A CIRCUIT AND METHOD FOR DETECTING FAILING NOZZLES IN AN EJECTION UNIT

Abstract

 The invention relates to a method for detecting failing nozzles in an ejection unit during the printing of an object of a print job, wherein the ejection unit is arranged to eject droplets of a liquid and comprises a plurality of nozzles, a plurality of liquid ducts each connected to one of the plurality of nozzles, and a plurality of electro-mechanical transducers arranged to create an acoustic pressure wave in the liquid in the plurality of ducts. The method comprises a first step of dividing the plurality of nozzles into a first group of nozzles and a second group of nozzles. The method also comprises actuating the plurality of electro-mechanical transducer to generate a pressure wave in the liquid in the plurality of ducts of the at least first group of nozzles and second group of nozzles. Then, it is possible to sense a residual pressure wave in the liquid in the plurality of ducts of the at least first group of nozzles and second group of nozzles. Then, it is possible to determine whether one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state. The method also comprises dividing the at least first group of nozzles and second group of nozzles into additional groups of nozzles if it is previously determined that one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state, or classifying the plurality of nozzles in the first group of nozzles and the plurality of nozzles in the second group of nozzles in a correctly functioning state if it is previously determined that all of the nozzles of the at least first group of nozzles and second group of nozzles are in a correctly functioning state. Finally, the method repeats all of the previous stepson the additional groups of nozzles until all the nozzles of the plurality of nozzles have been classified in a correctly functioning state or in a malfunctioning state.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention generally pertains to detecting failing nozzles in an ejection unit, in particular a piezo-actuated ejection unit.

[0002] In known methods, it is possible to detect failing nozzles by actuating the electro-mechanical transducer to generate a pressure wave in the liquid, and subsequently sensing a residual pressure wave in the liquid, which can be compared with the expected residual pressure wave to determine whether the nozzles are functioning correctly.

[0003] However, these known methods require a high number of actuations and processing of the generated waves, which require a significant amount of resources and time.

[0004] As a consequence, it is desired to have a method for detecting failing nozzles in an ejection unit that is capable of processing all the nozzles of an ejection unit with limited resources in a shorter time span.

SUMMARY OF THE INVENTION

[0005] In an aspect of the present invention, a method of operating an ejection unit according to claim 1 is provided. In another aspect of the present invention, a droplet ejection device is provided.

[0006] In an embodiment, the present invention relates to a method for detecting failing nozzles in an ejection unit during the printing of an object of a print job comprising one or more objects, wherein the ejection unit is arranged to eject droplets of a liquid and comprises a plurality of nozzles, a plurality of liquid ducts each connected to one of the plurality of nozzles, and a plurality of electro-mechanical transducers arranged to create an acoustic pressure wave in the liquid in the plurality of ducts. Said method comprises a step of dividing the plurality of nozzles into at least a first group of nozzles and a second group of nozzles. A person of skill in the art would readily recognize that it is possible to divide the plurality of nozzles into two or more groups. Depending upon the percentage of malfunctioning nozzles dividing the plurality of nozzles into a higher or lower number of groups allows the present invention to more significantly reduce the number of operations and cycles to determine the operating state of the whole plurality of nozzles.

[0007] Then, the plurality of electro-mechanical transducer is actuated to generate a pressure wave in the liquid in the plurality of ducts of the at least first group of nozzles and second group of nozzles, and the residual pressure waves in the liquid in the plurality of ducts of the at least first group of nozzles and second group of nozzles are sensed.

[0008] Further, it is determined whether one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state.

[0009] After said determination, the at least first group of nozzles and second group of nozzles are respectively divided into additional groups of nozzles if it is determined in the previous step that one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state. Alternatively, the plurality of nozzles in the first group of nozzles and the plurality of nozzles in the second group of nozzles are classified in a correctly functioning state if it is determined in the previous step that all of the nozzles of the at least first group of nozzles and second group of nozzles are in a correctly functioning state.

[0010] As a final step, all of the previous steps are repeated on the additional groups of nozzles, until all the nozzles of the plurality of nozzles have been classified in a correctly functioning state or in a malfunctioning state.

