FIELD OF THE INVENTION
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This invention relates to an ink-jet printer with at least
page-wide printhead structures and especially to a system for
aligning these printhead structures with respect to each other and
the image receiving substrate.
BACKGROUND OF THE INVENTION
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Ink-jet printing has become a widely used printing technique
especially in the digitally controlled electronic printing business.
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Many types of ink-jet printing mechanisms have been invented.
These can be categorised as either continuous inkjet (CIJ) or drop
on demand (DOD) ink-jet. Using one of these type of ink-jet
printing, colour printers have been designed, wherein from multiple
printhead structures different colours are printed. Properly
controlling the arrangement of various droplets of ink of different
colours will result in a wide spectrum of perceivable colours. The
clarity and quality of the resultant image is affected by the
accuracy of the placement of the ink droplets on the medium.
Printers which use multiple printhead structures to co-operatively
form a single image usually require mechanical or electronic
adjustment so that ink droplets printed by one printhead alight at
precise locations on the receiving medium relative to those printed
by another printhead in the printer. Several methods to achieve the
accurate alignment of the rows of droplets ejected by the different
printhead structures have been proposed.
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For example, in US-A-5 600 350 titled Multiple Inkjet Print
Cartridge Alignment By Scanning A Reference Pattern And Sampling
Same With Reference To A Position Encoder, US-A-5 448 269 titled
Multiple Inkjet Print Cartridge Alignment For Bi-directional
Printing By Scanning A Reference Pattern, US-A-5 451 990 titled
Reference Pattern For Use In Aligning Multiple Inkjet Cartridge,
US-A-5 404 020 titled Phase Plate Design For Aligning Multiple
Inkjet Cartridges By Scanning A Reference Pattern, US-A-5 350 929
titled Alignment System For Multiple Colour Pen Cartridges,
US-A-5 297 017 titled Print Cartridge Alignment In Paper Axis, and
US-A-5 250 956 titled Print Cartridge Bi-directional Alignment
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In US-A-5 534 895 the ink-jet printer is equipped with a source
of illumination that is passed across a test pattern having features
indicative of printhead structure alignment and discernible under
the illumination. The source of illumination is connected to
circuitry that determines the variation in light intensity of the
test pattern. A value indicative of the misalignment is calculated
and used to correct the timing of firing signals between the
sequentially fired banks of nozzles of a printbar.
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In US-A-5 751 305 it is disclosed to place a referencing
mechanism on the printer and a detector on the printhead in order to
dynamically align one or more printheads in a printer. The printhead
structure is moved at a known speed past two spaced apart reference
indicia of the referencing mechanism. The passing of a first of the
spaced apart reference indicia is detected and the passing of a
second of the spaced apart reference indicia is detected. The time
between the detection of the first reference indicia passage and the
detection of the second reference indicia passage is measured and a
delay time, related to the measured period of time, is created.
Energization of an ink drop ejection is delayed for the duration of
the delay time.
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In US-A-5 192 959 an alignment system for a pagewide printhead
structure is disclosed. The pagewidth printhead structure would
include a reference plate, a linear array of ink jet sub-units
affixed to the reference plate, and a plurality of alignment sub-units
affixed on opposite ends of the planar surface of said
reference plate. The ink jet printer would also include alignment
or reference points for engaging the alignment sub-units and thereby
aligning the pagewidth printhead structure with respect to the
frame. However once the printhead structure is aligned in the frame
no further fine tuning of the alignment is foreseen.
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In US-A-6 109 721 a bi-directional print position alignment
system for automatically aligning bi-directional printing position
of a printhead structure in a serial printer as a function of high
sensor accuracy and clock frequency of a CPU controlling the sensor.
The alignment system includes a sensing section for sensing a
position of a printhead structure for vertical alignment, a
misalignment detecting section for detecting mechanical misalignment
of the printhead structure, and a printing section for correcting
said mechanical misalignment of the printhead structure and printing
information on a printable medium after said mechanical misalignment
of the printhead structure is corrected.
