Method For Analyzing Image Data Corresponding To A Test Pattern

Method For Analyzing Image Data Corresponding To A Test Pattern
US 20140009527A1
(19) United States
(12) Patent Application Publication (10) Pub. No.: US 2014/0009527 A1
Mongeon et al.
(54)
(43) Pub. Date:
METHOD FOR ANALYZING IMAGE DATA
CORRESPONDING TO A TEST PATTERN
EFFECTIVE EoR EINE REGISTRATION OF
INKJET PRINTHEADS IN AN INKJET
Jan. 9, 2014
Publication Classi?cation
(51)
PRINTER
(52)
Int- Cl
B411 29/393
U-S- Cl
(2006-01)
CPC ................................... .. B41J29/393 (2013.01)
Xerox
_
Corporation, Norwalks
(72)
(57)
Inventors: Michael C. Mongeon, WalWorth, NY
..........................................................
ABSTRACT
..
A method anal
(US); Howard A. Mizes, Pittsford, NY
.
(Us)
.
da
. . yZeS Image
f
.
d
ta 0 .a test pattern pnm? on an
Image recelvmg member by a printer. The method Includes
identlfymg a process direction pos1t1on for each roW of dashes
in a test pattern printed on an image receiving member, iden
(73) Assigneej Xerox Corporation, Norwalk, CT (Us)
tifying a center of each dash in a cross-process direction,
identifying an inkjet ejector that formed each dash in the roW
(21) Appl' NO‘: 14/021’408
position for each printhead, a cross-process displacement for
of dashes. These data are used to identify a process direction
each column of printheads, and a stitch displacement in the
(22) Filed:
Sep. 9, 2013
cross-process direction between neighboring printheads in a
print bar unit that print a same color of ink. An actuator can be
.
.
operated With reference to the identi?ed process direction
Related U's' Apphcatlon Data
(62)
positions, cross-process displacements, and the identi?ed
Division of application No. 12/754,735, ?led on Apr.
stitch displacements to move at least some of the printheads in
6, 2010.
the printer.
IDENTIFY SCANLINES
THAT INTERSECT
TEST PATTERN DASHES
I
CALCULATE DASH
PROFILE
I
CALCULATE DASH
CENTERS
I
CORRECT DASH
INDEXING FOR
MISSING DASHES
I
CORRECT FOR
CROSS-PROCESS
IMAGE MEMBER MOTION
I
CALCULATE RELATIVE
PRINTHEAD POSITION
IN PROCESS DIRECTION
I
CALCULATE SERIES
DISPLACEMENT
I
CALCULATE STITCH
DISPLACEMENT
I30
Patent Application Publication
Jan. 9, 2014 Sheet 1 of8
IDENTIFY SCANLINES
THAT INTERSECT
TEST PATTERN DASHES
I
CALCULATE DASH
PROFILE
I
CALCULATE DASH
CENTERS
I
CORRECT DASH
INDEXING FOR
MISSING DASHES
I
CORRECT FOR
CROSS-PROCESS
IMAGE MEMBER MOTION
I
CALCULATE RELATIVE
PRINTHEAD POSITION
IN PROCESS DIRECTION
I
CALCULATE SERIES
DISPLACEMENT
I
CALCULATE STITCH
DISPLACEMENT
FIG. I
US 2014/0009527 A1
Patent Application Publication
AMPLITUDE
Jan. 9, 2014 Sheet 3 0f 8
US 2014/0009527 A1
‘T
40
50
60
70
80
SCANLINE NUMBER
90
100
FIG. 3
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SCANLINE COLUMN
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Patent Application Publication
Jan. 9, 2014 Sheet 4 0f 8
US 2014/0009527 A1
200 \
APPLY LOW PASS
FILTER T0 DASH
204
/
PROFILE
I
208
IDENTIFY LOCAL
_/
MINIMUMS
I
212
FIT LOCAL MINIMUM
WITH NEIGHBORING
/
IMAGE DATA
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IDENTIFY (ROSSPROCESS
216
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POSITION
FIG. 5
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HHHIIBEBE“IIBBHHHIIBBHHMIIBB
HEEEIIHEEEHIIBEHEHIIHEBHHIIEE
FIG. 6
Patent Application Publication
Jan. 9, 2014 Sheet 5 0f 8
IDENTIFY ALL DASHES
GENERATED BY A
PRINTHEAD
US 2014/0009527 A1
/
A
I
608
OBTAIN DASH
CENTER PROFILES
—/
IN PROCESS DIRECTION
I
612
AVERAGE THE EDGE
POSITIONS FOR DASHES
/
IN THE SAME ROW
I
614
IDENTIFY PRINTHEAD
POSITION IN THE
—/
PROCESS DIRECTION
CONTINUE IMAGE
ANALYSIS
FIG. 7
_/
Patent Application Publication
Jan. 9, 2014 Sheet 6 0f 8
PCORSITEOSN
DASH INDEX
FIG. 8
904
912
90a
7
US 2014/0009527 A1
Patent Application Publication
Jan. 9, 2014 Sheet 7 0f 8
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Jan. 9, 2014
US 2014/0009527 A1
METHOD FOR ANALYZING IMAGE DATA
CORRESPONDING TO A TEST PATTERN
EFFECTIVE FOR FINE REGISTRATION OF
INKJET PRINTHEADS IN AN INKJET
PRINTER
TECHNICAL FIELD
[0001]
This application claims priority to and is a divisional
application of co-pending US. patent application Ser. No.
12/754,735, Which is entitled “Test Pattern Effective For Fine
Registration Of Inkjet Printheads And Method Of Analysis
Of Image Data Corresponding To The Test Pattern In An
Inkjet Printer” and Was ?led on Apr. 6, 2010.
TECHNICAL FIELD
[0002]
This disclosure relates generally to identi?cation of
printhead orientation in an inkj et printer having one or more
printheads, and, more particularly, to analysis of image data
to identify the printhead orientation.
BACKGROUND
[0005] In order for the printed images to correspond closely
to the image data, both in terms of ?delity to the image objects
and the colors represented by the image data, the printheads
must be registered With reference to the imaging surface and
With the other printheads in the printer. Registration of print
heads is a process in Which the printheads are operated to eject
ink in a knoWn pattern and then the printed image of the
ejected ink is analyZed to determine the orientation of the
printhead With reference to the imaging surface and With
reference to the other printheads in the printer. Operating the
printheads in a printer to eject ink in correspondence With
image data presumes that the printheads are level With a Width
across the image receiving member and that all of the inkjet
ejectors in the printhead are operational. The presumptions
regarding the orientations of the printheads, hoWever, cannot
be assumed, but must be veri?ed. Additionally, if the condi
tions for proper operation of the printheads cannot be veri?ed,
the analysis of the printed image should generate data that can
be used either to adjust the printheads so they better conform
to the presumed conditions for printing or to compensate for
the deviations of the printheads from the presumed condi
tions.
[0006] Analysis of printed images is performed With refer
[0003] Ink jet printers have printheads that operate a plu
rality of inkjets that eject liquid ink onto an image receiving
member. The ink may be stored in reservoirs located Within
cartridges installed in the printer. Such ink may be aqueous
ink or an ink emulsion. Other inkjet printers receive ink in a
solid form and then melt the solid ink to generate liquid ink
for ejection onto the imaging member. In these solid ink
printers, the solid ink may be in the form of pellets, ink sticks,
granules or other shapes. The solid ink pellets or ink sticks are
typically placed in an ink loader and delivered through a feed
ence to tWo directions. “Process direction” refers to the direc
tion in Which the image receiving member is moving as the
imaging surface passes the printhead to receive the ejected
ink and “cross-process direction” refers to the direction
across the Width of the image receiving member. In order to
analyze a printed image, a test pattern needs to be generated
so determinations can be made as to Whether the inkj ets
operated to eject ink did, in fact, eject ink and Whether the
chute or channel to a melting device that melts the ink. The
melted ink is then collected in a reservoir and supplied to one
or more printheads through a conduit or the like. In other
ejected ink landed Where the ink Would have landed if the
printhead Was oriented correctly With reference to the image
receiving member and the other printheads in the printer. In
some printing systems, an image of a printed image is gener
ated by printing the printed image onto media or by transfer
inkjet printers, ink may be supplied in a gel form. The gel is
ring the printed image onto media, ejecting the media from
also heated to a predetermined temperature to alter the vis
cosity of the ink so the ink is suitable for ejection by a
the system, and then scanning the image With a ?atbed scan
ner or other knoWn o?Iine imaging device. This method of
printhead.
