Apparatus and method for obtaining color plane alignment in a single pass color printer

A system for controlling color plane image alignment in a multi-color, single pass laser printer achieves such alignment by imprinting of alignment marks directly on a belt which carries and/or drives media sheets past plural developer modules in a process direction. A pair of sensors are positioned adjacent the belt to enable a sensing of the alignment marks. A controller causes each of a plurality of developers to print a set of alignment marks on the belt, each set including plural marks that are positioned transverse to a print process direction. The controller, in response to the sensors' detecting the printed marks on the belt, determines times at which the marks pass beneath the sensors and, from such determined times, derives variations from expected sense times of the marks of each set. Thereafter, the controller adjusts data feed from color plane sub-images to one or more laser scanners in such a manner as to reduce color plane image misalignments.

FIELD OF THE INVENTION 
This invention relates to single pass multi-color laser printers and, more 
particularly, to a method and apparatus for achieving alignment of color 
plane images in such multi-color laser printers. 
BACKGROUND OF THE INVENTION 
Difficulties in achieving precise color plane alignments have hindered 
development of multi-color laser printers which employ single pass color 
printing processes. Subimages derived from color image planes must be 
precisely positioned, relative to each other, or else substantial image 
degradation results. For example, a subimage misalignment that exceeds 
about 50 microns produces a detectable degradation in print quality. 
Alignment of subimages is difficult to achieve in single pass color 
printers because precise alignment of the multiple imaging sources is 
required. Such alignments are subject to change with temperature 
variations, consumable servicing, printer handling, etc. 
Various methods have been proposed to reduce color plane alignment errors 
in single pass color printers. U.S. Pat. No. 5,287,162 to de Jong et al. 
describes a method and apparatus for correction of color alignment errors 
in such a printer. deJong et al. print plural chevrons on an intermediate 
photoreceptor belt or on a media sheet carried by a copy sheet conveyor. 
In order to achieve correction values for color alignment errors, de Jong 
et al. employ plural sensors, one for each color chevron that is printed 
and sense the relative positions of the chevrons. To achieve proper 
alignment correction values, each detector and its control circuitry is 
required to determine a centroid of each arm of a chevron being sensed. 
U.S. Pat. No. 5,339,150 to Hubble, III et al. describes a mark detection 
circuit for a multi-color, single pass, electrophotographic printer, 
wherein alignment marks are employed to achieve color plane subimage 
alignment. In one embodiment, Hubble, III et al. use four LED print bars 
to form a composite color image on a media sheet. A photosensor is placed 
beneath each print bar and a narrow target line is formed on the belt 
surface a few scan lines before the start of an exposure frame. The center 
of the target line is detected by each sensor which produces a 
corresponding detection signal. More specifically, the system includes 
multiple sensors placed at each print bar to detect the passage of 
alignment marks produced by the first print bar. An output signal is 
generated at each of the three downstream print bars, with the signals 
being utilized to commence image exposure sequence operations in 
synchronism with the first image exposure. 
In another embodiment, Hubble, III et al enable skew alignment adjustments 
by forming marks on opposite sides of the photoreceptor, detecting the 
center of each mark and making adjustments of the position of the 
downstream print bars, based on detected time differences between opposed 
marks. 
As indicated above, both de Jong et al. and Hubble, III et al. require 
multiple sensors to enable image alignment in a multicolor printer. Such 
multiple sensors, and the control circuitry associated with each sensor, 
add to the cost of the printer. Further, both de Jong et al. and Hubble, 
III et al. apply their respective marks to either a photoreceptor that is 
used as an intermediate carrier or directly to print media, the latter 
requiring a special feed of the print media through the printer to achieve 
an image alignment action. 
It is an object of this invention to provide an improved system and method 
for subimage color plane alignment in a single pass, color printer. 
It is another object of this invention to provide an improved system for 
subimage color plane alignment in a laser printer, wherein only two 
alignment mark sensors are required. 
It is a further object of this invention to provide an improved method for 
subimage color plane alignment in a single pass laser printer, wherein 
such alignment is enabled by the printing of alignment marks directly on a 
media sheet-carrying belt, obviating the need for use of an intermediate 
transfer medium. 