[0011] In an embodiment, the method of the present invention the step of determining whether one or more nozzles of a group of nozzles are in a malfunctioning state comprises aggregating the residual pressure wave sensed in the previous step in the liquid in the plurality of ducts of the nozzles of the group of nozzles, and determining that one or more nozzles of a group of nozzles are in a malfunctioning state if the aggregated residual pressure wave is below a predetermined threshold. In order to be able to perform these steps and reach accurate determinations the plurality of nozzles is normally actuated with a waveform confirming a jetting pulse and a quenching pulse, such that an almost negligible residual pressure wave is left in the liquid. In this way, the presence of a failing nozzle is easily detectable, as it entails the presence of a non-negligible residual pressure wave which can be detected in the aggregated waveform. As a consequence, the method of the present invention allows determining simultaneously whether there are malfunctioning nozzles amongst a plurality of nozzles.

[0012] In an embodiment, the present invention further comprises that a step of dividing the plurality of nozzles into a first group of nozzles and a second group of nozzles comprises grouping the nozzles of the plurality of nozzles that have a higher likelihood of being in a malfunctioning state in one of the first and second group of nozzles.

[0013] In an embodiment, the present invention comprises that the plurality of nozzles that have a higher likelihood of being in a malfunctioning state are determined based upon one or more of a failure rate higher than a predetermined threshold, a spatial adjacency to a nozzle with a failure rate higher than a predetermined threshold, and an observed correlation of failure during the execution of previous print jobs comprising one or more objects with a nozzle with a failure rate higher than a predetermined threshold.

[0014] In an embodiment, the present invention comprises that the step of dividing the plurality of nozzles into at least a first group of nozzles and a second group of nozzles comprises dividing the plurality of nozzles equitably into at least a first group of nozzles and a second group of nozzles.

[0015] In an embodiment, the present invention comprises that the step of dividing the plurality of nozzles into at least a first group of nozzles and a second group of nozzles comprises dividing the plurality of nozzles into at least a first group of nozzles and a second group of nozzles such that the nozzles of the plurality of nozzles that have a likelihood of being in a malfunctioning state higher than a predetermined threshold are allocated into the same group of nozzles from the at least a first group of nozzles and a second group of nozzles. This embodiment allows the present invention to cluster all, or at least most, of the malfunctioning nozzles in the same group, thereby also clustering the correctly functioning nozzles in another group. This process usually leads to uneven groups in which a small number of recurrently malfunctioning nozzles are clustered in a small group, while a greater number of correctly functioning nozzles are clustered in one or more other groups. Such configuration allows processing all of the groups of nozzles with a reduced number of operations and cycles, thereby speeding up the process for a given number of computational resources.

[0016] In an embodiment, the present invention comprises a software product comprising program code on a machine-readable non-transitory medium, the program code, when loaded into a processor of the droplet ejection device of the present invention, causes the processor to perform a method of the present invention.

[0017] In an embodiment, the present invention also comprises a droplet ejection device comprising a number of ejection units arranged to eject droplets of a liquid and each comprising a nozzle formed in a nozzle face, 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, characterized in that at least one of the number of ejection units is associated with a processor configured to perform any of the methods of the present invention.

[0018] Lastly, in an embodiment the present invention comprises a printing system comprising the droplet ejection device of the present invention, and 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 according the present invention, causes the control unit to perform the method according to any of the embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 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:

Figure 1

is a cross-sectional view of mechanical parts of an ejection unit according to the invention, together with an electronic circuit for controlling and monitoring the device.

Figure 2

shows a binary tree of the measurements needed to detect malfunctioning nozzles in a prior art configuration.

Figure 3

shows a graph of nozzle failure rates during the execution of a print job.

Figure 4

shows a binary tree of the measurements needed to detect malfunctioning nozzles in the method of the present invention.

Figure 5

shows a flow diagram of the method of the present invention for malfunctioning nozzles.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 related text

[0020] 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.

[0021] A single ejection unit of an ink jet print head is shown in Figure 1. The print head constitutes an example of a droplet ejection device according to the invention. The device comprises a wafer 10 and a support member 12 that are bonded to opposite sides of a thin flexible membrane 14.

