FIELD OF THE INVENTION
[0001] The present invention relates to a solution for increasing the reliability of ink
jet printing processes and apparatus. More specifically the invention is related to
a method for driving an ink jet print head and an ink jet printing apparatus embodying
the method.
BACKGROUND OF THE INVENTION
[0002] Ink jet printing has become an established technology for conveying information generated
by computing devices to the general public. One of the technologies frequently used
in the industrial printing environment, is the drop on demand (DOD) ink jet technology
wherein ink drops are ejected from a nozzle of a print head only upon request, depending
on the image data. There are two main categories of drop on demand ink jet technologies:
thermal DOD ink jet and piezo DOD ink jet. The difference between both technologies
is related to the actuating means driving the drop ejection process, i.e. the way
the ejection of a drop is initiated. In thermal DOD ink jet, a heater element within
the ink chamber causes rapid thermal expansion of a small volume of ink within an
ink chamber thereby creating a pressure pulse in the ink that causes a drop of ink
to be squeezed out through a nozzle at an end of the ink chamber. In piezo DOD ink
jet, piezoelectric material is used to construct part of the ink chamber walls. The
piezoelectric material may cause rapid changes of the ink chamber volume, thereby
creating a pressure pulse in the ink contained in the ink chamber and squeezing a
drop of ink through a nozzle at an end of the ink chamber. In DOD ink jet printing
there are several phenomena that may reduce the reliability of the drop ejection process.
A first phenomenon is nozzle blockage, caused by deposits of external dust (e.g. paper
dust) or precipitation of ink particles (e.g. pigments) in the nozzle.
[0003] Another problem may be causes by long periods of inactivity, leading to changes in
the physico-chemical characteristics of the ink located in the nozzle and therefore
also leading to a shift of the optimal operating conditions for the drop ejection
process. This problem is often referred to as latency. Still another problem for the
drop ejection process is heat dissipation by the thermal heating elements or the piezoelectric
material into the ink contained in the ink chamber. Because the amount of heat dissipated
varies with the printing activity, it contributes adversely to a well-defined and
reliable operating window for the drop ejection process. A third phenomenon is the
growth of gas bubbles in the ink chamber as a result of rectified mass diffusion caused
by the large acoustic pressure field during the drop ejection process. Gas bubbles
or seeds (nuclei) may be introduced in the ink chamber through an uncontrolled breakage
of the meniscus during drop ejection or because of an improper dissolved gas level
concentration of the ink resident in the ink chamber. Gas bubbles reduce the effectiveness
of the actuating means in creating the pressure waves driving the drop ejection process.
Gas bubbles absorb the acoustic energy. They often inhibit the ejection of drops from
the nozzle.
[0004] Solutions for temperature control of the print head have been proposed in
US 6 270 180 to Arakawa et al and
US 6 827 428 to Silverbrook. These patents disclose methods and apparatuses having means for print head temperature
measurement and operating with dedicated heating signals for driving the actuating
means prior to the printing or between periods of printing. The heating signals cause
heat dissipation in the ink without ejecting ink drops from the printing elements.
Latency control has been disclosed in
US 6 431 674 to Suzuki et al,
US 6 508 528 to Fujii et al and
US 6 619 777 to Chang. These documents disclose dedicated drive signals with the intention to create minute
vibrations of the nozzle meniscus of a printing element without ejecting a drop from
that nozzle. In
US 6 431 674 to Suzuki et al and
US 6 508 528 to Fujii et al, these latency signals are applied to the printing element actuators during a preset
period of time before or after a printing operation, i.e. in non-printing time. In
US 6 619 777 to Chang a latency signal may be applied during the printing operation and replaces the drive
signal corresponding with 'no drop ejection' at the specific pixel location.