SUMMARY OF THE INVENTION 
A system for controlling color plane image alignment in a multi-color, 
single pass laser printer achieves such alignment by imprinting of 
alignment marks directly on a belt which carries and/or drives media 
sheets past plural developer modules in a process direction. A pair of 
sensors are positioned adjacent the belt to enable sensing of the 
alignment marks. A controller causes each of a plurality of developers to 
print a set of alignment marks on the belt, each set including plural 
marks that are positioned transverse to a print process direction. The 
controller, in response to the sensors' detecting the printed marks on the 
belt, determines times at which the marks pass beneath the sensors and, 
from such determined times, derives variations from expected sense times 
of the marks of each set. Thereafter, the controller adjusts data feed 
from color plane sub-images to one or more laser scanners in such a manner 
as to reduce color plane image misalignments.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, print engine 10 incorporates apparatus for producing 
full color images on media sheets 12. Each media sheet 12 is selected from 
a media tray 14 by a pick roller 16 and is grabbed between a pair of 
follower rollers 18, 20 and a media transport belt 22 (which rides on 
rollers 24 and 26, respectively). Media transport belt 22 may be either a 
belt having a width of at least a media sheet or it may be plural, opposed 
narrow belts which grab opposite sides of a media sheet and propel it 
through a plurality of developer stations 28, 30, 32 and 34. It is 
necessary that media transport belt 22 include longitudinal portions which 
exhibit an insulating surface that is adapted to retain a charge state 
which will enable an attraction of toner particles from the respective 
developer stations. 
As will be hereafter understood, alignment marks are printed by each of the 
developer stations directly on media transport belt 22 and enable a 
control action (to be described below) to alter the positioning of 
subimages from respective color planes so as to assure proper color plane 
subimage alignment. 
Each of developer stations 28, 30, 32 and 34 is substantially physically 
identical, except that each contains a different color toner. For 
instance, developer station 28 includes black toner (K), developer station 
30 includes yellow toner (Y), developer station 32 includes magenta toner 
(M) and developer station 34 contains cyan toner (C). Each developer 
station further includes an organic photoconductor (OPC) that is 
positioned on an OPC roller 36. The toner supply for each developer 
station is maintained within a reservoir 38. 
OPC roller 36 is contacted by a charge roller 40 which applies the 
necessary charge state to OPC roller 36. Thereafter, a laser scanner 42 is 
controlled to scan OPC roller 36 and to impart charge states thereon in 
accordance with a particular color plane image. In the case of developer 
station 28, laser scanner 42 is controlled by data from a black color 
plane. 
As OPC roller 36 rotates the charged image, it passes by a developer roller 
44 which, in the known manner, enables toner to be taken up onto the 
surface of OPC roller 36 in accordance with the charge states resident 
thereon. Thereafter, the toned image is rotated into contact with a media 
sheet 12 which is pressed against OPC roller 36 by a transfer roller 46. 
Each of the additional developer stations operates in a substantially 
identical manner, using an associated laser scanner. 
To this point, the operation of print engine 10 is substantially consistent 
with full color prior art print engines. Difficulties arise in achieving 
(in such an engine) alignment of color plane subimages from each developer 
station. For example, the positioning of each of laser scanners 42 can 
change as a result of the handling of print engine 10, temperature 
changes, etc. Further, differences in OPC roller run-out and speed 
variations thereof can also cause color plane alignment changes. 
Accordingly, as will be described in detail below, each laser scanner 42, 
in combination with its associated developer station, causes the printing 
of a set of alignment marks directly on media transport belt 22, which 
alignment marks are sensed by an optical sensor 50 that is positioned 
downstream from the respective developer stations. Further, as transport 
belt 22 moves, the alignment marks are removed by a belt cleaner 52 to 
enable new sets of alignment marks to be imprinted thereupon on a next 
cycle. 