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In US-A-6 109 722 and US-A-6,076,915 test patterns are
disclosed that are useful for printhead structure alignment. The
test patterns are optically sensed and the sensed pattern are used
to electronically adjust the alignment, either by adjusting the
firing time of the nozzles, either by shifting the pattern of ink-jet
nozzles from which the ink is ejected.
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Although the teachings of the prior art do allow for a good
alignment of printhead structures, it is still desired to have a
system for printhead structure alignment that makes it possible to
align in more than one direction and/or over a fraction of the
nozzle pitch.
OBJECTS AND SUMMARY OF THE INVENTION
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It is an object of the invention to provide an ink jet printer
wherein including one or more mechanical means for aligning the
printhead structures with respect to each other over an integer
number of nozzle pitches and over fractions of the nozzle pitch.
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It is a further object of the invention to provide a method for
aligning printhead structures of an ink jet printer with respect to
each other over an integer number of nozzle pitches and over
fractions of the nozzle pitch.
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Further objects and advantage of the invention will become
clear from the detailed description herein after.
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The object of the invention is realised by providing an ink-jet
printer comprising
a frame (101),
at least two printhead structures (104, 104a) each with an array of
nozzles (105, 105a) said printhead structure being mounted in said
frame, so that said arrays of nozzles define an x-direction, and
means (123) for moving an image receiving substrate (100) at a
distance, DIS, past said at least two printhead structures in a
y-direction,
characterised in that said at least two printhead structures (104,
104a) each are coupled to at least one mechanical means(106, 107,
106a, 107a) for aligning said nozzle arrays with respect to each
other in at least one of said x- and y-direction.
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In a preferred embodiment said at least two printhead
structures (104, 104a) each are coupled to further mechanical
means(106, 107, 106a, 107a) for further aligning said nozzle arrays
with respect to each other in both said x- and y-direction.
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In a very preferred embodiment the printer is further equipped
with means for sensing an x- and/or y-edge of the image receiving
substrate, so that the printhead structures can not only be
mechanically aligned with respect to each other but also with
respect to an x- and/or y-edge of the image receiving substrate.
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A ink jet printer according to this invention may further
comprise means (112, 112a) for keeping a distance between said at
least two printhead structures (104, 104a) and said image receiving
substrate (100), said distance defining a z-direction, together with
means (112, 113) for adjusting the z-direction in accordance with
the thickness of the image receiving substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
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Figure 1 shows schematically an embodiment an ink jet printer
according to this invention with printhead structures equipped for
being mechanically aligned (for sake of clarity only one printhead
structure is shown).
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Figure 2 shows schematically an other embodiment of an ink jet
printer with printhead structures equipped for being mechanically
aligned (for sake of clarity only one printhead structure is shown).
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Figure 3 shows schematically a printer according to this
invention showing means for adjusting the distance between the
printhead structures and the image receiving substrate.
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Figure 4 and 5 show schematically a printer according to this
invention incorporating optical sensors for sensing a test image
together with a first (figure 4) and second stage (figure 5) of a
possible implementation of a method for aligning printhead
structures in a printer according to this invention.
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Figure 6 shows schematically a printer according to this
invention incorporating optical sensors for sensing a test image
together with a further possible implementation of a method for
aligning printhead structures in a printer according to this
invention.
DETAILED DESCRIPTION OF THE INVENTION
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It is in any ink jet printer comprising more than one printhead
structure desirable to have means and ways of aligning the printhead
structures with respect to each other and to the edge of the image
receiving member. In the printing business the trend to replace or
supplement classical (e.g. offset) printing by digital printing
techniques (e.g. electrostatic printing or ink jet printing) is
still growing. Due to this trend the demands on ink jet printing
have risen to higher standards than those demanded for SOHO (small
office/home) printing. Especially the registration of different
colour images in the print has to be very good. In digital printing
with ink jet printers in order to replace or supplement classical
(e.g. offset) printing page wide printheads are frequently used. In
such printers it is highly desired to have the possibility to align
the printheads - at least with respect to each other, preferably
also with respect to one or more of the edges of the image receiving
substrate - in a simple way that does not pose (too) high demands on
the computing power of the computer that drives the printer
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Therefore in an ink jet printer wherein at least two different
printhead structures are mounted in a frame, each of the printhead
structures is coupled to at least one mechanical means for aligning
the nozzles of said at least two different printhead structures in
at least one of the x- and y-direction.