[0022] A recess that forms an ink duct 16 is formed in the face of the wafer 10 that engages the membrane 14, e.g. the bottom face in Figure 1. The ink duct 16 has an essentially rectangular shape. An end portion on the left side in Figure 1 is connected to an ink supply line 18 that passes through the wafer 10 in thickness direction of the wafer and serves for supplying liquid ink to the ink duct 16.

[0023] An opposite end of the ink duct 16, on the right side in Figure 1, is connected, through an opening in the membrane 14, to a chamber 20 that is formed in the support member 12 and opens out into a nozzle 22 that is formed in a nozzle face 24 constituting the bottom face of the support member.

[0024] Adjacent to the membrane 14 and separated from the chamber 20, the support member 12 forms another cavity 26 accommodating a piezoelectric actuator 28 that is bonded to the membrane 14.

[0025] An ink supply system which has not been shown here keeps the pressure of the liquid ink in the ink duct 16 slightly below the atmospheric pressure, so as to prevent the ink from leaking out through the nozzle 22.

[0026] The nozzle face 24 is made of or coated with a material which is wetted by the ink, so that adhesion forces cause a pool 30 of ink to be formed on the nozzle face 24 around the nozzle 22. The pool 30 is delimited on the outward (bottom) side by a meniscus 32a.

[0027] The piezoelectric transducer 28 has electrodes 34 that are connected to an electronic circuit that has been shown in the lower part of Figure 1. In the example shown, one electrode of the transducer is grounded via a line 36 and a resistor 38. Another electrode of the transducer is connected to an output of an amplifier 40 that is feedback-controlled via a feedback network 42, so that a voltage V applied to the transducer will be proportional to a signal on an input line 44 of the amplifier. The signal on the input line 44 is generated by a D/A-converter 46 that receives a digital input from a local digital controller 48. The controller 48 is connected to a processor 50.

[0028] When an ink droplet is to be expelled from the nozzle 22, the processor 50 sends a command to the controller 48 which outputs a digital signal that causes the D/A-converter 46 and the amplifier 40 to apply an actuation pulse to the transducer 28. This voltage pulse causes the transducer to deform in a bending mode. More specifically, the transducer 28 is caused to flex downward, so that the membrane 14 which is bonded to the transducer 28 will also flex downward, thereby to increase the volume of the ink duct 16. As a consequence, additional ink will be sucked-in via the supply line 18. Then, when the voltage pulse falls off again, the membrane 14 will flex back into the original state, so that a positive acoustic pressure wave is generated in the liquid ink in the duct 16. This pressure wave propagates to the nozzle 22 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 16 in Figure 1. The oscillation will be damped due to the viscosity of the ink. Further, the transducer 28 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.

[0029] The electrodes 34 of the transducer 28 are also connected to an A/D converter 52 which measures a voltage drop across the transducer and also a voltage drop across the resistor 38 and thereby implicitly the current flowing through the transducer. Corresponding digital signals S are forwarded to the controller 48 which can derive the impedance of the transducer 28 from these signals. The measured electric response (current, voltage, impedance, etc.) is signaled to the processor 50 where the electric response is processed further.

Figure 2 related text

[0030] In Figure 2, a binary tree of the measurements needed to detect malfunctioning nozzles in a prior art configuration is shown.

[0031] As explained above, when an actuation pulse is generated and applied to the piezoelectric actuator an acoustic pressure wave is generated in the liquid ink in the ink channel. This pressure wave propagates to the nozzle 80 and causes an ink droplet to be expelled. It is known that a plurality of ink channels can be actuated at the same time. After said actuation, it is possible to aggregate the residual pressure waves caused by said actuations, and to detect whether one or more of the actuated ink channels relates to malfunctioning nozzles.

[0032] In these prior art configurations, it is first detected whether there are one or more malfunctioning nozzles amongst the plurality of nozzles in the ejection unit. In the likely event that there are malfunctioning nozzles, the plurality of nozzles is divided in two groups that are independently actuated again, and the aggregation of their residual pressure waves is checked again. This process is iteratively repeated by dividing the groups of nozzles in which one or more malfunctioning nozzles are detected into further smaller groups of nozzles, until all the malfunctioning nozzles have been detected. Once it is detected that there are no malfunctioning nozzles within a group of a plurality of nozzles, all of said plurality of nozzles are marked as correctly functioning nozzles. As the malfunctioning nozzles in an ejection unit are usually sparsely distributed, as is explained with more detail with reference to Figure 3 below, this prior art process usually leads to a significant number of iterations of the method to detect all the malfunctioning nozzles, as well as a to high number of actuations being needed in said iterations, as there usually are one or more malfunctioning nozzles in all the groups.