A problem of gas bubbles in the ink chamber is in the prior frequently tackled with
an active restoration operation, often referred to as a purging operation, wherein
the ink chamber is purged (flushed) with ink, with the purpose to drain the gas bubble
together with the ink. During this operation, a lot of ink is drained and a lot of
operating printing time is wasted because the purging operation requires the print
head to move and position itself relative a dedicated service station with appropriate
nozzle capping, cleaning and purging means. In
US 6 435 672 to Gröninger et al a solution has been proposed eliminating such an active restoration operation. According
to the
'672 patent, when a gas bubble is detected, the ink chamber is left on its own for a predetermined
period of time without driving the actuating means of the printing element. This time
allows the printing elements to re-establish its normal operating conditions before
resuming the printing operation. This approach could be referred to as self-restoration
instead of active restoration of a failing printing element. Although this solution
reduces the amount of active restoration operations that are required during a printing
operation, it has a disadvantage that the method reduces overall printer throughput
by interrupting the printing to allow self-restoration of the printing element, unless
the printing is continued anyhow and the printing artefact from the non-operational
printing element is accepted for the duration of the self-restoration time.
[0005] Patent
US 5 628 574 to Crowley discloses a web error recovery divert system, wherein a web is printed and scanned
for errors. An error is identified in a grouping of sections or pages. A replacement
grouping of sections or pages is printed, that is to replace the erroneous grouping.
The replacement grouping includes an identifying mark or banner page indicating the
presence of a replacement section. The web is redirected through a cutter and diverter
that identifies the banner page and, at the appropriate locations, cuts and removes
the grouping having the error. The replacement grouping and the remainder of the web
is driven to a post-processing unit for further operations. The web error recovery
divert system assumes self-recovery of the printer from its erroneous operation in
that the system automatically inserts the printing of a replacement grouping of sections
or pages, without dedicated actions to restore the erroneous operation of the printer.
[0006] It would be an advantage for industrial and/or high-throughput ink jet printing devices
to increase the reliability of the printing device without losing valuable print production
time that is to be allocated to print head maintenance or nozzle recovery.
SUMMARY OF THE INVENTION
[0007] A method and apparatus according to the invention overcome a number of the disadvantages
of the prior art in that they provide self-restoration of disturbed print elements
while the printing with the inkjet printing system is continued.
In one embodiment of the invention, the method detects a disturbance of a printing
element (either manually or automatically) and, upon detection, stops driving the
actuating means of the printing element for a predetermined period of time, while
continuing the consumption of print data for the printing element. Specific features
of preferred embodiments of the invention are set out in the claims. Further advantages
and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
[0008] Before describing in detail preferred embodiments of the invention, some printing
terms used hereinafter will be defined.
[0009] During a printing operation, a series of drops is ejected from a printing element
and deposited onto the receiving medium as a pattern of dots. The deposited pattern
is (at least partly) defined by the relative movement between the receiving medium
and the printing element. The drop ejection process in the printing element is synchronized
with the relative movement of the receiving medium versus the printing element (or
vice versa) such that the dots are printed at predefined locations on the receiving
medium i.e. a print raster. A print raster is used in the digital represent of an
image. A print raster typically is a two-dimensional grid of individual points, referred
to as pixels.
In single pass ink jet printers, that are characterized by having a fixed print head
and a transport system that moves a receiving medium passed the print head while the
print head is printing, the relative movement of the receiving medium versus the print
head results in the printing of dots in a first direction on the receiving medium
(further referred to as the scan direction) and the multiple printing elements of
the print head, often arranged in a linear array, simultaneously print dots in a second
direction (further referred to as the print direction). A series of printed dots along
the print direction is referred to as a print line and a series of dots along the
scan direction is referred to as a scan line.
Swath ink jet printers are characterized by having a limited width print head and
require additional stepping of the print head relative to the receiving medium (or
vice versa) in the print direction to cover the full print width. Typically a print
head moves in a scan direction back and forth across a receiving medium while printing
a swath of the digital image and the receiving medium moves stepwise between two print
head scans along a print direction perpendicular to the scan direction. Successively
printed swaths overlap or adjoin each other in the print direction to create a contiguous
image.