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A mechanical alignment of the nozzles in the print direction
(y-direction)forgoes the adaptation of the firing time of each
individual nozzle to the degree of parallelism between the nozzles
of two different print heads and/or to the difference in distance
between the nozzle arrays. This mechanical alignment has the
advantage that the computing power during printing can be lower.
This advantage is most pronounced in a printer that comprises
multiple printhead structures, e.g., six - four for the YMCK
printing and two for further supporting colours - because in such
printer the alignment of the nozzles of the six different printhead
structures based on adjustment of the firing time demands very much
of the computing power and on the electronics of the printhead.
Even if the computing power can be provided, it can be impossible to
adjust the firing time of each individual nozzle due to limitations
in the electronics of the printhead.
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A mechanical alignment in the x-direction, i.e. the possibility
of mechanically displacing the nozzles of the different printhead
structure in a direction perpendicular to the print direction has
the advantage that mechanical means can be introduced so that the
displacement of the nozzles can be effected over a fraction of the
nozzle pitch, whereas in prior art embodiment for alignment in the
x-direction, a "displacement" was always disclosed to go over an
integer number of nozzle pitches.
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Preferably in an ink jet printer according to this invention,
wherein at least two different printhead structures are mounted in a
frame, each of the printhead structures is coupled to at least one
mechanical means for aligning the nozzles of said at least two
different printhead structures in both said y- and x-direction.
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In figure 1, a first embodiment of an ink jet printer according
to this invention is schematically shown. For sake of clarity, only
one printhead structure is shown, it can however easily be
appreciated that it is possible to include any desired number of
printhead structures in a printer according to this invention. An
image receiving substrate (100) with and x-edge (100x) and a y-edge
(100y) is guided by a guiding means (123) past printhead structure
(104) with an array of nozzles (105). The guiding means and the
image receiving substrate are shown as being transparent for sake of
clarity. The printhead structure (104) is mounted in an y-frame
(103) so that the array of nozzles defines an x-direction,
perpendicular to the print direction, that defines an y-direction.
The y-frame (103) is mounted in an x-frame (102) by attachments
(110) so that it can be moved in a direction parallel to the print
direction (arrows A) and/or that it can get an angular movement
(arrows B) with respect to the x-frame. Therefore on both ends of
the end of the y-frame a linear actuator (106) coupled to a stepping
motor (106') is mounted in contact with the y-frame and the x-frame.
Opposite to each of the actuators (106) a play spring (109) is
present to avoid play of the printhead structure in the y-direction,
once it is aligned. The x-frame (102) is mounted in a master frame
(101) by fastening means (111), that allow for sliding movement in
the x-direction. At a side of the x-frame parallel with the x-direction,
a linear actuator (107) coupled to a stepping motor
(107') is mounted in contact with the x-frame (102) and the master
frame (101). A play spring (108) is mounted opposite to the linear
actuator (107) to avoid play of the printhead structure in the x-direction,
once it is aligned.
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When the attachment points (110) of the y-frame are designed so
as to allow for movement both in the direction of arrows A and of
arrows B, then an actuation of the actuators (106) in the same
direction and over the same distance will cause the y-frame (and
thus the printhead structure coupled to it) to be displaced in the
y-direction and an actuation of the actuators (106) in opposite
directions or actuation of only one actuator will cause the y-frame
to rotate. With the first type of actuation the distances between
different printhead structures are changed, by the second type of
actuation the parallelism of different printhead structures with
respect to each other and/or with respect to the x-edge (100x) of
the image receiving substrate is changed. It will be self-evident
for the person skilled in the art that it is possible to design the
attachment points of the y-frame (110) so as to allow only for a
movement according to arrows A, or only for a movement according to
arrows B or for allowing movement according to both arrows A and
arrows B.