[0004] A typical inkjet printer uses one or more printheads.
Each printhead typically contains an array of individual
generating a picture of the printed image suffers from the
inability to analysis the printed image in situ and from the
inaccuracies imposed by the external scanner. In some print
noZZles for ejecting drops of ink across an open gap to an
ers, a scanner is integrated into the printer and positioned at a
image receiving member to form an image. The image receiv
location in the printer that enables an image of an ink image
to be generated While the image is on media Within the printer
or While the ink image is on the rotating image member. These
ing member may be a continuous Web of recording media, a
series of media sheets, or the image receiving member may be
a rotating surface, such as a print drum or endless belt. Images
printed on a rotating surface are later transferred to recording
media by mechanical force in a trans?x nip formed by the
rotating surface and a trans?x roller. In an inkjet printhead,
individual piezoelectric, thermal, or acoustic actuators gen
erate mechanical forces that expel ink through an ori?ce from
an ink ?lled conduit in response to an electrical voltage signal,
sometimes called a ?ring signal. The magnitude, or voltage
level, of the signals affects the amount of ink ejected in each
drop. The ?ring signal is generated by a printhead controller
integrated scanners typically include one or more illumina
tion sources and a plurality of optical detectors that receive
radiation from the illumination source that has been re?ected
from the image receiving surface. The radiation from the
illumination source is usually visible light, but the radiation
may be at or beyond either end of the visible light spectrum.
If light is re?ected by a White surface, the re?ected light has
the same spectrum as the illuminating light. In some systems,
ink on the imaging surface may absorb a portion of the inci
dent light, Which causes the re?ected light to have a different
in accordance With image data. An inkjet printer forms a
spectrum. In addition, some inks may emit radiation in a
printed image in accordance With the image data by printing
different Wavelength than the illuminating radiation, such as
a pattern of individual ink drops at particular locations on the
When an ink ?uoresces in response to a stimulating radiation.
Each optical sensor generates an electrical signal that corre
image receiving member. The locations Where the ink drops
landed are sometimes called “ink drop locations,” “ink drop
positions,” or “pixels.” Thus, a printing operation can be
vieWed as the placement of ink drops on an image receiving
member in accordance With image data.
sponds to the intensity of the re?ected light received by the
detector. The electrical signals from the optical detectors may
be converted to digital signals by analog/digital converters
and provided as digital image data to an image processor.
Jan. 9, 2014
US 2014/0009527 A1
[0007] The environment in Which the image data are gen
erated is not pristine. Several sources of noise exist in this
scenario and should be addressed in the registration process.
For one, alignment of the printheads can deviate from an
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
The foregoing aspects and other features of a printer
that generates a test pattern that better identi?es printhead
types of imaging surfaces are used or When printheads are
orientations and characteristics and that analyZes the image
data corresponding to the generated test pattern are explained
in the folloWing description, taken in connection With the
replaced. Additionally, not all inkj ets in a printhead remain
accompanying draWings.
operational Without maintenance. Thus, a need exists to con
tinue to register the heads before maintenance can recover the
positions of markings in test pattern.
expected position signi?cantly, especially When different
missing jets. Also, some inkj ets are intermittent, meaning the
inkjet may ?re sometimes and not at others. Inkj ets also may
not eject ink perpendicularly With respect to the face of the
printhead. These off-angle ink drops land at locations other
than Were they are expected to land. Some printheads are
oriented at an angle With respect to the Width of the image
receiving member. This angle is sometimes knoWn as print
head roll in the art. The image receiving member also con
tributes noise. Speci?cally, structure in the image receiving
surface and/or colored contaminants in the image receiving
surface may be confused ink drops in the image data and
[0011]
FIG. 1 is a ?oW diagram of a method for identifying
[0012] FIG. 2 is a sample test pattern suitable for use With
the methods of FIG. 1.
[0013] FIG. 3 is an illustration of an amplitude response
signal for an optical detector imaging a dash in the test pattern
of FIG. 1.
[0014] FIG. 4 is an illustration of a portion of a dash pro?le
for a group of optical detectors imaging the test pattern of
FIG. 1.
[0015] FIG. 5 is a ?oW diagram of a method for locating the
cross-process position of a dash in a test pattern roW.
[0016]
FIG. 6 is a portion of a sample test pattern having a
lightly colored inks and Weakly performing inkjets provide
cross-process offset betWeen roWs of the test pattern.
ink drops that contrast less starkly With the image receiving
[0017] FIG. 7 is a ?oW diagram of a method for locating the
relative position of a printhead in the process direction.
[0018] FIG. 8 illustrates a method of computing a stitch
member than darkly colored inks or ink drops formed With an
appropriate ink drop mass. Thus, improvements in printed
images and the analysis of the image data corresponding to
the printer images are useful for identifying printhead orien
tation deviations and printhead characteristics that affect the
ejection of ink from a printhead. Moreover, image data analy
sis that enables correction of printhead issues or compensa
tion for printhead issues is bene?cial.
SUMMARY
[0008] A method analyZes image data corresponding to a
test pattern generated on an image receiving member by a
printer to identify positions for and registration betWeen
printheads in the printer. The method includes identifying a
process directionposition for each roW of dashes in a plurality
of roWs of dashes in image data of a test pattern printed on an
image receiving member, the test pattern being formed by
each printhead in a printer forming at least one dash in each
roW of dashes in the plurality of roWs of dashes, identifying a
center of each dash in a cross-process direction, identifying
an inkjet ejector that formed each dash in the roW of dashes,
identifying a process direction position for each printhead in
the printer, identifying a cross-process displacement for each
column of printheads, identifying a stitch displacement in the
cross-process direction betWeen neighboring printheads in a
print bar unit that print a same color of ink, and operating an
actuator to move at least some of the printheads in the printer
With reference to the identi?ed process direction positions,
cross-process displacements, and the identi?ed stitch dis
placements.
[0009]
To produce the test pattern that enables the printhead
positions to be identi?ed, the printheads of a printer are oper
displacement betWeen tWo printheads across a stitch inter
face.
[0019] FIG. 9 illustrates an alternative method of comput
ing a stitch displacement betWeen tWo printheads across a
stitch interface.
[0020] FIG. 10 is a schematic vieW of a prior art inkjet
imaging system that ejects ink onto a continuous Web of
media as the media moves past the printheads in the system.
[0021] FIG. 11 is a schematic vieW of a prior art printhead
con?guration.