[0010] In the description, reference is made to a piezo DOD ink jet print head, although
the invention is also applicable to thermal and other types of drop on demand ink
jet print heads. In general, the drop ejection process in ink jet print heads is controlled
in the nozzle of a printing element. For a drop to be ejected from a nozzle, the ink
surface tension at the nozzle meniscus must be overcome and the ink volume in the
nozzle is accelerated to provide enough kinetic energy to the ink drop ejected from
the nozzle. In piezo DOD ink jet print heads, the energy to eject a drop of ink from
a nozzle is provided through piezoelectric actuators. These piezoelectric actuators
are designed to quickly change the volume of the ink chamber by deformation of a wall
(or part of a wall) of the ink chamber. The sudden volume changes impose pressure
waves on the ink contained in the ink chamber, travelling towards the open nozzle
of the ink chamber and causing ejection of a volume of ink out of the nozzle. Since
the drop ejection process is linked to the action of pressure waves in the ink chamber,
air bubbles resident in the ink chamber can be real show stoppers for the drop ejection
process because they absorb the pressure and damp the pressure waves imposed by the
actuators, thereby leaving insufficient energy to eject a drop of ink through the
nozzle. Air bubbles may find their way into the ink chamber as a result of an uncontrolled
drop ejection process, e.g. uncontrolled meniscus breakage and restoration due to
accidental particles in or near the nozzle. Air bubbles may also get entrapped as
a result of insufficient degassing of the ink supplied to the ink chamber or the phenomenon
of bubble growth by rectified diffusion. They may also get entrapped during accidental
mechanical impact on the print head, e.g. impact with the receiving medium during
medium transport.
[0011] In
US 6 435 672 it is disclosed that some of the disturbances caused by entrapped air bubbles may
be resolved by interrupting the printing, i.e. not driving the printing element for
a predetermined period of time. According to the
'672 patent, such a period of inactivity of a printing element is launched after detection
of a drop ejection problem with the printing element (referred to in the
'672 patent as a disturbance in the ink chamber). We have found that regular periods of
not driving a printing element may be beneficial towards the prevention of disturbances
in the drop ejection process. It has been shown that the mean time between failure
of a printing element increases when periods of inactivity are introduced regularly,
irrespectively of drop ejection disturbances or failure. The introduction of regular
periods of inactivity may therefore postpone or even cancel unexpected interruptions
of the printing operation. Experiments have been conducted with arrays of 7440 printing
element (24 printhead having 310 nozzle each) on a .Factory single pass printing press
from Agfa-Gevaert NV (BE), allowing continuous printing. Solid areas where printed
(i.e. a printing element duty cycle of 100%). The printing elements operated at a
jetting frequency of 4.8 kHz. A UV-curable ink was used at an operating temperature
of 45°C. In a reference experiment, the average failing nozzle rate without the use
of inactivity periods was 1.1 failing nozzle per liter of ink jetted. The failing
rate decreased to 0.28 failing nozzle per liter ink jetted when inactivity periods
of 0.5 s were inserted every 19.85 s. The failing further decreased to 0.21 failing
nozzle per liter ink jetted when inactivity periods of 0.125 s were used every 2.1
s. These results also show that the frequency at which the periods of inactivity are
inserted is much more important than the duration of these periods. Different embodiments
for realizing regular periods of inactivity for printing elements during normal printing
operation, i.e. without interrupting the printing operation and losing valuable print
production time, may be thought of and depend on printer configuration, printing mode,
kind of print job, etc.
[0012] In swath printers, periods of inactivity are readily available between successive
scans of the print head during which the receiving medium is transported and the print
head is not printing. Therefore, print heads in swath printers may be regularly brought
in an idle state without interrupting the normal printing operation or reducing print
production throughput. In single pass printers, this is however not the case. A printing
operation in single pass printers may be regarded as a single scan of a page wide
print head across the receiving medium. In practice it is often the receiving medium
that does the single scan passed a fixed page wide print head. The print head is continuously
printing, ejecting drops in synchronism with the transport velocity of the receiving
medium, during the whole of the printing operation. A solution for realizing periods
of inactivity for print heads or printing elements in a single pass printer configuration,
without interrupting the printing operation, is provided by analyzing the print data
and searching for blanks in the print data during which no drops are to be ejected
from the print head or printing element. This solution will be first described with
a focus on a single printing element and will later on be broadened to cover a solution
for a complete multi-color printing apparatus. It will be clear from the description
hereinafter that, although the solution solves a specific problem of single pass printers,
the solution is not limited thereto and is also applicable to swath printers.
[0013] Print data for a digital printing press may be delivered by a printstreamer or press
server to the printing press at the printing press's nominal printing speed. The printing
press passes the print data on to a print head controller driving the print head.