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In figure 2 a second embodiment of an ink jet printer according
to this invention is very schematically shown. In this figure the
schematically shown ink jet printer comprises only one printhead
structures, it is however clear that it is possible to include any
desired number of printhead structures in a printer according to
this invention. An image receiving substrate (100) with and x-edge
(100x) and a y-edge (100y) is guided by a guiding means (123) past
printhead structure (104) with an array of nozzles (105). The
guiding means and the image receiving substrate are shown as being
transparent for sake of clarity. The printhead structure (104) is
mounted in an y-frame (103) so that the array of nozzles defines an
x-direction, perpendicular to the print direction, that defines an y
direction. The y-frame (103) is mounted in an x-frame (102) so that
it can rotate around an axis (110) located at one end of the
printhead structure (104). At the end of the printhead structure
opposite to the axis (110) a linear actuator (106) coupled to a
stepping motor (106') is mounted in contact with the y-frame and the
x-frame. Actuation of the actuator 106 causes the y-frame to rotate
around axis 110 and thus to move in the direction of arrow B. A
play spring (109) is present to avoid play of the printhead
structure in the y-direction, once it is aligned. The x-frame (102)
is mounted in a master frame (101) by fastening means (111), that
allow for a sliding movement in the x-direction. At a side of the
x-frame parallel with the x-direction, a linear actuator (107) is
coupled to a stepping motor (107') is mounted in contact with the x-frame
(102) and the master frame (101). A play spring (108) is
mounted opposite to the linear actuator (107) to avoid play of the
printhead structure in the x-direction, once it is aligned. In this
embodiment of a printer of this invention, the mechanical alignment
of the nozzles in the print direction (y-direction) is only an
alignment wherein the parallelism of different printhead structures
with respect to each other and/or with respect to the x-edge (100x)
of the image receiving substrate is changed. Thus, the possibility
of y-alignment in this second embodiment forgoes the need for
adapting the firing time of each individual nozzle to the degree of
parallelism between the nozzles of two different printhead
structures. Since the distance between the different printheads is
then not mechanically adjusted, (simplifying the design of the
mechanical means for y-alignment), it may be necessary to adjust the
firing time for each of the printhead structures taking in account
the difference in the distance between them. This adjustment is
however much less complicated than an adjustment of the firing time
of each individual nozzle and gives thus still a considerable
reduction of the computing power needed.
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An ink jet printer according to the present invention can
beneficially further include spacing means for keeping the distance
between the printhead structures and the image receiving substrate
constant (i.e. for keeping the distance in the z-direction
constant). If so desired, these spacing means can include movable
parts coupled to means for adjusting the distance in the z-direction.
In that case it is possible to adjust the distance in
the z-direction according to the thickness of the image receiving
substrate, so that a printer can be built wherein image receiving
substrates showing a large variety of thickness can be used and the
printer can be adjusted to the thickness of the substrate used, so
as to have an optimal "throw distance" (i.e. the distance between
the nozzle array and the image receiving substrate) for every
substrate thickness. A possible placement of the spacing means for
keeping the distance between the printhead structures and the image
receiving substrate constant (i.e. for keeping the distance in the
z-direction constant) is schematically shown in figure 3. This
figure is a view of the printer in figure 2 along arrow C. In this
figure the y-frame (102) is shown together with the printhead
structure (104) with nozzles (105) coupled to it. The axis 110
around which the y-frame can rotate upon actuation of actuator (106)
by a stepping motor (106') is also shown. The y-frame carries on
the side of it facing the guiding means (123) for guiding an image
receiving substrate past the printhead structure (104) a number of
spacers (e.g. three spacers) (112) each of the spacers having a
movable part (113). Both the guiding means and the image receiving
substrate are shown as being transparent. The movable part (113) of
the spacing means is in contact with the guiding means (123) and
keeps thus the distance, DIS, between y-frame and guiding means
constant. By moving the movable parts (113) of the spacing means
(112) in the z-direction, the distance, DIS, can be changed so as to
keep an optimum "throw distance" when the thickness of the image
receiving substrate is changed. In figure 3 the spacing means (112)
for keeping the distance between the printhead structures and the
image receiving substrate constant are shown as being present on the
side of the y-frame (102) facing the guiding means (123) and as
including a movable part (113). It is clear that the purpose of the
spacing means for keeping the distance between the printhead
structures and the image receiving substrate constant can be
achieved in other configurations. E.g., it is possible to have
spacing means, not including a movable part, between the master
frame (101) and the guiding means (123)for the image receiving
substrate. Then the y-frame is coupled to the x-frame in such a way
that it not only can be moved for adjusting the y-position of it,
but also for adjusting the z-position. When the y-frame is coupled
to the x-frame in this way, mechanical means, e.g., linear
actuators, for moving the y-frame in the z-direction can be
incorporated between the x- and y-frame.