DETAILED DESCRIPTION
[0022] A process 105 for analyZing image data of a test
pattern is depicted in FIG. 1. Process 105 employs a sensor to
analyZe image data obtained from the surface of an image
receiving member in a print system. This analysis enables the
positions of the dashes to be determined more accurately and
the positional information for the dashes may be used to
determine the position and orientation of the printheads more
accurately. The image data corresponding to a test pattern
printed on an image receiving member may be generated by
an optical sensor. The optical sensor may include an array of
optical detectors mounted to a bar or other longitudinal struc
ture that extends across the Width of an imaging area on the
image receiving member. In one embodiment in Which the
imaging area is approximately tWenty inches Wide in the
cross process direction and the printheads print at a resolution
of 600 dpi in the cross process direction, over 12,000 optical
detectors are arrayed in a single roW along the bar to generate
a single scanline across the imaging member. The optical
detectors are con?gured in association in one or more light
ated in accordance With a method for printing a test pattern.
sources that direct light toWards the surface of the image
The method includes operating at least one inkjet ejector in
each printhead in a plurality of printheads to eject at least one
head has been operated to eject ink to form at least one dash
receiving member. The optical detectors receive the light
generated by the light sources after the light is re?ected from
the image receiving member. The magnitude of the electrical
signal generated by an optical detector in response to light
being re?ected by the bare surface of the image receiving
member is higher than the magnitude of a signal generated in
in a roW of dashes in the test pattern.
response to light re?ected from a drop of ink on the image
dash in a roW of dashes of a test pattern on an image receiving
member, and continuing to operate the inkjet ej ectors in the
plurality of printheads until each inkjet ejector in each print
Jan. 9, 2014
US 2014/0009527 A1
receiving member. This difference in the magnitude of the
generated signal may be used to identify the positions of ink
tion. The distance imaged by an optical detector is dependent
upon the speed of the image member moving past the detector
drops on an image receiving member, such as a paper sheet,
and the line rate of the optical detector. A single roW of optical
media Web, or print drum. The reader should note, hoWever,
that lighter colored inks, such as yelloW, cause optical detec
tors to generate loWer contrast signals With reference to
uncovered portions of the image receiving member than the
contrast signals produced by darker colored inks, such as
black, With reference to uncovered portions of the image
receiving member. Thus, the contrast signal differences may
be used to differentiate betWeen dashes of different colors.
The magnitudes of the electrical signals generated by the
optical detectors may be converted to digital values by an
appropriate analog/digital converter. These digital values are
denoted as image data in this document and these data are
analyZed to identify positional information about the dashes
on the image receiving member as described beloW.
[0023] The ability to differentiate dashes of different ink
colors is subj ect to the phenomenon of missing or Weak inkjet
ejectors. Weak inkj et ejectors are ej ectors that do not respond
to a ?ring signal by ejecting an amount of ink that corresponds
to the amplitude or frequency of the ?ring signal delivered to
the inkjet ejector. A Weak inkjet ejector, instead, delivers a
lesser amount of ink. Consequently, the lesser amount of ink
ejected by a Weak jet covers less of the image receiving
member so the contrast of the signal generated by the optical
detector With reference to an uncovered portion of the image
receiving member is loWer. Therefore, ink drops in a dash
ejected by a Weak inkjet ejector may result in an electrical
signal having a magnitude that is different than that expected.
Missing inkjet ejectors are inkjet ej ectors that eject little or no
ink in response to the delivery of a ?ring signal. A process for
identifying the inkjet ejectors that fail to eject ink drops for
the test pattern is discussed in more detail beloW.
[0024] An example test pattern suitable for use With an
image analyZing process, such as process 105, is depicted in
FIG. 2. Test pattern 300 includes a plurality of dashes, Where
each dash is formed from ink ejected from a single inkjet
ejector in a printhead. The dashes 302 are formed in the print
process direction 332, With multiple roWs of dashes disposed
along the cross-process axis 336. Test pattern 300 is con?g
ured for use With a printer using cyan, magenta, yelloW, and
black (CMYK) coloring stations. Test pattern 300 is further
con?gured for use With ink coloring stations con?gured for
interlaced printing using tWo printhead arrays for each of the
CMYK colors. Dashes of the same color, one from each of the
detectors extending across the Width of the imaging area on
the image receiving member is called a scanline in this docu
ment. The dashes are generated With a length that is greater
than a single scanline in the process direction so the dash
image can be resolved in the image processing. Thus, mul
tiple scanlines are required to image the entire length of the
dashes in the process direction.
[0026]
RoWs in test pattern 300 may be grouped according
to the ladder formation used to space dashes 302, as seen by
groups 324A-324D. Each roW in one of groups 324A-324D is
offset by one inkjet ejector in the cross-process axis 336 from
the preceding roW. Each group has seven roWs, alloWing each
inkjet ejector in a seven inkjet ejector series to form one dash.
The number of groups is determined by the number of unique
colors the printing system generates, With test pattern 300
shoWing an example for a CMYK printing system providing
four groups, 324A, 324B, 324C, and 324D. The four groups
324A-324D alloW each inkjet ejector in the printheads for
every color (CMYK) to print a dash in test pattern 300. Thus,
line 340 that is parallel to process direction 332 may be
aligned to pass through the center of a dash of each color in the
same cross-process position. Line 340 passes through the
center of black dash 344A, and passes by the edge of black
dash 344B. In relative terms, black dash 344A is formed by an
inkjet ejector in ?rst black printhead at the ?rst position of a
group of seven consecutive inkjet ejectors in the ?rst print
head. Dash 344B corresponds to the seventh and ?nal inkjet
ejector of a previous group from the second black printhead,
Where the second black printhead is offset in the cross-pro
cess axis 336 by one-half the Width that separates ej ectors in
each printhead. This offset alloWs the tWo black printheads to
interlace dashes for full coverage of all locations under the
printheads in the print Zone.
[0027] Line 340 passes through yelloW dashes 344C and
344D, magenta dashes 344E and 344F, and cyan dashes 344G
and 344H in a similar manner to black dashes 344A and
344B. When aligned in the cross process direction, drops of
various colored inks may be placed in the same location for
color printing that produces secondary colors by mixing inks
from the CMYK colors. Additionally, the interlaced arrange
ment of printheads enables side-by-side printing of ink drops
to produce colors that extend the color gamut and hues avail
able With the printer. The test pattern 300 of FIG. 2 may be
aligned printheads in each coloring station, are spaced adja
repeated along the cross-process axis to include some or all of
cent to one another in each roW of test pattern 300, as seen
the inkjet ej ectors from each printhead in a printZone used to
form images on an image receiving member passing through
With cyan dashes 304, magenta dashes 308, yelloW dashes
312, and black dashes 316. In FIG. 2, the dashes in each roW
of test pattern 300 are arranged in a ladder including seven (7)
inkjet ejectors, such that one inkjet ejector in the inkjet print
head forrns a dash, and the next dash in the roW comes from an
inkjet ejector that is offset by six (6) positions in the cross
process axis 336. The space 320 betWeen consecutive dashes
in a roW of test pattern 300 is the Width of the six non-printing
the printZone.
[0028] The process of 105 of FIG. 1 begins by identifying
scanlines that intersect dashes in the test pattern (block 110).
One Way to extract the signal corresponding to the positions
of the dashes is to convolve the signal pro?le for an optical
detector in the cross-process direction With a cosine and a sine
inkjet ejectors. Alternative test patterns could employ ladders
function having a periodicity at the expected periodicity of
the dash pro?le. The squares of the individual convolutions
With a larger or smaller number of inkj et ej ectors in each
group producing a similar test pattern having multiple roWs of
dashes.
are then summed and compared to a predetermined threshold
to detect the presence of a dash. As used in this document,
“convolution” refers to the summation of the product of tWo
[0025]
functions. Thus, the summation of the product of the pro?le
function and sine function is computed and the summation of
the product of the pro?le function and cosine function is
The length of the dashes 302 corresponds to the
number of drops used to form a dash. The number of drops is
chosen to produce a dash that is suf?ciently greater in length
than the resolution of an optical detector in the process direc
computed. The squares of the magnitudes of these tWo con
Jan. 9, 2014
US 2014/0009527 A1
volutions are then added to produce a sum that is compared to
the predetermined threshold. As shoWn in FIG. 3, the
response of an optical detector for the scanlines prior to
scanline 67 has a relatively loW amplitude. For scanlines 67 to
about scanline 81, the amplitude indicates the presence of a
roW of dashes before returning to the loW amplitude value. A
sine and cosine function having a period corresponding to a
spacing betWeen dashes in a roW is selected for the convolu
tion operations. In one embodiment, the convolution opera
tion gives a maximum response When a period of 7 pixels is
chosen in the cross process direction. The summation of the
squares of the convolutions and the comparison to a threshold
help ensure the amplitude of the detector pro?le is su?icient
to indicate a dash line and not noise in the image data. The
operations described on the detector pro?le are equivalent to
a Fourier transform of the pro?le and detection of a peak at the
period of the ladder chart. If the pro?le data shoW a frequency
Within a prede?ned range of the expected frequency, then the
image data corresponds to dashes in the test pattern and the
top and the bottom of each dash can be determined With
reference to a scanline.