Print data is often structured per print line, the print data for each print line
comprising a series of pixel values, one pixel value for each pixel location of the
print line to be printed with the printing elements of the print head. The print head
controller translates the print data into driving signals (also referred to as waveforms)
for the printing element actuators, e.g. a piezoelectric element. The print data may
comprise binary pixel values for actuating binary print heads or grayscale pixel values
for actuating grayscale print heads.
Next to the print data, the print head controller also receives a print timing signal.
The print timing signal is provided in synchronism with the receiving medium transport.
It triggers the driving signals for the printing element actuators and therefore controls
the timing of the drop ejection process defining the moment a binary or grayscale
drop is ejected or 'fired' from the printing element, such that the ejected drop is
received on the receiving medium on its targeted pixel location. When the print head
controller receives the print data, it examines each pixel value for each individual
pixel location and checks whether a drop is to be ejected from a printing element
at that pixel location. If the pixel value corresponds to 'no drop to be ejected',
it is referred to as a 'zero pixel'. If the print data for a printing element comprises
a series of consecutive zero pixels, it corresponds to a part in the image where the
printing element is not supposed to print ink drops, i.e. a gap in the scan line.
A gap in a scan line provides an opportunity to install a period of inactivity for
the corresponding printing element. A period of inactivity for a printing element
may be realized by disabling the print timing signal for the printing element, thereby
preventing any drive signal from driving the printing element's actuator. It may also
be realized by driving the actuator with a 'inactive signal' that is designed to minimize
or disable energy input into the printing element. The first method may be required
if the print head controller, by default, always inserts a latency signal when no
drop is to be ejected from the printing element. Such a feature may be embedded in
the print head controller firmware to prevent latency problems. Experiments show that
the reliability of a printing element is significantly increased when periods of inactivity
of minimum about one tenth of a second can be inserted regularly during normal printing
operation In a preferred embodiment, periods of inactivity are chosen to be minimum
about 0.3 s. Depending on the print timing signal frequency and the receiving medium
transport velocity, this may already be feasible with gaps in the printed image of
a few centimeters. Besides gaps in the printed image, also other events may provide
an opportunity to install a period of inactivity or an idle time for a printing element.
These events may be blanks between successive pages in a print job, a receiving medium
standstill (e.g. web standstill or exchange of receiver roles), the preparation time
for a print job, etc. During such events, idle periods may be installed without giving
in print production time, which is an advantage compared to idle periods installed
after detection of a disturbance in the printing element's operation and interruption
of the printing operation as disclosed in the prior art.
[0014] Notwithstanding the beneficial effect of a period of inactivity onto the reliability
of a printing element in that it may allow air bubbles to dissolve again the ink or
evacuate from the ink chamber towards the ink manifold outside of the action radius
of the pressure waves in the ink chamber, the adverse effect of total inactivity of
a printing element - known as latency - should be avoided. Latency is the deterioration
of ink in the ink chamber and in the nozzle area over time, with a potential negative
impact on ink ejection or jetting reliability. Latency is very much depending on the
ink composition and the printing environment, e.g. UV curable inks seem to be less
susceptible to latency problems than water based inks. We have found that the reliability
of a printing element is further increased with the application of a latency signal
at regular intervals during a period of inactivity of the printing element. A latency
signal, furhter referred to as a precursor signal, is designed to stir the ink in
the ink chamber and create a short wobble of the meniscus without ejecting a drop
from the printing element. It prevents settling of the ink in the ink chamber, local
viscosity increases of the ink in the nozzle (as a result of evaporating of ink compounds),
and other physicochemical processes that reduce the fit-to-fire condition of the ink.