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It is also possible, if so desired, to equip a printer of this
invention with spacing means, not including a movable part, between
the master frame (101) and the guiding means (123)for the image
receiving substrate. Then the x-frame is coupled to the master
frame in such a way that it not only can be moved for adjusting the
x-position of it, but also for adjusting the z-position. When the
x-frame is coupled to the master frame in this way, mechanical
means, e.g., linear actuators , for moving the x-frame in the
z-direction can be incorporated between the master frame and the
x-frame.
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Preferably the mechanical means for adjusting the printhead
structures in the y-, x- and, if so desired, in the z-direction are
linear actuators. The linear actuators are preferably adjusted so
as to be able to displace the printhead structures over a distance
between about 1 µm and about 10 mm. The linear actuators are
preferably construed so as to allow for an alignment that is adapted
to the nozzle pitch of the nozzle arrays in the printhead. The
linear actuators are preferably designed so as to allow an alignment
- i.e. a displacement of the printheads - in steps as small as
1/20th of the nozzle pitch. Linear actuators allowing for a
displacement in steps as small as 1/10th of the nozzle pitch can
however also be beneficially used when high accuracy of the
alignment is desired. Thus in a printer according to this
invention, - depending on the accuracy of alignment that is desired
- linear actuators allowing for a displacement of the printheads in
steps between 1 to 100 µm (both limits included)can beneficially be
used. Preferably linear actuators allowing for a displacement
(alignment) in steps between 2 and 50 µm are used. E.g. a 720 dpi
printer has a nozzle pitch of 35 µm. Thus when using linear
actuators allowing for an alignment in steps of 3 µm, it is possible
to align the printhead structures in a 720 dpi printer to 1/10 of
the nozzle pitch. E.g. in a 250 dpi printer, the nozzle pitch is
100 µm, thus when using linear actuators allowing for displacement
in steps of 50 µm, it is possible to align the printhead structures
in a 250 dpi to 1/2 of the nozzle pitch.
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The actuators can be manually driven, e.g. it can be
micrometer screws or can, preferably, be powered by stepping motors.
In the latter case the linear actuators are preferably the spindles
of the stepping motors.
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When micrometer screws are used for the displacement
(alignment) of the printheads, it is preferred to use - in a printer
of this invention - micrometer screws allowing for a displacement
accuracy of the printheads between 1 to 100 µm (both limits
included). Preferably micrometer screws allowing for a displacement
(alignment) accuracy between 2 and 50 µm are used.
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When the spindles of the stepping motors are the linear
actuators coupled to the stepping motors, then the combination of
the step of the stepping motor and the pitch of the spindles is
preferably adapted to the nozzle pitch of the printhead. Thus,
stepping motors for use in an ink jet printer of this invention have
preferably a combination of motor step and spindel pitch so that a
linear displacement in steps between 1 µm and 100 µm (both limits
included),more preferably in steps between 2 µm and 50 µm (both
limits included) are possible.
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It is possible, if so desired, to use - in a printer according
to this invention - stepping motors with a rather large linear
displacement step due to either limited number of steps per rotation
of the motor or rather large pitch of the spindle, and
electronically create smaller steps, via so called "micro stepping".
This can have the advantage of using motors that are less expensive
and still proceed with a displacement of the printheads in equally
small steps than with motors having a small step and including a
spindle with a small pitch. Whatever the method that is used for
displacing the printheads - and thus the nozzle arrays contained in
them - it is important that the displacement can proceed in steps
between 1 µm and 100 µm (both limits included), more preferably in
steps between 2 µm and 50 µm (both limits included).