Figure 3 related text

[0033] In Figure 3, a graph of nozzle failure rates during the execution of a print job is shown. It can be observed that only a handful of the plurality of nozzles in a print head is typically malfunctioning during the execution of a print job. Moreover, an even smaller number of nozzles are normally malfunctioning a high percentage of the times (e.g. above 40% of the times) during the execution of a particular print job. As explained above in relation with Figure 2, when a prior art method is executed in an ejection unit showing the failure pattern of Figure 3, a first iteration takes place in which all of the nozzles are actuated and the resulting residual pressure waves sensed and aggregated. As this process leads to a determination that one or more nozzles are malfunctioning, the nozzles are divided into two mutually exclusive groups, in which the process is repeated. Further iterations in which the groups of nozzles are divided are performed until it is determined for all the nozzles in an ejection unit whether they are in a malfunctioning or in a correctly functioning state. The sparse distribution of failing nozzles amongst the plurality of nozzles of an ejection unit leads to a significant number of iterations and a significant number of actuation and sensing processes being needed in order to detect all those nozzles not functioning correctly. The present invention aims at reducing the number of iterations or the numbers of actuations and sensing needed to detect all the malfunctioning nozzles within an ejection unit.

Figure 4 related text

[0034] Next, Figure 4 shows a binary tree of the measurements needed to detect malfunctioning nozzles in the method of the present invention. As in the example above, all the nozzles in an ejection unit are tested in a first iteration with the aim of detecting whether there are any malfunctioning nozzles. In order to accelerate the detection process by either reducing the number of iterations needed, or the number of actuation needed to detect all the malfunctioning nozzles, the method of the present invention separates the nozzles in such a way that those with a higher likelihood of being malfunctioning nozzles are clustered in the same group of nozzles for the execution of further iterations of the method.

[0035] A person of skill in the art would readily appreciate that different occurrences can be used to designate a particular nozzle as having a higher likelihood to be a malfunctioning nozzle during the execution of a particular print job. Amongst said occurrences, the present invention takes into account different variables to classify a nozzle as having a higher likelihood of being a malfunctioning nozzle. Namely, the present invention takes into account whether during the execution of previous print jobs a nozzle has exhibited one or more of a failure rate higher than a predetermined threshold, a spatial adjacency to a nozzle with a failure rate higher than a predetermined threshold, or whether there is an observed correlation of failure of a nozzle during the execution of previous print jobs with another nozzle with a failure rate higher than a predetermined threshold.

[0036] In the example of Figure 4, the nozzles exhibiting a higher likelihood of failure have been clustered in the group shown in the left side of the binary tree. It can be observed that when the nozzles are divided into two groups following the criteria discussed above the nozzles in the right side of the tree can be quickly classified as correctly functioning nozzles, ideally in only one iteration as shown in Figure 4. As a consequence, either the number of actuations or the number of iterations needed to classify all malfunctioning nozzles is severely reduced.

Figure 5 related text

[0037] In the present invention, step S1 comprises dividing the plurality of nozzles into a first group of nozzles and a second group of nozzles.

[0038] Subsequently, step S2 comprises actuating the electro-mechanical transducer to generate a pressure wave in the liquid in the plurality of ducts of the at least first group of nozzles and second group of nozzles.

[0039] Step S3 of the present invention relates to sensing a residual pressure wave in the liquid in the plurality of ducts of the at least first group of nozzles and second group of nozzles; and.

[0040] Next, in step S4 the present invention determining whether one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state.

[0041] In step S5 of the present invention two different steps are performed depending upon whether it is determined in step S4 that one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state. If one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state the next step comprises dividing the at least first group of nozzles and second group of nozzles into additional groups of nozzles. To the contrary, if no nozzles of the at least first group of nozzles and second group of nozzles are determined to be in a malfunctioning state the plurality of nozzles in the first group of nozzles and the plurality of nozzles in the second group of nozzles are classified in a correctly functioning state.

