Technical domain
[0001] The present invention concerns an inkjet printhead of a printer, in particular a
piezoelectric printhead, as well as the corresponding circuitry. The printhead is
adapted to establish a fast status or diagnostic of the nozzles, based on the sensed
deformation of the piezo elements. The invention also covers a printer and/or a system
equipped with such a printhead. The present invention further concerns a method of
determining the status of the nozzles of an inkjet printhead, based on the sensed
deformation of the corresponding piezo elements. The present invention further concerns
a method of compensating potential defects of an inkjet printhead, in particular during
printing operations, so as to maintain a good quality of impression.
Related art
[0002] In the field of inkjet printers, significant development has been made to improve
the reliability and the durability of the printhead.
[0004] Despite the efforts, there is still room for improvement of inkjet printheads, in
particular to increase the variety of sensed defects, to improve the sensing to inks
having unknown properties, to provide more accurate corrective actions, to automatically
implement some adjustments and/or any other improvements which become self-evident
in the following details.
Short disclosure of the invention
[0005] An aim of the present invention is the provision of a system and a method that overcomes
the shortcomings and limitations of the state of the art. It is in particular an aim
of the present invention to provide a system for inkjet system adapted to sense the
nozzles during a printing operation so that no time is wasted to determine the status
of the nozzle beside regular printing jobs. For industrial applications, maintenance
operations can have a significant cost due to the non functioning time of the printhead.
In particular, it is a further aim of the present invention to keep the possibility
of scanning the nozzle status between prints, when the printer is at idle.
[0006] Another aim is to provide a system for inkjet printhead which is adapted to accurately
discriminate several types of defects of the nozzles. It is in particular necessary
to improve the sensing quality and the diversity of defects.
[0007] Another aim of the present invention is to provide a system for inkjet printhead
allowing to automatically initiate curative or preventive operations, or compensation
programs, so as to maintain a good quality of impression despite some nozzle defects.
It is more particularly an aim to provide a system more reliable than the current
ones, wherein defects are precisely detected and automatically compensated, limited,
or avoided so that manual maintenance is limited.
[0008] It is another aim to provide a system, or modular elements adapted to be implemented
onto existing printers, so as to upgrade them to an improved level of performance,
reliability and durability.
[0009] Another aim of the invention is to provide an inkjet printer adapted to address all
or part of the above-mentioned problems. It is in particular an aim to provide an
inkjet printer adapted for industrial applications.
[0010] Another aim is to provide a method to detect, discriminate and characterized nozzle
defects of an inkjet printhead in an accurate and automatic way. The method is furthermore
aimed at being applicable during printing operations and in automatic way.
[0011] Another way is to provide a method of accurately sensing a variety of nozzle defects
of an inkjet printer and automatically provide adaptative responses of the printer
so as to anticipate, cure or compensate such nozzle defects.
[0012] According to the invention, these aims are attained by the object of the attached
independent claims, and better defined through the dependent claims. In particular,
the system according to the present description comprises one or several sensing units
each provided with a signal generator and a compensation network adapted to simulate
an ideal virtual nozzle. Each sensing unit is also provided with a mean to compare
the current of such an ideal virtual nozzle with the one of a real nozzle and a mean
to compute the signal differences so that a nozzle defect can be detected and characterized.
The system according to the present invention further comprises means to detect phase
differences, magnitude and/or capacitance so as to improve the sensing accuracy. In
particular, the system allows not only to detect nozzle defects but also surrounding
parameters having an impact on the impression, such as the ink characteristics, the
aging, the temperature etc...
[0013] The present method allows to sense some or all the nozzles of a printhead during
a printing operation. In particular, the sensing step can occur immediately after
a drop of ink is ejected. Furthermore, non ejecting sensing can still be performed,
for an improved flexibility, wherein the operator does not have to worry about unwanted
drops.
[0014] With respect to what is known in the art, the invention provides the advantage of
a more accurate and more complete sensing of an inkjet printhead. It further provides
the advantage of limiting or avoiding manual control and maintenance operations.
