred to as a “scan path”, produced by the laser beam is indicated by arrow  72 . The alignment fixture processing/feedback unit  42  determines, at each end of each non-black color scan, the process direction distance from the nominal calibration spot, indicated by dot  70 , to the centroid of the laser beam as it strikes fixed sensor  50 . This distance is indicated by Y 1f  at the start-of-scan end and by Y 2f  at the end-of-scan end. Tester  10  also determines, at each end of each non-black color scan, the process direction distance from the nominal calibration spot  70  to the centroid of the beam as it strikes floating sensor  58 . This distance is indicated by Y 1v  at the start-of-scan end and by Y 2v  at the end-of-scan end. “Y” values are considered to be positive if they are on the side of the nominal calibration point  70  that is away from the black photoconductive drum axis datum  56 . 
     The calculated skew distance of the laser beam relative to fixed sensors  50  is: 
     
       
           D   f =( Y   2f   −Y   1f ). 
       
     
     The calculated skew distance of the laser beam relative to the V-block sensors  58  is: 
     
       
           D   v =( Y   2v   −Y   1v ). 
       
     
     As can be appreciated, if D v  is nonzero, then scan path  72  is not linear in three dimensions. Rather, scan path  72  is slightly arcuate due to the cylindrical outer surface of the photoconductive drum. 
     The skew between a reference line through the V-blocks  60  of that particular color and the black reference photoconductive drum datum  56  is: 
     
       
           S   v =( D   f   −D   v ). 
       
     
     In a similar manner the skew can be found for each of the non-black color V-blocks  60  relative to the black V-block reference datum  56 . 
     These measurements can also be corrected for the distance X 1  between fixed sensor  50  and floating V-block sensor  58 . Distance X 1  is known from alignment fixture  62 . An angle, ∀, of scan line  72  across fixed sensors  50  is: 
     
       
         ∀= tan   −1 ( D   f /215.9) 
       
     
     where D f  is in mm to be consistent with the units of the 215.9 mm distance between fixed sensors  50 . Thus, the predicted location of the laser beam on the V-block sensor  58  at fixed sensor  50  is: 
     
       
           Y′   1v   =Y   1v   +X   1   tan∀=Y   1v   +X   1 *( D   f /215.9) 
       
     
     or 
     
       
           Y′   1v   =Y   1v +( X   1 /215.9)* D   f   
       
     
     and 
     
       
           Y′   2v   =Y   2v −( X   1 /215.9)* D   f   
       
     
     where D f  is in units consistent with Y and X 1  is in mm. Clearly, this same technique works for distances between fixed sensors  50  that are different than the desired nominal print line distance of 215.9 mm. In a similar manner, all the Y iv  can be corrected to Y′ iv  values projected onto the fixed sensor location. Using the laser beam itself with the algorithm described above corrects for the skew of the laser beam itself and separates out the desired information about the skew in V-blocks  60  alone. 
     Black printhead  32  is adjusted mechanically to be in line with the black photoconductive drum datum  56  using corresponding fixed sensors  50  located at each end of the scan line. The non-black color printheads  34 ,  36 ,  38  are mechanically adjusted using the respective fixed sensors  50  located on each end of the scan line until the skew of the printhead relative to the black photoconductive drum datum  56  is: 
     
       
           S   b =( Y   2f   −Y   1f )=2 *S   v.   
       