As will be later understood, each developer station imprints four marks on 
transport belt 22. A first pair of marks (e.g., lines) are printed so that 
they are adjacent either edge of transport belt 22 and are positioned so 
as to orient their long dimensions orthogonal to the process direction 
(i.e., direction of belt movement). A second set of marks, printed by each 
developer station, include a pair of lines that are positioned along 
opposed edges of the belt and are oriented at oblique angles to the 
process direction of transport belt 22. Accordingly, developer stations 
28, 30, 32 and 34 imprint a total of sixteen alignment marks on transport 
belt 22, which alignment marks are sensed by a pair of optical sensors 50, 
50' (see FIG. 2). Sense circuitry determines the timing between the 
sensing of the alignment marks of each pair and the sensing of a pair of 
alignment marks which are printed by one developer station and serve as 
reference marks (e.g., the marks from K developer station 28). Error 
values are derived from the mark timing measurements, which error values 
are representative of timing differences between (i) expected time 
intervals between marks and (ii) measured time intervals between marks. 
The derived error values are then used to control the rates of data feed 
that modulate the respective laser scanners so as to correct color plane 
image misalignments. Importantly, no mechanical adjustments are required 
to correct for such misalignments, only alterations in timing of data fed 
to the respective laser scanners. 
FIG. 2 illustrates a plan view of media transport belt 22 with a pair of 
media sheets 12 positioned thereon. Optical sensors 50 and 50' are 
positioned close to belt drive roller 26 and interrogate a single pixel 
strip along transport belt 22. The center lines of the respective OPC 
rollers are illustrated by the dashed lines that are transverse to 
transport belt 22. 
As indicated above, each developer station writes four alignment marks onto 
transport belt 22, two of which are orthogonal to process direction 53 and 
two of which are slanted with respect to process direction 53. The marks 
shown in FIG. 2 are representative of when only two of four developer 
stations have been passed, with the remaining developer stations yet to 
print their alignment marks on transport belt 22. 
Turning now to FIG. 3, a high level block diagram is shown of a controller 
60 which is utilized to operate print engine 10 and, further, to control 
the color subimage alignment process that comprises the invention hereof. 
Controller 60 includes a central processing unit (CPU) 62 which 
communicates via a bus system 64 with print engine 10, a random access 
memory (RAM) 66 and a read only (ROM) 68. For exemplary purposes, it will 
be assumed that certain procedures are contained within either RAM 66 or 
ROM 68. However, one skilled in the art will realize that such procedures 
are not necessarily stored as separate code segments, but may be 
integrated with other code that is operatable to control print engine 10. 
Accordingly, the specific positioning and arrangement of the code 
procedures is to be understood as exemplary only. 
RAM 66 stores an image to be printed as individual color subimages in C, M, 
Y and K color plane raster buffers 70. A buffer control procedure 72 
controls the output of data from color plane raster buffers 70 to print 
engine 10. A printer control procedure 74, in ROM 68, provides overall 
control of print engine 10 and institutes calls for the various procedures 
shown in RAM 66, as they are needed. An alignment mark procedure 76 
periodically causes the alignment marks, referenced above, to be printed 
on transfer belt 22. Alignment mark procedure 76 may be caused to operate 
between individual media sheets passing through print engine 10 or 
intermittently, as the need arises. 
An alignment mark calculation procedure 78 (in RAM 66) is invoked to 
calculate timing and timing variations of the sensed alignment marks and 
to further derive adjustment parameters that are stored in image plane 
adjustment parameters region 80 of RAM 66. Those adjustment parameters are 
utilized to control buffer control procedure 72 so that any offset, skew, 
or width variations that are sensed for an image color plane are corrected 
by alteration of image data flow from color plane raster buffers 70. 
Turning now to FIG. 4, a detailed view is shown of printed alignment marks 
100. One group of alignment marks is positioned on a side of transport 
belt 22 that is near the start of the laser scan position and another 
group of alignment marks is positioned on a side of transport belt 22 that 
is near the end of the laser scan position (only one side is shown). 