[0029]
A dash pro?le is then identi?ed With reference to the
optical detector responses (block 114). The gray level
responses of the optical detector betWeen the top and the
bottom of each detected dash are averaged and these averages
are mapped across the optical detector array. An example of
this mapping is shoWn in FIG. 4. In the portion shoWn in FIG.
4, the optical detectors corresponding to a local minimum in
[0031] The process 105 of FIG. 1 continues by correcting
the detected dash indices for missing dashes (block 122). A
dash may be missing from the image data for a variety of
reasons, but frequently a dash is absent because the inkjet
ejector intended to print a dash fails to eject ink in response to
a ?ring signal. The absence and identi?cation of missing
dashes may be obtained using several knoWn properties of the
test pattern. For one, a larger than expected distance separates
the centers of detected dashes in the neighborhood of a miss
ing dash or dashes. If the inter-dash distance exceeds the
expected distance by a Wide enough margin, then one or more
ejectors are deemed to be missing from the test pattern.
Another property that may be used is the contrast demon
strated by a dash pro?le. As noted above, the dash centers
correspond to different local minimum values by ink color.
Thus, the process is able to use these differing contrast values
to identify the color of a missing dash. Accordingly, the
number of dashes in an area, the distance betWeen dashes in
the area, and the contrast values for the dashes in the area may
be used to identify missing dashes and the inkj et ejectors that
should have printed the missing dash or dashes. The indices of
the identi?ed inkj et ej ectors are adjusted to take into account
the missing dashes. For example, in an array of seven
expected dashes Where dashes expected at indices 4 and 5 are
missing, the centers of dashes 3 and 6 are separated by a
distance of approximately three times the normally expected
distance. Instead of incorrectly identifying ejector 6 as ejector
4, the process 105 detects the missing dashes and assigns the
correct index to ejector 6. Inkjet ejectors that do not generate
the gray level function are identi?ed as corresponding to the
detected dashes may be indexed separately in order to com
dash positions in the cross-process direction. That is, the gray
level is higher at detectors sensing a portion of the image
pensate for inoperable inkjet ejectors or to signal that a print
head is faulty.
receiving member that has little or no ink on it and the loWer
[0032]
values occur Where ink drops are present. Thus, the yelloW
dashes Y1 and Y2 present local minima that have an average
gray level that is higher than the average gray level for other
including all inkjet ejectors in every printhead has a plurality
inks C1, C2, M1, M2, B1, and B2 that provide more contrast.
tern moves in the process direction 332 under the ink stations
The mapping shoWn in FIG. 4 depicts a pro?le through the
in the print Zone. HoWever, the image receiving member may
dashes and may be called a dash pro?le.
also drift along the cross-process axis 336 as the dashes for
the test pattern are formed. Cross-process drift errors may
accumulate betWeen roWs in the test pattern, resulting in
[0030]
The generated dash pro?le is further analyZed to
determine the cross-process locations corresponding to the
centers of each dash in the dash pro?le (block 118).A ?ltering
As seen in FIG. 2, a full test pattern arrangement
of roWs, such as the tWenty-eight roWs depicted in test pattern
300. The image receiving member that receives the test pat
inaccurate measurements of the cross-process positions for
and interpolation process, such as the one shoWn in FIG. 5,
may be used to locate the center of each dash. In FIG. 5,
dashes in different roWs.
process 200 begins by convolving the dash pro?le data With a
displacement caused by drift in the image receiving member
loW-pass ?lter kernel function (block 204). The loW-pass
(block 126). To measure the magnitude and direction of
media drift, the average detected cross-process positions of
?ltering convolution serves to smooth the scanline data fur
ther, eliminating sudden spikes in image data values that are
caused by noise instead of by dashes in the image data. A
series of local minima are located in the ?ltered image data
(block 208). Each local minimum, identi?ed by the dots in
[0033]
Process 105 measures and corrects for cross-process
every dash in a roW of test dashes are compared to the
expected average positions for the dashes With reference to
the ?rst roW of dashes. Cross-process displacement is the
difference betWeen the measured average position and the
FIG. 3, corresponds to a center of a dash in the ?ltered image
expected average position. Averaging the positions of the
data at the resolution of the optical detectors. To identify the
entire roW of dashes distinguishes errors in imaging the test
pattern that occur due to media drift from errors that may
occur With misalignment in a smaller group of ejectors or a
center of a dash more speci?cally, the local minimum is
interpolated With reference to the gray level values from the
neighboring pixels on each side of the identi?ed local minima
single printhead.
(block 212). This interpolation may be performed by ?tting
[0034]
these three data values to a curve to identify the local mini
mum more precisely. In one interpolation scheme, a quadratic
displaced due to cross-process media drift is depicted in FIG.
curve is used for the interpolation. The cross-process position
member, and subsequent cross-process direction drift causes
of the minimum value of the ?tted curve is calculated and
stored as the center of a dash in the test pattern (block 216).
The processing of blocks 208-216 are carried out for each
an offset for all subsequent roWs including roWs 408 and 412.
RoW 408 is offset as indicated by arroW 416. The cross
local minimum identi?ed in the ?ltered image data.
An example of a portion of a test pattern With a roW
6. Test pattern roW 404 is formed on an image receiving
process offset calculations determine that the average posi
tion of dashes in roW 408 is offset from the expected average
Jan. 9, 2014
US 2014/0009527 A1
position, even though the dashes in roW 408 are in the correct
positions relative to each other. Subsequent roWs such are roW
412 are then in a relative position that aligns With roW 408.
mum at the start of a dash in a column in the process direction.
[0035]
The process 105 cancels out the effects of media
portion of the image receiving member underlying the optical
drift by adjusting the detected cross-process positions of
dashes in the opposite direction and magnitude of the
detector. Similarly, the end position of each dash may be
detected offset. From the example of FIG. 6, if roW 408 has a
cross-process offset of 30 um in the direction of arroW 416,
then the center positions of each dash in the roW 408 are
adjusted by 30 pm in the opposite direction of arroW 416. The
same correction may be applied to subsequent roWs such as
roW 412 to remove errors introduced from cross-process drift
The convolution With the edge detection kernel identi?es a
local minimum Where the start position of dash occurs on the
identi?ed by a convolution With an end edge detection kernel.
The end edge kernel is the inverse of the start edge kernel. For
the dashes generated by the inkj et noZZles in the same roW of
the printhead being evaluated, the detected edge positions of
the dashes are averaged to reduce the impact of alignment
variances in individual ej ectors (block 612). From these roW
positions, the center of the printhead in the process direction
is calculated (block 614). If the process direction position of
additional printheads needs to be computed for other print
for the remaining portion of the test pattern.
[0036] The determination of cross-process positions for
each ejector in a printing system detailed in blocks 114-126
alloWs for adjustment of the locations of each droplet crossing
an imaging receiving member moving in the process direc
heads (block 618), the process continues (block 604). Other
Wise, the image analysis process of FIG. 1 continues (block
tion. Each dash in a test pattern also occupies a position in the
process direction. Unlike the cross-process direction Where
heads are determined, the analysis process 105 identi?es the
absolute positions for each ejector are determined, the deter
mination of printhead positions in the process direction is
based on the relative positions of the respective printheads.
