FIELD OF THE INVENTION
[0001] 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
[0002] Ink-jet printing has become a widely used printing technique especially in the digitally
controlled electronic printing business.
[0003] 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.
[0004] 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
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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
[0011] 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.
[0012] 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.
[0013] Further objects and advantage of the invention will become clear from the detailed
description herein after.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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
[0018] 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).
[0019] 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).
[0020] 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.
[0021] 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.
[0022] 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
[0023] 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
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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).
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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 n
1 to n
x, the array of nozzles (105a) in the printhead (104a) has a number of nozzles n
1a to n
xa. 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 (n
1a) and the last nozzle (n
xa) 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 (n
xa - 1)NPa, the target value for distance, w
tar. The actual distance w is then compared with the target distance , w
tar. 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.
[0046] 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.
[0047] 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
[0048]
- 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.
1. 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.
2. An ink jet printer according to claim 1, wherein each of said printhead structures
is coupled to mechanical means (106, 107, 106a, 107a) for aligning said nozzle arrays
in both said x- and y-direction.
3. An ink jet printer according to claim 1 or 2, wherein each of said printhead structures
is equipped with at least one further mechanical means for adjusting said distance,
DIS, between said printhead structures and said image receiving substrate.
4. An ink jet printer according to any of claims 1 to 3, wherein said mechanical means
are linear actuators are coupled to stepping motors.
5. An ink jet printer according to claim 4, wherein said stepping motors have a step
between 2 µm and 50 µm, both limits included.
6. An ink jet printer according to any of claims 1 to 4, wherein said printer further
comprises means for sensing (114, 115, 116, 117, 118, 119) a position of said printhead
structures.
7. An ink jet printer according to claim 6, wherein said means for sensing said position
are optical means.
8. An ink jet printer according to claim 6 or 7, wherein said means for sensing said
position are coupled to a computer for detecting a difference between a sensed position
and a target position stored in a memory of said computer.
9. An ink jet printer according to claim 8, wherein said computer is further coupled
to said stepping motors for automatically actuating said stepping motors in function
of said difference between said sensed position and said target position .
10. 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 said image receiving substrate for testing an y-alignment
and/or an x-alignment of said printhead structures, creating actual alignment data
of said printhead structures ,
- comparing said actual data and target data concerning said y-and x-alignment of
said printhead structures and
- actuating mechanical means for aligning said printhead structures according to said
difference.
11. A method according to claim 10, wherein after said step of printing a test image,
a further step of off-line or on-line sensing said actual alignment data on said test
image with optical sensors is inserted.
12. A method according to claim 10, wherein said method further comprises a step of sensing
said y-edge of said image receiving substrate.
13. A method according to claim 11, wherein said method further comprises a step of actuating
mechanical means for aligning said printhead structures parallel with respect to said
y-edge.
14. A method according to claim any of claims 10 to 13, wherein said method further comprises
a step of sensing said x-edge of said image receiving substrate.
15. A method according to claim 13, wherein said method further comprises a step of actuating
mechanical means for aligning said printhead structures parallel with respect to said
x-edge.