Alignment marks 100 comprises four sets of marks, each set including four 
marks. Two marks of each set are oriented parallel to the laser scan 
direction (and orthogonal to the process direction), and the other two 
marks of a set are oriented at an angle to both the laser scan direction 
and the process direction. A pair of marks 102, (that are orthogonal to 
the process direction) and a pair of slanted marks 104 comprise a set that 
are printed by each developer station on transport belt 22. 
An optical sensor 50 is mounted in a fixed position above one side of 
transport belt 22 and another optical sensor is similarly positioned over 
the other side. The positioning of optical sensors 50 and 50' is such that 
each is directly over the centerline of the respective set of printed 
alignment marks 100. Each optical sensor preferably comprises a blue light 
emitting diode, as all toner colors respond well to its wavelength. A 
photodiode (not shown) is used as the photodetector and a lens is used to 
focus the alignment mark image plane onto the photodiode as transport belt 
22 moves each alignment mark beneath an optical sensor 50, 50'. 
FIG. 5 illustrates a high level logic flow diagram that describes the 
procedure employed for deriving offset, skew and width errors for each of 
the color plane images. Initially, each developer station is caused to 
print a set of alignment marks onto transport belt 22 (step 120). 
Thereafter, as each mark passes a respective optical sensor 50, 50', the 
time of its passage is sensed (step 122). Using, for instance, the black 
marks as reference marks, any offset in the expected time of arrival of 
subsequent alignment marks to the alignment marks printed by the black 
developer station is calculated as a "timing error" for the sensed marks 
(step 124). Next, any offset, skew and/or width errors are calculated 
(step 126) based upon the timing error values calculated in step 124. 
Using the calculated error values, adjustment factors are calculated (step 
128) and are stored in image plane adjustment parameters region 80 of RAM 
66. Thereafter, (step 130) the adjustment parameters are utilized by 
buffer control procedure 72 to control data flow from the respective color 
planes to the laser scanners in such a manner as to reduce the calculated 
misalignment parameters. 
FIG. 6 shows the effect of image plane misalignments on alignment mark 
positions. The black (K) mark set is used for reference positioning. In 
the example shown in FIG. 6, the alignment marks printed by the Cyan (C) 
developer station are offset in the process direction only. The Magenta 
(M) plane alignment marks are offset in the scan direction only and the 
Yellow (Y) plane alignment marks are offset in both the process and the 
scan direction. Timing pulse waveforms 140 and 142 respectively illustrate 
outputs from optical sensor 50 (in a first case 140) when all of the 
alignment marks are perfectly positioned and (in second case (142) when 
alignment errors are present. 
The sensed pulse variations are utilized to calculate four alignment error 
values, i.e., X-position or scan direction error, Y-position or process 
direction error, image width error and image skew error. 
To calculate the Y-position error (process direction), note that cyan 
alignment marks 144 and 146 both show process direction misalignments 
(with the shaded areas being the actual sensed alignment marks and the 
outlined areas illustrating proper positioning of the marks). The 
Y-position error is calculated by subtracting the mark expected time T1C 
from the actual mark time T2C. This difference is multiplied by the speed 
of transport belt 22 to give a process direction error. Process direction 
errors for the magenta and yellow image planes are derived in a similar 
manner. Recall that alignment marks 150 and 152, printed by the K 
developer station, are utilized to determine the reference timing. 
Skew error is the error which results from a lack of parallelism between 
scan lines of one image plane relative to scan lines of the black image 
plane. To determine skew error, the process direction position error 
values from each side of media transport belt 22 are compared. The skew 
error is the process direction error from one side subtracted from the 
process direction error of the opposite side. 
X-position error is misalignment of an image plane in a direction that is 
orthogonal to the process direction. The angled alignment marks produced 
by each developer station are utilized to determine the X-position error. 
In FIG. 6, magenta marks 154 and 156 are shown with X-position errors 
only. It can be seen that angled alignment mark 156 shows an X-position 
error while alignment mark 154 does not. Accordingly, the timing 
difference is derived from the sensing of angled alignment marks 156 which 
enables a timing difference T2M-T1M to be sensed. This difference varies 
with process position errors, however, the process position error is 
already known from the process position error calculations and can be 
subtracted out, leaving the X-position error only. Accordingly, the 
X-position error is expressed: (T2M-T1M) (s/k)-Y error, where: s is the 
media transport belt speed and k is a constant, dependent upon the angle 
of angled alignment marks 156. If the angled alignment marks are 
positioned at 45.degree. to the process direction, the constant is equal 
to one, otherwise, the constant is equal to the tangent of the mark angle. 