     
     The alignment fixture processing/feedback unit  42  presents the operator with a display which shows where the desired laser scan beam should be for the required skew, S b =2 *S   v , and where the beam is currently located. This is a real time presentation to allow the operator to make the appropriate interactive skew adjustment and then tighten the screws to mount the printhead in the proper position. A fine adjustment may also be made after the printhead coarse adjustment and tie down has been completed. This skew adjustment is made for each color sequentially. 
     Raster image processor  66  generates a second special page to turn on a constant laser scan line (100% duty cycle) which starts at the first PEL location at the start-of-scan and stops at the last PEL location at the end-of-scan, corresponding to a nominal 215.9 mm location. These actual values can be changed if advantageous to the alignment process. 
     Fixed sensors  50  can be disposed along the lengths of the photoconductive drums at respective locations corresponding to desired side printing margins. The alignment fixture processing/feedback unit  42  determines from the start-of-scan fixed sensor  50  whether the laser scan beam is completely off sensor  50 , extends completely across sensor  50 , or begins on sensor  50 . Based upon this information, the fixture processing/feedback unit  42  determines whether the first PEL of the scan beam is beyond the desired target, short of the desired target, or on the desired target. Processing/feedback unit  42  then commands raster image processor  66  to either decrease or increase the number of counts that occur after the horizontal synchronization signal and before the start-of-scan. The number of counts is decreased in order to pull the first PEL back onto sensor  50  or, if the first PEL is on sensor  50  but beyond the target, to move the first PEL backward towards the target on sensor  50 . The number of counts is increased in order to push the first PEL onto sensor  50 , or, if the first PEL is on sensor  50  but short of the target, to move the first PEL forward towards the target on sensor  50 . 
     Based upon the alignment fixture processing/feedback unit command, raster image processor  66  increases or decreases the number of counts dynamically down the page. Once the first PEL of a particular color laser scan line is in the correct position on the start-of-scan end sensor  50 , processing/feedback unit  42  informs raster image processor  66 . Raster image processor  66  then stops increasing or decreasing the number of counts and stores that count value in printer NVRAM  46  as the correct starting margin value Ns. This process is carried out simultaneously for all four colors. 
     Next, alignment fixture processing/feedback unit  42  determines, from the end-of-scan fixed sensor  50 , whether the laser scan beam is totally off sensor  50 , extends completely across sensor  50 , or, if the last PEL of the scan beam is on sensor  50 , whether the last PEL is short of the desired target or beyond the desired target. 
     Based upon these data, processing/feedback unit  42  then commands raster image processor  66  to either increase the number of counts that occur after the horizontal synchronization signal and before the start-of-scan in order to push the end-of-scan PEL onto sensor  50 , decrease the number of counts in order to pull the end-of-scan PEL back onto sensor  50 , or increase or decrease the number of counts based upon the location of the end-of-scan PEL on sensor  50  relative to the desired target (increase if short of the target, decrease if beyond the target). 
     Based upon the alignment fixture processing/feedback unit command, raster image processor  66  decreases or increases the number of counts dynamically down the page. Once the last PEL of a particular color laser scan line is in the correct position on the end-of-scan end sensor  50 , processing/feedback unit  42  informs raster image processor  66 . Raster image processor  66  then stops increasing or decreasing the number of counts. Raster image processor  66  then uses this count value Ne and the count value that was stored in NVRAM  46  as the starting margin count value Ns to calculate how many PEL slices are to be inserted or deleted from the respective scan line. The number inserted, Ni, is calculated as: Ni=Ne−Ns. Because Ne and Ns are measured in clock pulses which are PEL slices, Ni is the number of PEL slices to be added if it is positive, and deleted if it is negative. Raster image processor  66  then stores that Ni value in printer NVRAM  46  as the correct starting PEL slice insertion value for that color. This process is carried out simultaneously for all four colors. 
     In another embodiment, the black line length is left unchanged from its initial value, and all the non-black color line lengths are adjusted to be equal to the length of the black line, rather than to the nominal 215.9 mm line length. The exposure to adjustment fixture  10  is that the black line length might be such that the end-of-scan PEL does not land on sensor  50  in the adjustment fixture. Thus, a larger sensor  50  is required in order to insure that the location of the last PEL is always sensed. 
     Next, the alignment fixture processing/feedback unit  42  uses the fixed sensors  50  at both ends of all four colors to determine the process direction displacement Y 1f  of the skew-adjusted laser beam from the nominal target at the start-of-scan end and the displacement Y 2f  in the process direction from the nominal target at the end-of-scan end for each color. Likewise, the alignment fixture processing/feedback unit  42  determines the corrected process direction displacement Y′ 1v  and Y′ 2v  of the skew adjusted laser beam from the nominal target at each end of the V-block aligned sensors  58  for each color. 
     Assuming that each photoconductive drum  22 ,  24 ,  26 ,  28  has the same diameter and is rotating at the same constant angular velocity, and that transfer belt  30  passes under black photoconductive drum  22  last, each color will image on transfer belt  30  relative to the nominal spacing between the black photoconductive drum datum  56  and that color &#39;s nominal spacing as described by the following relationship: 
     
       
           Y   vfi =( Y   1f   +Y   2f ) i /2−( Y′   1v   +Y′   2v ) i   
       
     
     where “i” represents the i th  color image plane. 
     In the case of black photoconductive drum  22 , the procedure is to align the black laser beam parallel with the black photoconductive drum V-block axis, so the equation for the process direction location of the black laser beam relative to the reference black photoconductive drum axis datum  56  is: 
     
       
           Y   vfK =( Y   1f   +Y   2f ) K /2  
       
     
     The alignment fixture processing/feedback unit  42  calculates the measured drum-to-drum spacing Y Di  for the i th  color using the nominal drum-to-drum spacing Y NDi  corrected by the measured distance Y vfi  from the sensor nominal target and the laser beam off-set Y vfk  of the black photoconductive drum  22  itself: 
     
       
         
           Y 
           Di 
           =Y 
           ND 
           i +Y 
           vfi 
           +Y 
           vfK 
         
       
     
     This drum-to-drum spacing, relative to the black photoconductive drum image, is calculated for all three non-black colors. 
     The alignment fixture processing/feedback unit  42  passes these Y Di  values for each non-black color to the printer raster image processor  66 . Based upon this Y D  number, raster image processor  66  determines the appropriate number of whole PEL&#39;s (scans) between photoconductive drum image points on transfer belt  30 , and the fractional PEL needed to determine the horizontal synchronization phasing between black printhead  32  and that particular non-black color printhead. These values are then stored in NVRAM  46  in printer  12 . The printer registration for all four image planes is now complete except for the absolute top-of-page location of the entire four-color image plane. Only this remaining top-of-page synchronization relative to the paper sensor location requires a test page to be run at the final assembly test station to set the proper vertical synchronization value into NVRAM  46  in printer  12 . 
     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.