Different types of precursor signals have been disclosed in the prior art. Their specification
strongly depends on the ink jet technology and the actuating means that are used,
and configuration details of the printing element (ink chamber length and cross-section,
nozzle diameter, etc.). A suitable precursor signal can readily be selected from the
prior art. The frequency of application of a precursor signal during a period of inactivity
of a printing element is a tradeoff between, on the one hand, avoiding energy input
to the printing element to benefit maximally from the inactivity of the printing element
and, on the other hand, applying enough stirring of the ink to keep the ink in a fit-to-fire
condition. A frequency choice may also depend on the ink type used (e.g. a low frequency
for UV curable ink versus a higher frequency for water based ink), the degassing level
of the ink, the design of the print head and its susceptibility to the creation and
retention of air bubbles in the ink chamber (e.g. through flow print heads are less
susceptible to the retention of air bubbles in the ink chamber than end shooter print
heads), the operating temperature, etc. Experiments showed , during a period of inactivity
and for a UV curable ink having a viscosity between 3 and 15 mPa.s and operated at
a temperature of about 40 °C, a precursor signal is preferably applied at a frequency
of about 100 Hz. A solution for applying a precursor signal to a printing element
during a period of inactivity may be to replace the 'inactive signal' with a 'precursor
signal' for one or a number of consecutive zero pixels for the printing element, or
enable the print timing signal for the printing element for one or a number of consecutive
zero pixels and have the embedded precursor signal in the print head controller drive
the printing element actuator.
[0015] The concept as discussed above for a single printing element, may now be extended
to a full print head. Embodiments that may be thought of are very much depending on
hard- and/or firmware features of the print heads themselves: ink jet print heads
may comprise multiple printing elements arranged in one or more arrays, printing element
actuators or drive parameters may be individually controllable or en bloc for the
entire print head, multiple ink jet print heads may abut each other to create a single
page wide print head configuration having a contiguous array of printing elements
across the full width of a page, etc. Some embodiments are discussed hereinafter,
without the intention to limit the list of possible embodiments to these examples.
- Assume an ink jet print head having a number of printing elements for which the timing
of the drop ejection process is controlled with a common print timing signal. Assume
further that a precursor signal is provided in the print head controller firmware
that is automatically inserted when the pixel value for a given printing element corresponds
with a 'zero pixel'. In this case, the inactivity of a printing element when drop
ejection is not required from the printing element is overruled by the precursor signal
that is automatically inserted. An embodiment may therefore include the use of the
print timing signal to enforce a period of inactivity to all printing elements of
the print head at the same time. The print head controller therefore needs to analyze
the print data and detect the absence of a requirement for ejecting an ink drop from
every printing element of the print head in a print line, a so called 'zero pixel
line', before disabling the print timing signal for the print line to enforce a period
of inactivity for all of the printing elements of the print head.
- Assume an ink jet print head having a number of printing elements for which the drop
ejection process is synchronized using a single print timing signal for all the printing
elements and for which drive signals or drive voltages for the printing element actuators
are individually controllable. In this case the drive voltage for a printing element,
for which the print head controller has detected a 'zero pixel', may be set to a non-energizing
voltage (e.g. 0 Volt) putting the printing element de facto in a state of inactivity,
independent of the activity of other printing elements in the print head that may
still be ejecting drops when a print timing signal is applied. The embodiment has
an additional advantage that a preconfigured firmware precursor signal in the print
head controller may easily be blocked/overruled when a period of inactivity for the
printing element is to be installed by setting a non-energizing drive voltage, and
likewise may easily be passed when a precursor signal is to interrupt the period of
inactivity for the printing element by setting a normal drive voltage.
- A print head may have several banks or groups of printing elements, each bank or group
being controlled with a separate driver circuitry, e.g. 64 printing elements per driver
chip. Depending on the specific features of these driver chips and the way the print
head controller accesses these driver chips, the level of controllability of the inactivity
of printing elements of the print head may vary from the individual printing element
to a group of printing elements associated with a driver chip or to all printing elements
in the print head.
- In 'shared wall' piezo DOD print head actuators operating in 'shear mode', the piezoelectric
actuators are designed as part of the walls separating neighboring ink chambers. Print
head actuators of this type have been described extensively in the prior art, e.g.
