Patent Description:
During the execution of print processes several faults can disturb the jetting of a nozzle, leading to ejection abnormalities. For example, blocking of an ink nozzle due to the presence of a dirt particle is one of the most common causes of malfunction in ink jetting. In order to identify whether a nozzle jetting abnormally, it is customary actuating the one or more electro-mechanical transducers in the print head to generate a pressure wave in the liquid in the plurality of ducts, in order to subsequently sense a residual pressure wave in the liquid in the plurality of ducts.

After the above mentioned process, the sensed residual pressure wave is compared with the residual pressure wave of a correctly functioning nozzle after manufacturing. From said comparison, a plurality of abnormalities along with their root cause can be detected, such as the presence of dirt particles, air bubbles, or dry ink.

However, the comparison with the residual pressure wave of a correctly functioning nozzle after manufacturing does not allow detecting the malfunctioning of nozzles that arises due to prolonged use of a print head. Said prolonged use may cause a drift in the behavior of one or more nozzles, which may cause ejection abnormalities such as side-shooting nozzles. These abnormalities are difficult to detect by means of a comparison of a residual pressure wave resulting from an actuation of an electro-mechanical transducer and the residual pressure wave of a correctly functioning ejection unit at the time of manufacturing. However, they may still lead to visible artifacts in the printed image.

As a consequence, it is desired to have a method for detecting ejection abnormalities in an inkjet print head that is capable of detecting the ejection abnormalities caused by prolonged use.

Documents <CIT>, <CIT> and <CIT> disclose methods of detecting malfunctioning of nozzles in printing devices.

In claim <NUM>, a method of detecting a failing nozzle in an ejection device during the printing of a print job is provided. In claim <NUM>, a droplet ejection device is provided comprising a plurality of ejection units. Said ejection unit is arranged to eject droplets of a liquid and comprises one or more of nozzles, one or more liquid ducts each connected to one of the one or more nozzles, and one or more electro-mechanical transducers each arranged to create an acoustic pressure wave in the liquid in one or more ducts, and further arranged to sense a residual pressure wave in the liquid in each of the one or more ducts. In claim <NUM> a software product is disclosed.

The method of the present invention comprises actuating the electro-mechanical transduce to generate a pressure wave in the liquid in one or more ducts. Said actuation typically causes the ejection of a liquid through the one or more nozzles in the ejection unit. Subsequently, the method of the present invention comprises sensing a residual pressure wave in the liquid in each of the one or more ducts. The sensed residual pressure wave allows performing different analyses in order to ascertain the jetting quality of an ejection unit.

In another step, the method of the present invention comprises comparing the residual pressure wave previously sensed in the one of the one or more ducts with the residual pressure wave of the one of the one or more ducts sensed in one or more previous executions of the method by determining the difference of one or more parameters of the residual pressure wave previously sensed and one or more parameters of the residual pressure wave sensed in one or more previous executions of the method.

In another step, the method of the present invention comprises determining whether the one of the one or more of nozzles is in an operative state or in a malfunctioning state, wherein the one of the one or more of nozzles is determined to be in a malfunctioning state when the difference of one or more parameters of the residual pressure wave previously sensed in the liquid of the one of one or more ducts and one or more parameters of the residual pressure wave sensed in the one of the one or more ducts in one or more previous executions of the method exceeds a predetermined threshold.

In an embodiment, all of the steps of the present invention previously described are performed for more than one of the one or more liquid ducts such that a determination is made about whether each of the more than one of the one or more of nozzles is in an operative state or in a malfunctioning state by performing the comparing step for the more than one of the one or more liquid ducts with their residual pressure wave sensed in one or more previous executions of the method exceeds a predetermined threshold.

In an embodiment, the method of the present invention comprises actuating the electro-mechanical transducer (<NUM>) to generate a pressure wave in the liquid in the one or more liquid ducts (<NUM>) comprises actuating the electro-mechanical transducer (<NUM>) with a waveform that causes the ejection of a droplet.

In an embodiment, the method of the present invention comprises that actuating the electro-mechanical transducer to generate a pressure wave in the liquid in the one or more liquid ducts (<NUM>) comprises actuating the electro-mechanical transducer (<NUM>) with a plurality of waveforms.