Relative positions are determined because an image receiving
member moves past the printheads in a print Zone in the
process direction, alloWing a printhead to eject ink onto any
position along the process direction by timing When each ink
droplet is ejected. Proper timing alloWs droplets from mul
tiple printheads to be aligned in even roWs, preventing unin
tended over-prints or uneven roWs Where different printheads
?re either too early or too late to form a uniform roW. Print
heads that are aligned in the process direction also alloW for
intentional overprinting, or drop-on-drop printing, Where a
drop from one printhead mixes With a drop from a different
printhead to produce a neW color. For example, a drop from a
cyan printhead may be ejected ?rst, With a later drop from a
corresponding yelloW printhead depositing on the cyan drop
to form an ink mass that appears to be green. If the relative
positions of the printheads are knoWn, the printing system
may adjust the operations of the cyan and yelloW ejectors to
produce the drop-on-drop result.
[0037] The registration process 105 determines the relative
position of each of the printheads in the process direction
622).
[0038]
Once the process direction positions of the print
series alignment of different printheads in the print Zone
(block 134). Series alignment is de?ned as the cross-process
alignment of corresponding ejectors used in corresponding
printheads in the print Zone. In the test pattern shoWn in FIG.
2, line 340 passes through a single print column including the
center of black dash 344A, yelloW dash 344C, magenta dash
344E, and cyan dash 344G. Each of these dashes is generated
by an inkj et ejector having the same target position in a
printhead of each of the CMYK colors. The dashes in a print
column are in series alignment because they each have the
same cross-process positions, alloWing line 340 to pass
through the center of each dash.
[0039] While test pattern 300 shoWs dashes aligned along
cross-process axis 336, dashes belonging to corresponding
inkjet ejectors in a print column may be misaligned due to
variances in the cross-process positions of different print
heads. Using the detected cross-process pro?les of test pat
tern dashes, process 105 compares the cross-process posi
tions from a reference printhead to the cross-process pro?les
of a second printhead in a print column. A print column
corresponds to the printheads arranged in the process direc
tion that are opposite roughly the same portion of the image
receiving member. If there is a misalignment betWeen the tWo
(block 130). A test pattern such as test pattern 300 from FIG.
2 may be used to detect the offset of each printhead relative to
printheads, then a portion of the printhead inkjet ejectors
other printheads in the process direction. An example process
600 for determining the relative position of each printhead in
the process direction is shoWn in FIG. 7. Process 600 begins
by identifying all dashes belonging to a single printhead in a
test pattern, such as test pattern 300 from FIG. 2 (block 604).
printhead is selected as a reference printhead and a common
As an example, tWo cyan dashes 304 shoWn as a pair come
each printhead are in the overlap region. Next, the difference
betWeen the measured noZZle spacing and the expected
noZZle spacing is calculated for eachpair of noZZles in the tWo
printheads in the overlap region. These measured differences
are averaged to give the relative head offset in each print
from different cyan printheads, With the pattern of dash pair
304 repeated throughout test pattern 300. The left-most
detected dash in every pair of cyan dashes present in the test
pattern belongs to a single cyan printhead, While the right
most dash belongs to another cyan printhead. Once each dash
belonging to a single printhead is identi?ed, a pro?le of the
optical detector closest to the center of each dash, as previ
ously identi?ed by the interpolation around the local minima
of FIG. 4, for example, is obtained in the process direction
(block 608). Each pro?le is convolved With an edge detection
overlap one another. To determine series alignment, one
set of noZZles printed betWeen the reference head and any
other head in the print column are identi?ed. For example, if
each head has 880 noZZles, and noZZle 1 on the reference head
is aligned With noZZle 11 on another head, then 870 noZZles in
column. The relative head offsets betWeen each head in the
print column and the reference head are adjusted so the mean
of the relative head offsets sum to Zero. The relative head
offsets are adjusted by modifying the positions of one or more
of the printheads in the print column.
[0040] The printheads may be adjusted in the cross-process
kernel to identify a top or a bottom of each dash in process
direction using actuators, such as electrical motors, that are
direction. As used in this document, “edge detection kernel”
refers to a function that is de?ned so the convolution of the
operatively connected to a printhead or to a mounting mem
ber to Which a printhead is mounted. These actuators are
dash pro?le and the edge detection kernel function is a mini
typically electro-mechanical devices that respond to control
Jan. 9, 2014
US 2014/0009527 A1
signals that may be generated by a controller con?gured to
implement process 105. In one embodiment, each printhead
may be operatively connected to an independent actuator. In
heads to compensate for drift that occurs during normal
operation. The adjustment process may also be conducted in
response to a signal to print test patterns and adjust the print
alternative embodiments, a group of tWo or more printheads,
heads generated by a user of the printer. In some embodi
typically mounted to a single printhead bar, may be opera
ments, the test pattern arrangements depicted herein may be
printed on portions of an image receiving member that are
normally discarded after the printing process. For example,
inter-document gaps in Web printing systems may include
arrangements of test patterns used for registering printheads.
An inter-document gap may be the small region betWeen
tively connected to a single actuator to enable movement of
the printhead group With the single actuator. All but one of the
printheads are further mechanically coupled to independent
secondary actuators, With the printhead not having an inde
pendent actuator being adjusted solely by the ?rst actuator.
This arrangement alloWs the ?rst actuator to adjust all of the
coupled printheads simultaneously, With the secondary inde
pendent actuators providing further adjustments to their
respective printheads.
document regions that is cut aWay When a continuous Web of
paper is cut into individual sheets. The roWs of the test pattern
may be distributed among the individual regions that are cut
aWay. One or more roWs of the test pattern may be printed in
[0041] Another form of printhead alignment in the cross
process direction is knoWn as stitch alignment. Stitch align
the cut aWay region.
ment occurs at the interface boundaries betWeen adjacent
tem 120 is shoWn. For the purposes of this disclosure, the
[0045]
Referring to FIG. 10, a prior art inkjet imaging sys
printheads in a print array. Many printhead con?gurations
imaging apparatus is in the form of an inkj et printer that
arrange multiple printheads on different roWs in a single array
to span the entire cross-process Width of an image receiving
employs one or more inkjet printheads and an associated solid
ink supply. HoWever, the systems and methods described
member that passes through the print Zone. The multiple
herein are applicable to any of a variety of other imaging
printheads are “stitched” together to form a seamless line in
apparatus that use inkjets to eject one or more colorants to a
the cross process direction. For example, the rightmost inkjet
ejectors of printhead 1040 in FIG. 11 can eject ink drops that
medium or media. The imaging apparatus includes a print
are adjacent ink drops ejected by the leftmost inkj et ejectors
engine to process the image data before generating the control
signals for the inkjet ej ectors. The colorant may be ink, or any
of printhead 1036. Stitch error arises When a gap or overlap
suitable substance that includes one or more dyes or pigments
exists betWeen edge noZZles of neighboring heads of the same
color.
and that may be applied to the selected media. The colorant
may be black, or any other desired color, and a given imaging
[0042]
apparatus may be capable of applying a plurality of distinct
In process 105 of FIG. 1, X-stitch alignment is cal
culated from the measurements of the dash position measure
ments in the cross process direction (block 138). One method
of calculating this alignment is illustrated in FIG. 8. For each
stitch interface betWeen printheads, the cross process position
of the rightmost sixteen noZZles of the printhead on the left
side of the stitch interface is plotted against the noZZle index.
colorants to the media. The media may include any of a
variety of substrates, including plain paper, coated paper,
glossy paper, or transparencies, among others, and the media
may be available in sheets, rolls, or another physical formats.