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Possible misalignment of the printheads can be detected off-line.
E.g. A template of a test image can provided with the
printer. The operator of the printer can then compare an actual
print of the test image on the printer with the target output as
shown in a template of the test image. If the operator detects
misalignment - i.e. differences between the print of the test image
and the template of it - he can either manually adjusts the
micrometer screws to align the printheads so as to have an actual
output corresponding to the target output or he can activate the
stepping motors to align the printheads. It is also possible to
scan the printed (actual) test image with an optical scanner and to
input the scanned data into a computer memory, wherein the target
data, if so desired with tolerances, for the test image are saved.
The computer can then compare the data of the actual test image with
the target data and e.g. display the differences on a screen. Based
on the figures presented on the screen, the operator of the printer
either adjusts the micrometer screws or actuates the stepping
motors. It is however also possible to couple the computer wherein
the actual data of the test image are compared with the target data
to the stepping motors that can the automatically be actuated to
adjust the alignment.
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Preferably the possible misalignment of printheads in a printer
of this invention is automatically detected on the printer and then
either manually or automatically corrected. Therefore, an ink jet
printer according to this invention is preferably further equipped
with means for sensing the relative position of the printhead
structures with respect to each other. In a still further preferred
embodiment an ink jet printer according to this invention is
equipped with means for sensing the relative position of the
printhead structures not only with respect to each other, but also
with respect to one or more edges of the image receiving substrate.
The means for sensing the relative position of the printhead
structures and/or the edge(s) of the image receiving member can
beneficially be optical means, e.g. CCD-cameras, that are placed in
the printer such as to read a printed test image and/or the edges of
the images receiving substrate. In this way possible misalignments
between the nozzles of the different printhead structures and/or the
edge of the paper are detected. The means for sensing the position
of the printhead structures can be coupled to a computer so as to
compare the actual data of the test image with the target data and
to display the degree of misalignment on the computer screen. An
operator of the printer then reads this information and actuates the
linear actuators for aligning the printhead structures. In a very
preferred embodiment the computers wherein the target positions and
tolerances thereon in the y-, x- and, if so desired, the z-direction,
are stored and these values are compared with the actual
values sensed by the sensing means, is further coupled to stepping
motors for actuating the linear actuators automatically to a degree
depending on the difference between actual positions sensed by the
means for sensing the position of the printhead structures and the
target positions. In this way the alignment can proceed
automatically.
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The invention further encompasses a method for aligning
printhead structures in an ink jet printer comprising the steps of :
- providing an image receiving substrate with an x- and a y-edge,
- printing a test image on an image receiving substrate for
testing a y-alignment and of an x-alignment of said printhead
structures, creating actual data from said test image,
- comparing said actual data with target data concerning said
y- and x-alignment of said printhead structures and
- actuating mechanical actuators for aligning said printhead
structures according to said target values.
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Preferably after the step of printing a test image, a further
step of sensing the actual data of the test image with optical
sensors is inserted.
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More preferably, in said step of sensing the test image, also a
y-edge and/or an x-edge of said image receiving substrate is sensed.
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It is possible in a method according to this invention to align
the printheads only with respect to each other, but in a very
preferred embodiment of a method according to this invention a step
of sensing the edge of the image receiving substrate that is
substantially orthogonal to the print direction (herein after called
"x-edge") and/or a step of sensing one of the edges of the image
receiving substrate that is substantially parallel to the print
direction (herein after called "y-edge") is included, then the
printheads can be aligned with respect to each other and to an edge
of the image receiving substrate.
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In a highly preferred embodiment of a method of this invention,
said actual data of the test image sensed with optical sensors are
sent to a computer memory and said step of comparing the actual data
with target data is executed in said computer memory. In the most
preferred embodiment of the invention said computer wherein the
actual data are compared with target data is also coupled to the
mechanical actuators and when in said computer a difference between
the actual data and the target data of the test image is found, the
computer automatically executes the step of actuating the mechanical
actuators.