Short description of the drawings
[0015] Exemplar embodiments of the invention are disclosed in the description and illustrated
by the following drawings :
Figure 1 : Schematic representation of a system according to an embodiment of the
present description,
Figure 2 : Details of a sensing unit according to an embodiment of the present description.
Figure 3: Schematic representation of a system according to an embodiment of the present
description.
Figure 4: Schematic representation of a system according to an embodiment of the present
description.
Figure 5: Schematic representation of a system according to an embodiment of the present
description.
Figure 6: Example of an arrangement of the sensing unit according to an embodiment
of the present description.
Figure 7: Example of an arrangement of the sensing unit according to an embodiment
of the present description.
Figure 8: Schematic representation of a system according to an embodiment of the present
description.
Figure 9 : Schematic representation of a system according to an embodiment of the
present description.
Figures 10, 10a, 10b, 10c, 10d: schematic representation of the different unit of
the system according to different embodiments of the present description.
Figure 11a: Schematic representation of an arrangement according to the present description
during an activation step.
Figure 11b: Schematic representation of an arrangement according to the present description
during a sensing step.
Figure 12a: Schematic representation of the driving and sensed signal for a non-printing
activation step according to the present description.
Figure 12b: Schematic representation of the driving and sensed signal for a printing
activation step according to the present description.
Examples of embodiments of the present invention
[0016] With reference to figure 1, the system according to the present description comprises
a printhead 1, at least one nozzle bank 10 comprising several nozzles 11 and a nozzle
command unit 12 adapted to pilot the nozzles according to a determined printing operation.
The system also comprises at least one nozzle sensing unit 20 adapted to detect potential
defects of a nozzle or a nozzle bank. The system further comprises at least one printing
power amplifier 30, which activates the nozzles or some of the nozzles to provide
a suitable inkjet, and a printing amplifier switch 31 so as to be connected to a nozzle
or a nozzle bank.
[0017] A nozzle bank here denotes a group or an array of several nozzles. A system according
to the present description can comprise 1 or 2 banks of nozzles or a higher even number
of banks such as 4, 8, 16 banks. Independently of the number of banks, each nozzle
can be activated by means of one or several piezoelectric actuators, better described
below. For the purpose of the present description, a pair of nozzles or a pair of
nozzle banks defines a pair of nozzles or nozzle banks which are both connected to
a given sensing circuit.
[0018] Unless specified differently, a nozzle is here understood as comprising or being
combined to the necessary piezo actuator or piezo actuators, adapted to eject the
ink through the nozzle.
[0019] The nozzle sensing unit
20 is here described in line with figure 2. It comprises a signal generator
22 adapted to generate a sensing excitation signal to one or several nozzles. The signal
generator
22 can be coupled to a sensing amplifier
23. The sensing amplifier
23 is designed to have low power and low noise. The ensemble defined by the signal generator
22 and the sensing amplifier
23 can be disconnected or connected to a sensing line
210 by means of sensing amplifier switch
21. The signal generator
22 is particularly adapted for a non-printing activation or for calibration of the corresponding
nozzle or nozzle bank.
[0020] The sensing line
210 can be connected to the printing line
200 at the connection point
220.
[0021] The sensing unit
20 further comprises a mean to compare the two currents passing through the printing
line
200 and through the sensing line
210. Such a mean can be a difference amplifier
25, connected to the printing line
200 through a first connection line
240 and to the sensing line
210 through a second connection line
241. In such a way, the mean to compare the two currents can detect and measure difference
between the two currents passing through the printing line and through the sensing
line. Alternatively, the mean to compare the two currents passing through the printing
line
200 and through the sensing line
210 can be a high input common mode difference amplifier. It is understood that any suitable
device adapted to compare two currents, detect a difference between two currents and/or
measure such a difference can be used.
[0022] The sensing unit
20 comprises or is combined with an analogue to digital conversion device
27..