[0042] Finally, all of the previous steps are repeated on the additional groups of nozzles until all the nozzles of the plurality of nozzles have been classified either in a correctly functioning state or in a malfunctioning state.

[0043] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A method for detecting failing nozzles in an ejection unit during the printing of an object of a print job, wherein the ejection unit is arranged to eject droplets of a liquid and comprises a plurality of nozzles (22), a plurality of liquid ducts (16) each connected to one of the plurality of nozzles (22), and a plurality of electro-mechanical transducers (28) arranged to create an acoustic pressure wave in the liquid in the plurality of ducts (16), the method comprising:

a) dividing the plurality of nozzles (22) into at least a first group of nozzles and a second group of nozzles; and

b) actuating the plurality of electro-mechanical transducers (28) of the at least first group of nozzles and second group of nozzles to generate a pressure wave in the liquid in the plurality of ducts (16); and

c) sensing a residual pressure wave in the liquid in the plurality of ducts (16) of the at least first group of nozzles and second group of nozzles; and

d) determining whether one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state; and

e) if it is determined in step d) that one or more nozzles of the at least first group of nozzles and second group of nozzles are in a malfunctioning state dividing the at least first group of nozzles and second group of nozzles into additional groups of nozzles, or
if it is determined in step d) that all of the nozzles of the at least first group of nozzles and second group of nozzles are in a correctly functioning state classify the plurality of nozzles in the at least first group of nozzles and the plurality of nozzles in the at least second group of nozzles in a correctly functioning state; and

f) repeating steps a) to e) on the additional groups of nozzles until all the nozzles of the plurality of nozzles (22) have been classified in a correctly functioning state or in a malfunctioning state.

2. The method of claim 1, wherein determining whether one or more nozzles of a group of nozzles are in a malfunctioning state comprises aggregating the residual pressure wave sensed in step c) in the liquid in the plurality of ducts (16) of the nozzles of the group of nozzles, and determining that one or more nozzles of a group of nozzles are in a malfunctioning state if the aggregated residual pressure wave is below a predetermined threshold.

3. The method of any preceding claim, wherein dividing the plurality of nozzles (22) into a first group of nozzles and a second group of nozzles comprises grouping the nozzles of the plurality of nozzles (22) that have a higher likelihood of being in a malfunctioning state in one of the first and second group of nozzles.

4. The method of claim 3, wherein the plurality of nozzles (22) that have a higher likelihood of being in a malfunctioning state are determined based upon one or more of a failure rate higher than a predetermined threshold, a spatial adjacency to a nozzle with a failure rate higher than a predetermined threshold, and an observed correlation of failure during the execution of previous print jobs with a nozzle with a failure rate higher than a predetermined threshold.

5. The method of any preceding claim, wherein dividing the plurality of nozzles (22) into at least a first group of nozzles and a second group of nozzles comprises dividing the plurality of nozzles (22) equitably into at least a first group of nozzles and a second group of nozzles.

6. The method of any of claims 3 to 4, wherein dividing the plurality of nozzles (22) into at least a first group of nozzles and a second group of nozzles comprises dividing the plurality of nozzles (22) into at least a first group of nozzles and a second group of nozzles such that the nozzles of the plurality of nozzles (22) that have a likelihood of being in a malfunctioning state higher than a predetermined threshold are allocated into the same group of nozzles from the at least a first group of nozzles and a second group of nozzles.

7. A droplet ejection device comprising a number of ejection units arranged to eject droplets of a liquid and each comprising a nozzle (22), a liquid duct (16) connected to the nozzle (22), and an electro-mechanical transducer (28) arranged to create an acoustic pressure wave in the liquid in the duct (16), wherein each of the ejection units is associated with a processor (50) configured to perform the method according to any of the claims 1 to 6.

8. A printing system comprising the droplet ejection device according to claim 7 as an ink jet print head and a control unit suitable for executing the method according to any of the claims 1 to 6.

9. A software product comprising program code on a machine-readable non transitory medium, the program code, when loaded into a control unit of a printing system according to claim 8, causes the control unit to execute any of the methods of claims 1 to 6.

Drawing