[0046]
FIG. 10 is a simpli?ed schematic vieW of a direct
to-sheet, continuous-media, phase-change inkjet imaging
NoZZle index refers to a number assigned to an inkjet ejector
system 120, that may be modi?ed to generate the test patterns
to identify each inkjet ejector uniquely. For example, in a
printhead having 880 inkj et ejectors, the inkjet ej ectors may
and adjust printheads using the methods discussed above. A
media supply and handling system is con?gured to supply a
long (i.e., substantially continuous) Web of media W of “sub
strate” (paper, plastic, or other printable material) from a
be uniquely assigned a number in the range of l-880. In this
plot, the cross process position of the sixteen noZZles of the
printhead on the right side of the stitch interface is plotted
against the noZZle index. A line is ?t through each group of
sixteen noZZles and extrapolated to the interface. The differ
media source, such as spool of media 10 mounted on a Web
stitch displacement.
roller 8. For simplex printing, the printer is comprised of feed
roller 8, media conditioner 16, printing station 20, printed
Web conditioner 80, coating station 100, and reWind unit 90.
For duplex operations, the Web inverter 84 is used to ?ip the
[0043]
An alternative calculation of stitch displacement is
Web over to present a second side of the media to the printing
shoWn in FIG. 9. In this process, the mean position 904 of the
rightmost sixteen noZZles on the printhead on the left side of
the stitch interface may be calculated and the mean position
908 of the leftmost sixteen noZZles on the printhead on the
right side of the stitch interface may also be calculated. The
expected spacing betWeen the mean positions should corre
station 20, printed Web conditioner 80, and coating station
ence betWeen the tWo extrapolated lines is de?ned as the
spond to sixteen jets. The difference betWeen the measured
spacing 912 and the expected spacing is the stitch displace
ment. Although tWo processes are described for the compu
tation of stitch displacement, other processes are possible.
While the method for computation of the stitch method has
been discussed With reference to a group of sixteen noZZles in
each printhead on either side of the stitch interface, other
numbers of noZZles may be used. Regardless of method, the
stitch displacement calculation is performed for each stitch
100 before being taken up by the reWind unit 90. In the
simplex operation, the media source 10 has a Width that
substantially covers the Width of the rollers over Which the
media travels through the printer. In duplex operation, the
media source is approximately one-half of the roller Widths as
the Web travels over one-half of the rollers in the printing
station 20, printed Web conditioner 80, and coating station
100 before being ?ipped by the inverter 84 and laterally
displaced by a distance that enables the Web to travel over the
other half of the rollers opposite the printing station 20,
printed Web conditioner 80, and coating station 100 for the
printing, conditioning, and coating, if necessary, of the
reverse side of the Web. The reWind unit 90 is con?gured to
Wind the Web onto a roller for removal from the printer and
interface in the printer (block 138, FIG. 1).
[0044] In operation, the image analysis process 105 of FIG.
subsequent processing.
1 may be carried out at regular intervals to alloW the print
needed and propelled by a variety of motors, not shoWn,
[0047]
The media may be unWound from the source 10 as
Jan. 9, 2014
US 2014/0009527 A1
rotating one or more rollers. The media conditioner includes
rollers 12 and a pre-heater 18. The rollers 12 control the
tension of the unwinding media as the media moves along a
side of the media. Each backing member is used to position
the media at a predetermined distance from the printhead
opposite the backing member. Each backing member may be
path through the printer. In alternative embodiments, the
con?gured to emit thermal energy to heat the media to a
media may be transported along the path in cut sheet form in
Which case the media supply and handling system may
predetermined temperature Which, in one practical embodi
include any suitable device or structure that enables the trans
port of cut media sheets along a expected path through the
imaging device. The pre-heater 18 brings the Web to an initial
predetermined temperature that is selected for desired image
characteristics corresponding to the type of media being
printed as Well as the type, colors, and number of inks being
used. The pre-heater 18 may use contact, radiant, conductive,
or convective heat to bring the media to a target preheat
temperature, Which in one practical embodiment, is in a range
of about 30° C. to about 70° C.
[0048] The media are transported through a printing station
20 that includes a series of printhead modules 21A, 21B, 21C,
and 21D, each printhead module effectively extending across
the Width of the media and being able to place ink directly
ment, is in a range of about 40° C. to about 60° C. The various
backer members may be controlled individually or collec
tively. The pre-heater 18, the printheads, backing members 24
(if heated), as Well as the surrounding air combine to maintain
the media along the portion of the path opposite the printing
station 20 in a predetermined temperature range of about 40°
C. to 70° C.
[0051]
As the partially-imaged media moves to receive inks
of various colors from the printheads of the printing station
20, the temperature of the media is maintained Within a given
range. Ink is ejected from the printheads at a temperature
typically signi?cantly higher than the receiving media tem
the Web as moves past the printheads. The controller 50 uses
perature. Consequently, the ink heats the media. Therefore
other temperature regulating devices may be employed to
maintain the media temperature Within a predetermined
range. For example, the air temperature and air ?oW rate
behind and in front of the media may also impact the media
temperature. Accordingly, air bloWers or fans may be utiliZed
to facilitate control of the media temperature. Thus, the media
temperature is kept substantially uniform for the jetting of all
inks from the printheads of the printing station 20. Tempera
ture sensors (not shoWn) may be positioned along this portion
of the media path to enable regulation of the media tempera
ture. These temperature data may also be used by systems for
these data to generate timing signals for actuating the inkjet
measuring or inferring (from the image data, for example)
ejectors in the printheads to enable the four colors to be
ejected With a reliable degree of accuracy for registration of
the differently color patterns to form four primary-color
images on the media. The inkj et ej ectors actuated by the ?ring
hoW much ink of a given primary color from a printhead is
being applied to the media at a given time.
signals corresponds to image data processed by the controller
contact, radiant, conductive, and/or convective heat to control
a temperature of the media. The mid-heater 30 brings the ink
placed on the media to a temperature suitable for desired
properties When the ink on the media is sent through the
spreader 40. In one embodiment, a useful range for a target
temperature for the mid-heater is about 35° C. to about 80° C.
The mid-heater 30 has the effect of equaliZing the ink and
substrate temperatures to Within about 15° C. of each other.
(i.e., Without use of an intermediate or offset member) onto
the moving media. As is generally familiar, each of the print
heads may eject a single color of ink, one for each of the
colors typically used in color printing, namely, cyan,
magenta, yelloW, and black (CMYK). The controller 50 of the
printer receives velocity data from encoders mounted proxi
mately to rollers positioned on either side of the portion of the
path opposite the four printheads to compute the position of
50. The image data may be transmitted to the printer, gener
ated by a scanner (not shoWn) that is a component of the
printer, or otherWise generated and delivered to the printer. In
various possible embodiments, a printhead module for each
primary color may include one or more printheads; multiple
printheads in a module may be formed into a single roW or
multiple roW array; printheads of a multiple roW array may be
staggered; a printhead may print more than one color; or the
printheads or portions thereof can be mounted movably in a
direction transverse to the process direction P, such as for
spot-color applications and the like.
[0049] The printer may use “phase-change ink,” by Which
is meant that the ink is substantially solid at room temperature
[0052]
FolloWing the printing Zone 20 along the media path
are one or more “mid-heaters” 30. A mid-heater 30 may use
LoWer ink temperature gives less line spread While higher ink
temperature causes shoW-through (visibility of the image
from the other side of the print). The mid-heater 30 adjusts
substrate and ink temperatures to 0° C. to 20° C. above the
temperature of the spreader.