In an embodiment, the method of the present invention comprises that actuating the electro-mechanical transducer with a plurality of waveforms comprises actuating the electro-mechanical transducer with a plurality of waveforms with a waveform period between <NUM>,<NUM> milliseconds and <NUM> milliseconds.

In an embodiment, the method of the present invention comprises that actuating the electro-mechanical transducer with a plurality of waveforms comprises actuating the electro-mechanical transducer with a plurality of identical waveforms.

In an embodiment, the method of the present invention comprises that actuating the electro-mechanical transducer with a plurality of waveforms comprises actuating the electro-mechanical transducer with a plurality of different waveforms.

In an embodiment, the method of the present invention comprises that wherein the electro-mechanical transducer is actuated with one or more waveforms suitable for causing the ejection of liquid.

In an embodiment, the method of the present invention comprises that the one or more parameters of the residual pressure wave sensed in the liquid in each of the one or more liquid ducts comprise at least one or more of frequency, phase, amplitude, and damping factor of the residual pressure wave.

In an embodiment, the method of the present invention comprises that actuating the electro-mechanical transducer to generate a pressure wave in the liquid in the one or more liquid ducts comprises actuating the electro-mechanical transducer with a different waveform or plurality of waveforms in different executions of the method.

In an embodiment, the method of the present invention comprises that when it is determined in the step of determining whether the one of the one or more of nozzles is in an operative state or in a malfunctioning state that one or more of nozzles are in a malfunctioning state the method further comprises determining the root cause of the malfunctioning state based upon the difference of one or more parameters of the residual pressure wave sensed in the liquid in each of the one or more liquid ducts in the step of sensing a residual pressure wave in the liquid in the one of the one or more liquid ducts and one or more parameters of the residual pressure wave sensed in each of the one or more liquid ducts in one or more previous executions of the method exceeds a predetermined amount.

In an embodiment, the method of the present invention comprises that the root cause of the malfunctioning state is one of a side-shooting nozzle, the presence of dried ink in a nozzle, the presence of excess water in the ink, the presence of water in the nozzle face, or the presence of dirt in the nozzle.

Further, the present invention comprises a droplet ejection device comprising a number of ejection units arranged to eject droplets of a liquid and each comprising a nozzle, a liquid duct connected to the nozzle, and an electro-mechanical transducer arranged to create an acoustic pressure wave in the liquid in the duct, wherein each of the ejection units is associated with a processor configured to perform the method according to any of the methods of the present invention.

Further, the present invention relates to a printing system comprising the droplet ejection device of the present invention as an ink jet print head and a control unit comprising a processor suitable for executing the method according to any of the methods of the present invention.

Also, the present invention relates to a software product comprising program code on a machine-readable non transitory medium, the program code, when loaded into a control unit of the printing system of the present invention, causes the control unit to execute any of the methods of the present invention.

The present invention will become more fully understood from the detailed description given below, and the accompanying drawings which are given by way of illustration only, and are thus not limitative of the present invention, and wherein:.

The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral.

A single ejection unit of an ink jet print head is shown in <FIG>. The print head constitutes an example of a droplet ejection device according to the invention. The device comprises a wafer <NUM> and a support member <NUM> that are bonded to opposite sides of a thin flexible membrane <NUM>.

A recess that forms an ink duct <NUM> is formed in the face of the wafer <NUM> that engages the membrane <NUM>, e.g. the bottom face in <FIG>. The ink duct <NUM> has an essentially rectangular shape. An end portion on the left side in <FIG> is connected to an ink supply line <NUM> that passes through the wafer <NUM> in thickness direction of the wafer and serves for supplying liquid ink to the ink duct <NUM>.

An opposite end of the ink duct <NUM>, on the right side in <FIG>, is connected, through an opening in the membrane <NUM>, to a chamber <NUM> that is formed in the support member <NUM> and opens out into a nozzle <NUM> that is formed in a nozzle face <NUM> constituting the bottom face of the support member.

Adjacent to the membrane <NUM> and separated from the chamber <NUM>, the support member <NUM> forms another cavity <NUM> accommodating a piezoelectric actuator <NUM> that is bonded to the membrane <NUM>.