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A printer according to this invention incorporating optical
sensors for sensing a test image together with a first stage of a
possible implementation of a method for aligning the printhead
structures is shown in figure 4. In figure 4 two printhead
structures (104 and 104a) are schematically shown. In both printhead
structure the same numericals as in figure 1 to 3 are used for
designating the same parts of the printhead structure, the
numericals of the second printhead structure have been provided with
the letter "a". For sake of clarity the printer, shown in figure 1,
is further schematised in this figure 4. In figure 4 the master
frame and the x- and y-frames and the spacers are omitted for
clarity and the figure 4 shows two printhead structures (104, 104a)
each with an array of nozzles (105, 105a), the array of nozzles
(105) in the printhead (104) has a number of nozzles n1 to nx, the
array of nozzles (105a) in the printhead (104a) has a number of
nozzles n1a to nxa. Both printhead structures are coupled to linear
actuators (106, 106a, 107, 107a) for aligning them in the y- and
x-direction respectively. Play springs (108, 108a, 109, 109a) are
placed in the printer so as to press the printhead structures firmly
against the linear actuators. The printhead structure can rotate
around an axis (110, 110a) and are supported in the x-direction by
fastening means (111, 111a) leaving the possibility for sliding the
printhead structures in the x-direction. The printhead structures
are shown as deviating from the target position, in the x-direction
the deviation is half the nozzle pitch (NP, NPa) and in the y-direction
the non-parallelism of the printhead structures is
exaggerated for sake of clarity. An image receiving substrate (100)
with y- edges (100y) and an x-edge (100x) passes the printhead
structures in the y-direction. A sensor (114) senses the arrival of
the image receiving substrate in the printing zone and signals the
arrival of the image receiving substrate so as to start the
printing. Two lines (120a, 120'a) substantially parallel to the y-edge
of the image receiving substrate are printed using the first
nozzle (n1a) and the last nozzle (nxa) of printhead 104a. Then the
image receiving substrate passes image sensors (115 and 116) so that
the lines 120a and 120'a, printed by the first printhead structure
(104a) are sensed and a distance, w, between both lines is detected.
When the printhead is orthogonal to the y-direction this distance,
w, equals (nxa - 1)NPa, the target value for distance, wtar. The
actual distance w is then compared with the target distance , wtar.
When a difference is observed, the mechanical actuator 106a is
actuated so as to displace the printhead 104a perpendicular to the
y-direction. This situation is shown in figure 5, where printhead
104a is placed perpendicular to the y-direction In a second stage
both printhead structures (104, 104a) print a line (121, 121a)
substantially parallel to the x-edge of the image receiving
substrate and a line (120, 120a) substantially parallel to the y-edge
of the image receiving substrate. The image receiving substrate
passes again image sensors (115 and 116) so that the line 121a,
printed by the first printhead structure (104a) is sensed first and
the line 121 printed by the second printhead structure (104) is
sensed secondly. The time difference between the passage of line
121a and the passage of line 121 under sensor 115 and under sensor
116 is measured, this translates in a distance between lines 121a,
and 121 at sensor 115 of h and in a distance between lines 121a, and
121 at sensor 116 of h'. If h - h' ≠ 0, then the actuator 106 is
actuated for adjusting h and h' so that h - h' = 0. The lines 120
and 120a are sensed by the sensor 118, and it is determined if both
lines are in line, if a difference, d is found, then the actuators,
107 and 107a are actuated for bringing both lines, 120 and 120a in
line. It is preferred that the alignment proceeds first to bring
the printhead structures parallel to each other (y-alignment) and
that then the printhead structures are aligned in the x-direction.
Although the method has been explained with only 2 printhead
structures, it is clear that the method can be used for aligning
more than two printhead structures, e.g., when the first two
printhead structures are aligned, then the third is aligned with
reference to the already aligned printhead structures and so on
until all printhead structures are aligned with respect to each
other.