[0053] FolloWing the mid-heaters 30, a ?xing assembly 40
and substantially liquid When heated to a phase change ink
is con?gured to apply heat and/ or pressure to the media to ?x
melting temperature for jetting onto the imaging receiving
surface. The phase change ink melting temperature may be
any temperature that is capable of melting solid phase change
the images to the media. The ?xing assembly may include any
suitable device or apparatus for ?xing images to the media
ink into liquid or molten form. In one embodiment, the phase
change ink melting temperature is approximately 70° C. to
140° C. In alternative embodiments, the ink utiliZed in the
imaging device may comprise UV curable gel ink. Gel ink
may also be heated before being ejected by the inkj et ejectors
of the printhead. As used herein, liquid ink refers to melted
including heated or unheated pres sure rollers, radiant heaters,
heat lamps, and the like. In the embodiment of the FIG. 1 0, the
?xing assembly includes a “spreader” 40, that applies a pre
determined pressure, and in some implementations, heat, to
the media. The function of the spreader 40 is to take What are
essentially droplets, strings of droplets, or lines of ink on Web
solid ink, heated gel ink, or other knoWn forms of ink, such as
W and smear them out by pres sure and, in some systems, heat,
so that spaces betWeen adjacent drops are ?lled and image
aqueous inks, ink emulsions, ink suspensions, ink solutions,
solids become uniform. In addition to spreading the ink, the
or the like.
spreader 40 may also improve image permanence by increas
ing ink layer cohesion and/or increasing the ink-Web adhe
sion. The spreader 40 includes rollers, such as image-side
[0050] Associated With each printhead module is a backing
member 24A-24D, typically in the form of a bar or roll, Which
is arranged substantially opposite the printhead on the back
roller 42 and pressure roller 44, to apply heat and pressure to
Jan. 9, 2014
US 2014/0009527 Al
the media. Either roll can include heat elements, such as
heating elements 46, to bring the Web W to a temperature in a
range from about 35° C. to about 80° C. In alternative
embodiments, the ?xing assembly may be con?gured to
spread the ink using non-contact heating (Without pressure)
of the media after the print Zone. Such a non-contact ?xing
assembly may use any suitable type of heater to heat the
by reWind unit 90. Alternatively, the media may be directed to
other processing stations that perform tasks such as cutting,
binding, collating, and/or stapling the media or the like.
[0058]
Operation and control of the various subsystems,
components and functions of the device 120 are performed
With the aid of the controller 50. The controller 50 may be
implemented With general or specialiZed programmable pro
media to a desired temperature, such as a radiant heater, UV
cessors that execute programmed instructions. The instruc
heating lamps, and the like.
[0054] In one practical embodiment, the roller temperature
tions and data required to perform the programmed functions
may be stored in memory associated With the processors or
in spreader 40 is maintained at a temperature to an optimum
temperature that depends on the properties of the ink such as
circuitry con?gure the controllers and/or print engine to per
55° C.; generally, a loWer roller temperature gives less line
spread While a higher temperature causes imperfections in the
form the functions, such as the difference minimiZation func
tion, described above. These components may be provided on
gloss. Roller temperatures that are too high may cause ink to
offset to the roll. In one practical embodiment, the nip pres
a printed circuit card or provided as a circuit in an application
sure is set in a range of about 500 to about 2000 psi lbs/side.
implemented With a separate processor or multiple circuits
may be implemented on the same processor. Alternatively, the
circuits may be implemented With discrete components or
LoWer nip pressure gives less line spread While higher pres
sure may reduce pressure roller life.
[0055] The spreader 40 may also include a cleaning/oiling
station 48 associated With image-side roller 42. The station 48
cleans and/or applies a layer of some release agent or other
material to the roller surface. The release agent material may
be an amino silicone oil having viscosity of about 10-200
centipoises. Only small amounts of oil are required and the oil
carried by the media is only about 1-10 mg per A4 siZe page.
In one possible embodiment, the mid-heater 30 and spreader
40 may be combined into a single unit, With their respective
functions occurring relative to the same portion of media
simultaneously. In another embodiment the media is main
tained at a high temperature as it is printed to enable spreading
of the ink.
[0056] The coating station 100 applies a clear ink to the
printed media. This clear ink helps protect the printed media
from smearing or other environmental degradation folloWing
removal from the printer. The overlay of clear ink acts as a
sacri?cial layer of ink that may be smeared and/or offset
during handling Without affecting the appearance of the
image underneath. The coating station 100 may apply the
clear ink With either a roller or a printhead 104 ejecting the
clear ink in a pattern. Clear ink for the purposes of this
disclosure is functionally de?ned as a substantially clear
overcoat ink that has minimal impact on the ?nal printed
color, regardless of Whether or not the ink is devoid of all
colorant. In one embodiment, the clear ink utiliZed for the
coating ink comprises a phase change ink formulation With
out colorant. Alternatively, the clear ink coating may be
formed using a reduced set of typical solid ink components or
a single solid ink component, such as polyethylene Wax, or
controllers. The processors, their memories, and interface
speci?c integrated circuit (ASIC). Each of the circuits may be
circuits provided inVLSI circuits. Also, the circuits described
herein may be implemented With a combination of proces
sors, ASICs, discrete components, or VLSI circuits.
[0059]
The imaging system 120 may also include an optical
sensor 54. The drum sensor is con?gured to detect, for
example, the presence, intensity, and/or location of ink drops
jetted onto the receiving member by the inkj ets of the print
head assembly. In one embodiment, the optical sensor
includes a light source and a light detector. The light source
may be a single light emitting diode (LED) that is coupled to
a light pipe that conveys light generated by the LED to one or
more openings in the light pipe that direct light toWards the
image substrate. In one embodiment, three LEDs, one that
generates green light, one that generates red light, and one
that generates blue light are selectively activated so only one
light shines at a time to direct light through the light pipe and
be directed toWards the image substrate. In another embodi
ment, the light source is a plurality of LEDs arranged in a
linear array. The LEDs in this embodiment direct light
toWards the image substrate. The light source in this embodi
ment may include three linear arrays, one for each of the
colors red, green, and blue. Alternatively, all of the LEDS may
be arranged in a single linear array in a repeating sequence of
the three colors. The LEDs of the light source may be coupled
to the controller 50 or some other control circuitry to activate
the LEDs for image illumination.
[0060]
The re?ected light is measured by the light detector
in optical sensor 54. The light sensor, in one embodiment, is
a linear array of photosensitive devices, such as charge
coupled devices (CCDs). The photosensitive devices gener
polyWax. As used herein, polyWax refers to a family of rela
ate an electrical signal corresponding to the intensity or
tively loW molecular Weight straight chain poly ethylene or
poly methylene Waxes. Similar to the colored phase change
linear array that extends substantially across the Width of the
inks, clear phase change ink is substantially solid at room
temperature and substantially liquid or melted When initially
jetted onto the media. The clear phase change ink may be
may be con?gured to translate across the image substrate. For
example, the linear array may be mounted to a movable
heated to about 100° C. to 140° C. to melt the solid ink for
jetting onto the media.
amount of light received by the photosensitive devices. The
image receiving member. Alternatively, a shorter linear array
carriage that translates across image receiving member. Other
devices for moving the light sensor may also be used.