An ink supply system which has not been shown here keeps the pressure of the liquid ink in the ink duct <NUM> slightly below the atmospheric pressure, so as to prevent the ink from leaking out through the nozzle <NUM>.

The nozzle face <NUM> is made of or coated with a material which is wetted by the ink, so that adhesion forces cause a pool <NUM> of ink to be formed on the nozzle face <NUM> around the nozzle <NUM>. The pool <NUM> is delimited on the outward (bottom) side by a meniscus 32a.

The piezoelectric transducer <NUM> has electrodes <NUM> that are connected to an electronic circuit that has been shown in the lower part of <FIG>. In the example shown, one electrode of the transducer is grounded via a line <NUM> and a resistor <NUM>. Another electrode of the transducer is connected to an output of an amplifier <NUM> that is feedback-controlled via a feedback network <NUM>, so that a voltage V applied to the transducer will be proportional to a signal on an input line <NUM> of the amplifier. The signal on the input line <NUM> is generated by a D/A-converter <NUM> that receives a digital input from a local digital controller <NUM>. The controller <NUM> is connected to a processor <NUM>.

When an ink droplet is to be expelled from the nozzle <NUM>, the processor <NUM> sends a command to the controller <NUM> which outputs a digital signal that causes the D/A-converter <NUM> and the amplifier <NUM> to apply an actuation pulse to the transducer <NUM>. This voltage pulse causes the transducer to deform in a bending mode. More specifically, the transducer <NUM> is caused to flex downward, so that the membrane <NUM> which is bonded to the transducer <NUM> will also flex downward, thereby to increase the volume of the ink duct <NUM>. As a consequence, additional ink will be sucked-in via the supply line <NUM>. Then, when the voltage pulse falls off again, the membrane <NUM> will flex back into the original state, so that a positive acoustic pressure wave is generated in the liquid ink in the duct <NUM>. This pressure wave propagates to the nozzle <NUM> and causes an ink droplet to be expelled. The pressure wave will then be reflected at the meniscus 32a and will oscillate in the cavity formed between the meniscus and the left end of the duct <NUM> in <FIG>. The oscillation will be damped due to the viscosity of the ink. Further, the transducer <NUM> is energized with a quench pulse which has a polarity opposite to that of the actuation pulse and is timed such that the decaying oscillation will be suppressed further by destructive interference.

The electrodes <NUM> of the transducer <NUM> are also connected to an A/D converter <NUM> which measures a voltage drop across the transducer and also a voltage drop across the resistor <NUM> and thereby implicitly the current flowing through the transducer. Corresponding digital signals S are forwarded to the controller <NUM> which can derive the impedance of the transducer <NUM> from these signals. The measured electric response (current, voltage, impedance, etc.) is signaled to the processor <NUM> where the electric response is processed further.

A graph showing the ratio between the amplitude measured in the residual pressure wave of a nozzle and the amplitude of a correctly jetting nozzle for a plurality of burst lengths is shown in <FIG>. The result obtained for measurements of the amplitude of nozzles for different burst lengths can be observed in <FIG>. Further, the nozzles may be classified in different categories (e.g. side shooter, acceptable functioning behavior, correct functioning behavior, and non-jetting nozzle) depending upon their jetting behavior based upon the observed result while performing a printing operation. As explained below in relation with <FIG> and <FIG> it is possible to perform the same operation using different parameters amongst those observable in a residual pressure wave: phase, amplitude, etc. A person of skill in the art would readily understand that any other parameter, e.g. damping factor, etc., may be also used. This procedure is used to determine the proper functioning of the nozzles for different burst lengths, such that those leading to a more reliable jetting are decided. Once the most reliable bursts of pulses have been determined, the method of the present invention can compare the residual pressure wave sensed in a step of the method in the one of the one or more liquid ducts with the residual pressure wave of the one or more liquid ducts sensed in one or more previous executions of the method. This process may be performed by determining the difference of one or more parameters of the residual pressure wave sensed and one or more parameters of the residual pressure wave sensed in one or more previous executions of the method. This process allows the method of the present invention to detect subtle variations in the behavior of the print heads, commonly known as drift.