EP 0 364 136 to Temple. In this type of print head actuators, opposite side walls of an ink chamber can
bend inward to reduce the volume of the ink chamber or outward to increase the volume
of the ink chamber. The opposite side walls are shared with neighboring ink chambers,
i.e. a left side wall shared with a left neighbor ink chamber and a right side wall
shared with a right neighbor ink chamber. Driving the piezoelectric side walls of
the ink chamber of a printing element not only affects the volume of the ink chamber
of that printing element, it also affects the volume of the ink chamber of its neighboring
printing elements (although limited to the effect of only one side wall of the neighboring
ink chamber). That is one of the reasons why, with ink jet print heads based on a
shared wall piezoelectric actuator, neighboring printing elements are preferably not
driven simultaneously. Instead, printing elements may be driven every one out of three
at the same time. Every first out of three printing elements in the linear array may
be allocated to a first set A, every second to a second set B, and every third to
a third set C. A print head as described above may be controlled with three individual
print timing signals, one for each set of printing elements in the linear array. The
print head may also be controlled with a single print timing signal for the complete
print head, that is split and delayed internally in the print head controller to trigger
the drop ejection process for each of the sets of printing elements in the linear
array sequentially. Because of the mutual influence of neighboring printing elements,
an embodiment for providing periods of inactivity for printing elements in a shared
wall piezo DOD print head may be somewhat different from what has been described before.
It may include the detection, in the print head controller, of groups of three neighboring
zero pixels in a print line. The middle one of each group of three zero pixels reflects
an opportunity for a state of inactivity of the corresponding printing element. In
more general terms, the embodiment may include analyzing the print data, detecting
a set consecutive zero pixels in a print line, and putting the corresponding printing
elements of the set on inactive, except for the utmost printing elements from the
set. If the printing elements can not be put inactive on an individual basis, the
analysis of the print data may detect the presence of an entire print line of zero
pixels and provide a period of inactivity for all the printing elements via disabling
of the print timing signal if this situation occurs.
[0016] It is important to note the difference between print data and image data. To explain
this difference, assume the printing of a continuous tone (contone) and single color
image, e.g. a black & white photograph. This image is made available in digital format
for preparing the image for printing during a number of prepress steps (e.g. color
management, rendering, etc.). The digital representation of the image typically comprises
a number of image pixels (e.g. 1024 x 768 pixels), each image pixel having a gray
value (e.g. 0 to 255). The more image pixels used, the more image detail is preserved
in the digital representation of the photograph; the more gray values used, the more
shades can be represented. Ink jet print heads and printing devices typically have
a number of limitations to print the digital representation of an image, e.g. the
printing device may have a limited print resolution or the print head may have limited
gray scale capability. Therefore a digital image is additionally processed before
printing to map the digital representation to the capability of the printing device.
This digital image processing is typically executed on a front end system. Some examples:
- The number of image pixels may have to be mapped to a number of print pixels to fit
the print resolution of a printing device.
- The range of available gray values per image pixel (e.g. 0-255) may have to be mapped
to a range of printable gray values per printed pixel (e.g. 0-15). This may for example
result in all image pixels with a gray value between 0 and 15 being mapped into a
'zero pixel' value for the corresponding print pixels.
- Advanced digital image processing techniques such as rendering and error diffusion
may be required to compensate for the image quality loss as a result of the above
discussed print limitations.
[0017] Aside of prepress actions to preserve as much as possible the original image quality
during printing, the preprocessed pixel data needs to be allocated to a print head
and to printing elements within the print head for actually printing the pixel data
on the receiving medium. It is not always possible to decide in advance which of the
pixel data will be printed by a given printing element. This decision may for example
depend on the specific print head configuration of the printing device, on the composition
of multiple images in a single print job or the imposition of multiple print jobs
on a single printing device, on dedicated shingling techniques used in swath printers
to reduce the visibility of a number of print artefact, etc. It may therefore be preferable
to analyze the print data at the print head controller level instead of analyzing
image date at the front end system, that is after allocation of print data to printing
elements.
[0018] The use of periods of inactivity to increase the reliability of the printing elements
of a printing device is also applicable to color printing devices after decomposition
of a full color image into multiple monochrome image layers (e.g. a Cyan Magenta Yellow
and BlacK image layer). At the printing device, each monochrome image layer is printed
with dedicated pint head or print head setup allocated for printing the color of that
specific monochrome image layer.