[0057] FolloWing passage through the spreader 40 the
[0061]
printed media may be Wound onto a roller for removal from
the system (simplex printing) or directed to the Web inverter
84 for inversion and displacement to another section of the
in optical sensor 54 that corresponds to each ink jet and/ or to
each pixel location on the receiving member. The light sensor
rollers for a second pass by the printheads, mid-heaters,
spreader, and coating station. The duplex printed material
may then be Wound onto a roller for removal from the system
A re?ectance may be detected by the light detector
is con?gured to generate electrical signals that correspond to
the re?ected light and these signals are provided to the con
troller 50. The electrical signals may be used by the controller
50 to determine information pertaining to the ink drops
Jan. 9, 2014
US 2014/0009527 A1
ejected onto the receiving member as described in more detail
below. Using this information, the controller 50 may make
adjustments to the image data to alter the generation of ?ring
signals to either retard or quicken the ejection of an ink drop
or drops from an inkjet ejector.
identifying a process direction position for each printhead
in the printer;
identifying a cross-process displacement for each column
of printheads;
[0062] A schematic vieW of a prior art print Zone 1000 that
may be modi?ed to use the test patterns described above is
identifying a stitch displacement in the cross-process
direction betWeen neighboring printheads in a print bar
unit that print a same color of ink; and
depicted in FIG. 11. The print Zone 1000 includes four color
operating an actuator to move at least some of the print
units 1012, 1016, 1020, and 1024 arranged along a process
direction 1004. Each color unit ejects ink of a color that is
different than the other color units. In one embodiment, color
unit 1012 ejects cyan ink, color unit 1016 ejects magenta ink,
color unit 1020 ejects yelloW ink, and color unit 1024 ejects
black ink. The process direction is the direction that an image
receiving member moves as travels under the color unit from
color unit 1012 to color unit 1024. Each color unit includes
tWo print arrays, Which include tWo print bars each that carry
multiple printheads. For example, the printhead array 1032 of
heads in the printer With reference to the identi?ed pro
cess direction positions, cross-process displacements,
and the identi?ed stitch displacements.
2. The method of claim 1, the identi?cation of the process
direction position for each roW in a plurality of roWs further
comprising:
convolving a portion of the image data of the test pattern
that corresponds to a response of an optical detector to
light re?ected by the image receiving member With a
the magenta color unit 1016 includes tWo print bars 1036 and
1040. Each print bar carries a plurality of printheads, as
cosine function and a sine function having a period
corresponding to spacing betWeen dashes in a roW;
summing a square of each convolution; and
exempli?ed by printhead 1008. Print bar 1036 has three print
heads, While print bar 1040 has four printheads, but alterna
identifying the position of the dash as corresponding to the
position Where the sum of the squares of the convolu
tive print bars may employ a greater or lesser number of
tions is greater than a threshold.
3. The method of claim 2, the identi?cation of the center of
printheads. The printheads on the print bars Within a print
array, such as the printheads on the print bars 1036 and 1040,
are staggered to provide printing across the image receiving
member at a ?rst resolution. The printheads on the print bars
With the print array 1034 Within color unit 1016 are interlaced
With reference to the printheads in the print array 1032 to
enable printing in the colored ink across the image receiving
member in the cross process direction at a second resolution.
The print bars and print arrays of each color unit are arranged
in this manner. One printhead array in each color unit is
aligned With one of the printhead arrays in each of the other
color units. The other printhead arrays in the color units are
similarly aligned With one another. Thus, the aligned print
head arrays enable drop -on-drop printing of different primary
colors to produce secondary colors. The interlaced printheads
also enable side-by-side ink drops of different colors to
extend the color gamut and hues available With the printer.
[0063] It Will be appreciated that variants of the above
disclosed and other features, and functions, or alternatives
thereof, may be desirably combined into many other different
systems or applications. Various presently unforeseen or
each dash further comprising:
generating a pro?le through a roW of dashes;
identifying a minimum image data value for each dash in
the generated pro?le in a cross-process direction and an
optical detector that generated the minimum image data
value;
?tting a curve to the identi?ed minimum image data value
for a dash and tWo image data values, the tWo image data
values corresponding to responses of tWo optical detec
tors, one detector being positioned on each side of the
optical detector that generated the minimum image data
value; and
identifying a minimum value of the ?tted curve as the
center of the dash corresponding to the minimum image
data value.
4. The method of claim 3 Wherein the curve is a quadratic
curve.
5. The method of claim 1 further comprising:
identifying a position in a roW of dashes corresponding to
a missing dash in the roW of dashes; and
unanticipated alternatives, modi?cations, variations, or
improvements therein may be subsequently made by those
identifying an inkjet ejector that failed to eject ink for the
skilled in the art, Which are also intended to be encompassed
6. The method of claim 5 further comprising:
by the folloWing claims.
adjusting the identi?cation of the inkjet ej ectors that
What is claimed is:
1. A method for analyZing image data of a test pattern
generated by a printer comprising:
generating With an optical sensor image data of a test
pattern having a plurality of roWs of dashes, the test
missing dash.
formed each dash in the roW of dashes With reference to
the inkjet ejector identi?ed With the missing dash.
7. The method of claim 1 further comprising:
identifying a cross-process direction displacement for a
roW of dashes in the image data corresponding to the test
pattern being formed on an image receiving member by
each printhead in a plurality of printheads Within a
pattern, the cross-process direction displacement being
printer that form the test pattern With at least one dash in
each roW of dashes in the plurality of roWs of dashes;
identifying a process direction position for each roW of
dashes in the plurality of roWs of dashes in the image
plurality of roWs of dashes; and
adjusting identi?ed dash positions in the roW of dashes
data of the test pattern on the image receiving member;
identifying a center of each dash in a cross-process direc
tion;
identifying an inkjet ejector that formed each dash in the
roW of dashes;
identi?ed With reference to one roW selected from the
With reference to the identi?ed cross-process direction
displacement for the roW of dashes.
8. The method of claim 7, the identi?cation of the cross
process displacement for a roW of dashes further comprising:
computing an average center of mass for a roW of dashes;
generating the cross-process direction displacement for the
roW of dashes as a difference betWeen the computed
Jan. 9, 2014
US 2014/0009527 A1
average center or mass for the roW of dashes and an
expected center of mass for the roW of dashes.
9. The method of claim 1, the identi?cation of the process
direction position for each printhead further comprising:
identifying each dash in the image data corresponding to
the test pattern that Was formed With ink ejected from
one printhead in the printer;
generating a density pro?le through a center of each dash;
convolving a kernel With each density pro?le to identify a
minimum value corresponding to the kernel;
averaging the minimum values for each convolution to
identify the process direction position for the one print
head; and
adjusting ?ring signals generated to operate inkjet ejectors
in a printhead to decrease the identi?ed positional dif
ferences betWeen ink drops ejected by different print
heads.
10. The method of claim 1, the identi?cation of the cross
process displacement further comprising:
selecting a reference printhead from the printheads in the
column of printheads;
computing a difference betWeen inkj et ejector positions in
11. The method of claim 1, the identi?cation of the stitch
displacement betWeen neighboring printheads further com
prising:
associating a cross-process position for each leftmost ink
jet ejector in a ?rst printhead With an index for each
leftmost inkjet ejector;
associating a cross-process position for each rightmost
inkjet ejector in a second printhead that is a next nearest
printhead left of the ?rst printhead in the cross-process
direction With an index for each rightmost inkjet ejector;
and
identifying the stitch displacement by computing a vertical
displacement betWeen the tWo associations at an inter
face betWeen the ?rst and the second printheads.
12. The method of claim 1, the identi?cation of the stitch
displacement betWeen neighboring printheads further com
prising:
computing a mean cross-process position for each leftmost
inkjet ejector in a ?rst printhead;
another printhead in the column of printheads for the
computing a mean cross-process position for each right
most inkjet ejector in a second printhead that is a next
nearest printhead left of the ?rst printhead in the cross
inkjet ejectors in the reference printhead that overlap
With the inkjet ejectors in the other printhead;
measuring a difference betWeen the tWo mean cross-pro
averaging the computed differences to identify the cross
process displacement for each printhead other than the
reference printhead in the column of printheads; and
identifying the stitch displacement by computing a differ
the reference printhead and inkjet ejector positions in
operating a plurality of actuators to move the printheads
other than the reference printhead in the column of print
heads by distances that sum the average computed dif
ferences to Zero.
process direction;
cess positions; and
ence betWeen the measured difference betWeen the tWo
mean cross-process positions and an expected spacing
betWeen the tWo mean cross-process positions.
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