Further, a person skilled in the art would readily also understand that a step of comparing the residual pressure wave sensed in a step of the method in the one of the one or more liquid ducts with the residual pressure wave of the one or more liquid ducts sensed in one or more previous executions of the method can be performed for the different parameters shown in <FIG> or other additional factors such as damping factor. As a consequence, the step of determining whether the one of the one or more of nozzles is in an operative state or in a malfunctioning state can be performed using information about one or more of the mentioned parameters of the residual pressure wave.

A graph showing the ratio between the phase measured in the residual pressure wave of a nozzle and the phase of a correctly jetting nozzle for a plurality of burst lengths is shown in <FIG>. Further, the nozzles have been classified in four different categories (side shooter, acceptable functioning behavior, correct functioning behavior, and non-jetting nozzle) depending upon their jetting behavior based upon the observed result while performing a printing operation.

Further, a person skilled in the art would readily also understand that the method of the present invention can be applied to each and all of the plurality of nozzles of a print head. Also, person skilled in the art would readily also understand that the method of the present invention may comprises actuating the electro-mechanical transducer to generate a pressure wave in the liquid in the one or more liquid ducts comprises actuating the electro-mechanical transducer with a plurality of waveforms, known in the art as bursts.

A graph showing the ratio between the frequency measured in the residual pressure wave of a nozzle and the frequency of a correctly jetting nozzle for a plurality of burst lengths is shown in <FIG>. Further, the nozzles have been classified in four different categories (side shooter, acceptable functioning behavior, correct functioning behavior, and non-jetting nozzle) depending upon their jetting behavior based upon the observed result while performing a printing operation.

Based on the determinations above, the method of the present invention is able to determine whether each of the one or more of nozzles is determined to be in a malfunctioning state when the difference of one or more parameters of the residual pressure wave sensed in the liquid in each of the one or more ducts in a previous step and one or more parameters of the residual pressure wave sensed in each of the one or more ducts in one or more previous executions of the method exceeds a predetermined threshold. In one embodiment, the method of the present invention determines that one nozzle is in a malfunctioning state if the difference of one or more parameters of the residual pressure wave sensed in the liquid in each of the one or more ducts in a previous step and one or more parameters of the residual pressure wave sensed in each of the one or more ducts in one or more previous executions of the method exceed a first predetermined threshold. Further, in an embodiment the method of the present invention determines that one nozzle is in a malfunctioning state if two more parameters of the residual pressure wave sensed in each of the one or more ducts in one or more previous executions of the method exceed a second predetermined threshold, wherein said second predetermined threshold is smaller than the first predetermined threshold. Further, a person skilled in the art would readily also understand that the method of the present invention can make determination about the jetting state based on whether a combination of parameters differ from those measured in previous executions of the method for the same nozzle more than a threshold, based on the information gathered in simulations.

Claim 1:
A method of detecting a failing nozzle in an ejection device comprising a number of ejection units during the printing of a print job, each ejection unit being arranged to eject droplets of a liquid and comprising a nozzle (<NUM>), a liquid duct (<NUM>) connected to the nozzle (<NUM>), and an electro-mechanical transducer (<NUM>), arranged to create an acoustic pressure wave in a liquid in the liquid duct (<NUM>) and further arranged to sense a residual pressure wave in the liquid, the method comprising:
a) actuating an electro-mechanical transducer (<NUM>) to generate a pressure wave in the liquid in a corresponding liquid duct (<NUM>); and
b) sensing a residual pressure wave in the liquid in said liquid duct (<NUM>); and
c) comparing the residual pressure wave sensed in step b) with another residual pressure wave in the liquid of the same or another liquid duct (<NUM>), sensed in a previous execution of the method, by determining the difference of a parameter of the residual pressure wave sensed in step b) and a parameter of the other residual pressure wave; and
d) determining whether the nozzle (<NUM>) corresponding to said liquid duct is in an operative state or in a malfunctioning state, wherein said nozzle (<NUM>) is determined to be in a malfunctioning state when the difference of said parameter of the residual pressure wave sensed in step b) and said parameter of said other residual pressure wave sensed in a previous execution of the method exceeds a predetermined threshold.