[0019] So far, periods of inactivity have been created to prevent the creation and retention
of air bubbles in the ink chamber that may disturb the drop ejection process of the
printing element. It has been described that periods of inactivity may be inserted
without interrupting the printing process. Therefore the print data is analyzed and
'zero pixels' or sequences of 'zero pixels' in the print data for a given printing
element are detected. The printing of 'zero pixels' may then be replaced with periods
of inactivity of the printing element. Periods of inactivity may be interrupted with
regular precursor signals to preserve a fit-to-fire condition of the ink in the ink
chamber and in the nozzle.
[0020] A period of inactivity for a printing element may also be used as a solution to restore
a disturbance or a failure in the ejection process of a printing element, as disclosed
in
US 6 435 672 to Gröninger et al. A state of disturbed operation or failure can be detected visually (either manually
or with a visual inspection system) or via piezoelectric sensing methods disclosed
in that same document. The
'672 patent further suggests to interrupt the printing, after detection of a disturbance,
and resume the printing after a period of inactivity of the printing element(s) has
elapsed. In one aspect of the present invention, the stream of print data for the
printing elements restoring from a disturbance continues to be consumed from the print
server, although the output of the printing elements that are supposed to print this
data is suppressed during their restoration. This approach has a major advantage towards
print job management, especially for single pass printing devices, in that for example
the synchronization/registration of the web (receiving medium) transport of the printing
operation of the print engine is preserved. Interrupting the printing, as suggested
in the prior art, breaks the synchronization between these two processes and it is
known that loss of registration of printed images on a receiving medium may be a problem
for post-processing equipment of the printed job, such as cutting and folding of the
printed material. It is a further advantage of this aspect of the invention that it
is possible to suppress only part of a print line, and preferably only during a preset
period of time, so that essential marks for post-processing equipment may still be
printed, while the rest of the print line is being suppressed.
[0021] Having described in detail preferred embodiments of the current invention, it will
now be apparent to those skilled in the art that numerous modifications can be made
therein without departing from the scope of the invention as defined in the appending
claims.
1. A method for increasing the reliability of an ink jet printing system comprising a
printing element having an ink chamber provided with an ink and a nozzle for ejecting
ink drops therefrom and actuating means for controlling the drop ejection process,
the method comprising the steps of:
- detecting a disturbance in the drop ejection process of the printing element;
- leaving the actuating means of the printing element inactive for a predetermined
period of time, if a disturbance in the drop ejection process of the printing element
is detected;
characterised in that the method further comprises continuing the consumption of print data for the printing
element while leaving the actuating means of the printing element inactive.
2. The method according to claim 1, wherein the step of leaving the actuating means of
the printing element inactive includes disabling a print timing signal for controlling
the timing of the drop ejection process in the printing element or driving the actuating
means of the printing element with an inactive signal for avoiding energy input into
the ink chamber.
3. The method according to any one of the previous claims, wherein the predetermined
period of time is determined by an ink parameter or a printing element operating parameter.
4. The method according to any one of the previous claims, wherein the ink jet printing
system comprises a plurality of printing elements each having an ink chamber provided
with an ink and a nozzle for ejecting ink drops therefrom and actuating means for
controlling the drop ejection process, the ink drops ejected from the plurality of
printing elements forming a print line; the method further comprising suppressing
a part of the print line, associated with a group of printing elements, if a disturbance
is detected in the drop ejection process of at least one of the group of printing
elements, associated with the part of the print line.
5. An ink jet printing system comprising:
- a print head having a printing element with an ink chamber provided with a nozzle
for ejecting an ink drop therefrom and actuating means for controlling the drop ejection
process;
- a print head controller for receiving print data and driving the print head according
to the print data;
- detecting means for detecting a disturbance in the drop ejection process of the
printing element, and;
- control means for leaving the actuating means of the printing element inactive for
a predetermined period of time, if a disturbance in the drop ejection process of the
printing element is detected;
characterised in that the print head controller further comprises means for continuing a consumption of
print data for the printing element while leaving the actuating means of the printing
element inactive.
6. The system according to claim 5, wherein the control means for leaving the actuating
means of the printing element inactive includes means to disable a print timing signal
used for controlling the timing of the drop ejection process in the printing element
or means to drive the actuating means of the printing element with an inactive signal
for avoiding energy input into the ink chamber.
7. The system according to claim 5 or 6, wherein the period of time is adjustable as
a function of an ink parameter or a printing element operating parameter.