-
Using figure 6, a further implementation of the method of this
invention is shown, wherein the printhead structures are aligned
with respect to the edges of the image receiving substrate. The
figure is basically the same as figures 4 and 5, both printhead
structures (104, 104a) print a line (121, 121a) substantially
parallel to the x-edge of the image receiving substrate and a line
(120, 120a) substantially parallel to the y-edge of the image
receiving substrate. The image receiving substrate passes image
sensors (115 and 116) so that de x-edge of the image receiving
substrate is sensed (see dashed line 100'x). The sensors 115 and
116 sense the line 121a, printed by the first printhead structure
(104a). The time difference between the passage of x-edge of the
image receiving substrate and the passage of line 121a under sensor
115 and under sensor 116 is measured, this translates in a distance
between the x-edge of the image receiving substrate and line 121a at
sensor 115 of h1 and in a distance between the x-edge of the image
receiving substrate and line 121a, at sensor 116 of h'1. If h1 -
h'1 ≠ 0, then the actuator 106a is actuated for adjusting h1 and
h'1 so that h1 - h'1 = 0. Then the sensors 115 and 116 sense also
the line 121 printed by the second printhead structure (104). The
time difference between the passage of x-edge of the image receiving
substrate and the passage of line 121 under sensor 115 and under
sensor 116 is measured, this translates in a distance between the
x-edge of the image receiving substrate and line 121 at sensor 115
of (h1 + h) and in a distance between the x-edge of the image
receiving substrate and line 121, at sensor 116 of (h'1 + h'). When
(h1 + h) - (h'1 + h') ≠ 0 linear actuator 106 is actuated to adjust
the distances so that (h1 + h) - (h'1 + h') = 0. Sensor 117 senses
an y-edge (100'y) of the image receiving substrate. The lines 120
and 120a are sensed by the sensor 118, and it is determined if both
lines are at the same distance from the y-edge of the image
receiving substrate. If d' ≠ d, then the actuators, 107 and 107a
are actuated for bringing both lines, 120 and 120a in line. It is
preferred that the alignment proceeds first to bring the printhead
structures parallel to each other (y-alignment) and that then the
printhead structures are aligned in the x-direction.
-
Although the method has been explained with only 2 printhead
structures, it is clear that the method can be used for aligning
more than two printhead structures, e.g., when the first two
printhead structures are aligned with respect of the edges of the
image receiving substrate, then the third is aligned with reference
to the already aligned printhead structures and so on until all
printhead structures are aligned with respect to each other and with
respect to the edges of the image receiving substrate. Although the
method according to this invention has been explained with the use
of 3 sensors (figures 4 and 5), 4 sensors (figure 6), the number of
optical sensors is basically determined by the quality of alignment
of the printhead structures that is desired. When e.g. only the
parallelism between the printhead structures is deemed necessary,
then the method of this invention can be executed with only two
sensors, e.g., sensors 115 and 116. The sensors as shown in figures
4, 5 and 6 have a certain range so as to be able to sense lines that
are a number of nozzle pitches apart and have a resolution as to be
able to sense a misalignment of at least one tenth of the nozzle
pitch NP. It is however possible to execute a method according to
this invention using smaller sensors that , e.g., are designed to
sense over the width of a nozzle pitch when these are placed in
close proximity.
Parts list
-
- 100
- Image receiving substrate
- 100x, 100y :
- x- and y-edge of the image receiving substrate
- 101
- Master frame
- 102, 102a
- x-frame
- 103, 103a
- y-frame
- 104, 104a
- printhead structure.
- 105, 105a
- nozzle array
- 106, 106a
- linear actuator for alignment in the y-direction
- 107, 107,
- linear actuator for alignment in the x-direction
- 108, 108a, 109, 109a :
- anti play springs
- 110, 110a
- attachment and pivoting point in the y-frame
- 111, 111a
- attachment points of the x-frame to the master frame
- 112, 112a
- spacing means between the printhead structures and the
image receiving substrate
- 113, 113a
- movable parts in the spacing means for aligning in the z-direction
- 114
- sensor of x-edge of the image receiving substrate
- 115, 116
- sensors for sensing the x-edge of the image receiving
substrate and for sensing the test image
- 117, 119
- sensor for sensing a y-edge of the image receiving
substrate
- 118
- sensor for sensing the test image
- 123
- guiding means for guiding the image receiving substrate past the
printhead structure.