Width variations from one image plane to the next are determined from 
differences in X-position error determined from a timing signal derived 
from alignment marks on one side of transport belt 22, as compared with 
the timing signals derived from angled alignment marks on the other side 
of transport belt 22. The difference in width errors from one side to the 
opposite side is the width error. 
Corrections are made to each colored image plane based on the detected 
errors to insure that the remaining image planes align to the black image 
plane. Corrections are made for all four of the errors described above in 
the following manner: 
X-Position Error: Laser scanners require a start-of-scan optical detector 
to indicate the beginning of each scan line. The starting point for each 
image plane is determined by a fixed number of clock cycles after the scan 
detect signal has been received. The X-position error is corrected by 
incrementing or decrementing this constant by the number of clock cycles 
that occur between scan detect and image start. The formula for the change 
required for this constant is: Cycles=Fclock*Xerror/Scan Velocity, where 
Fclock is the clock frequency and scan velocity is the velocity of the 
scan beam. 
Y-Position Error: Laser printers determine the top of each page from a 
fixed number of scan cycles after a start- of-page signal has been 
detected. This value is different for each scanner in a single pass 
printer based on the timing between each color developer station. 
Y-position error correction adjusts this start position based on the 
measured error. The correction to the number of scan cycles delay is equal 
to: Y error * scan resolution. For example, if Y error=0.015 inch and the 
scan resolution is 1200 scan lines per inch, then the correction is 1200 * 
0.015=18 lines. 
Width Error: Width error is corrected by changing the spacing between dots 
in the scan line. This can be accomplished by varying the frequency of the 
data clock or preferably by inserting or subtracting spaces at fixed 
increments. The capability exists in laser printers for subpixel 
modulation. A pixel is divided into subpixels to allow dot shifting, gray 
scaling, curve smoothing, etc. Typically, a pixel is divided into 64 
subpixels. To compensate for width error, a subpixel can be added or 
subtracted at calculated intervals to correct for the error. Changing a 
pixel by such a small amount is not perceivable in the image, but corrects 
for the error. 
For example, if the width between sensors is 8.0 inches, then at 1200 dots 
per inch, 1200.times.8 or 9,600 dots exist between the sensors. The total 
number of subpixels is 9,600 * 64 or 614,400. Each subpixel is about 13 
microinches wide. Correction for width error needs to occur at a subpixel 
increment determined by the width between sensors, divided by the width 
error. If the width error is determined to be 0.010 inch, then the 
correction increment is 8.0/0.010=800. A subpixel is then added every 800 
subpixel to correct for the width error. 
Skew Error: Skew error correction requires a buffering of a predetermined 
number of rows of raster pixel data and retrieving the data by jumping 
from row to row at increments based on the measured skew. For example, if 
the printer is designed such that the maximum skew error that can occur is 
0.020 inches, at 1200 scan lines per inch resolution, 0.020 * 1200=24 
lines of data need to be buffered. The number of jump points is determined 
by the skew error divided by the row spacing. For example, if the skew 
error is measured to be 0.010 inch and the row spacing 1/1200 inch, then 
the number of jump points required is 0.010 * 1200=12. Raster pixel data 
is then pulled from row buffers by jumping to a new row buffer at width 
increments determined by total width/number of jump points or 8/12=0.67 
inch for this example, with 8.0 inches being the width. Several algorithms 
for jumping from row to row in the buffered data can be devised by those 
skilled in the art, by varying how the data is either written into the 
buffers or pulled from the buffers or a combination thereof. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications can be devised by 
those skilled in the art without departing from the invention. 
Accordingly, the present invention is intended to embrace all such 
alternatives, modifications and variances which fall within the scope of 
the appended claims.