[0023] The mean to compare the two currents passing through the printing line
200 and through the sensing line
210, in particular the difference amplifier
25 or the alternative high input common mode difference amplifier, can be combined to
a filtering unit
26, adapted to filter the measured currents of the printing line
200 and the sensing line
210, and/or to condition or transform the signal in a way to be properly analysed.
[0024] The printing line
200 comprises a first bypass
201 adapted to bypass the mean to compare the two currents passing through the printing
line
200 and through the sensing line
210. The first bypass
201 is typically activated when the corresponding printing power amplifier
30 is activated so as to operate the printing. The printing line
200 further comprises a first shunt
202 adapted to connect the printing line
200 and the first connection line
240. The first shunt
202 allows the mean to compare the two currents passing through the printing line
200 and through the sensing line
210 to measure the current passing through the printing line
200. Typically, the first shunt
202 is activated soon after the corresponding printing power amplifier
30 is deactivated, so as to detect the residual current. A sensing step can occur as
soon as few microseconds after the drop ejection. To this end, the switches, in particular
the sensing amplifier switch 21 and the printing power amplifier switch
31 have leakage and parasitic capacitance as low as possible.
[0025] The sensing line
210 comprises a second bypass
211 and a second shunt
212 at the connection point between the sensing line
210 and the second connection line
241.
[0026] The sensing line
210 comprises a sensing compensation network Such a sensing compensation network can
take the form of an impedance matching circuit
24. It can comprise for example a variable capacitor
24a and a resistor
24b. It allows to mimic the current signal of a virtual nozzle having an ideal response
during and/or after an activation of the corresponding nozzle
11 or nozzle bank
10. The sensing compensation network can allow to mimic a virtual ideal nozzle during
the activation of the corresponding printing power amplifier
30. It is however necessary that the sensing compensation network mimics an ideal virtual
nozzle during the sensing period, through the activation of the corresponding signal
generator
22 and sensing amplifier
23. Thus, the sensing compensation network can be active permanently during a printing
operation, or at least activated for the sensing step. This is particularly convenient
in case of a sensing step following a printing activation step, wherein an ink drop
is ejected by the corresponding nozzle
11 or nozzle bank
10.
[0027] For a better performance, the sensing compensation network can comprise or being
combined with an inductive element
28. Such an inductive element
28 is adapted to simulate the lead inductance of the cable connections which are used
to connect the sensing unit
20 to the other parts of the printhead.
[0028] Coming back to figure 1, the sensing unit
20 can further comprise isolated shunt amplifiers allowing a better performance of the
sensing. A first isolated shunt amplifier
203 can be combined to the first shunt
202, on the printing line
200. A second shunt amplifier
213 can be combined to the second shunt
212 on the sensing line
210. The difference amplifier
25 thus receives the currents from the first shunt amplifier
203 and the second shunt amplifier
213 so that a more accurate measure can be performed.
[0029] Figures 2 represents a simplified version of figure 1, comprising only one nozzle
bank.
[0030] Figure 3 illustrates an embodiment wherein the present printhead comprises 2 nozzles
banks
10, 10' and 2 printing power amplifiers
30, 30'. The same reference numbers correspond to the same features already described. Under
such a configuration, the sensing line corresponds to a second printing line
200' connected to a non active nozzle bank
10' at the time when the sensed nozzle bank
10 is activated. In particular, the corresponding printing power amplifiers
30' remains inactive, so that no ink is ejected. As previously described, the sensing
unit
20 comprises a signal generator
22 and a sensing amplifier
23, which are connected to the printing line
200 by means of a sensing amplifier switch
21 and a connection point
220. A second printing power amplifier
30' is connected to a second printing line
200', by means of a second printing power amplifier switch
31'. The second printing line
200' is linked to a second nozzle bank
10', independent from the first nozzle bank
30, and comprising several second nozzles
11' and a second nozzle command unit
12'. The ensemble of signal generator
22 and sensing amplifier
23 is also connected to the second printing line
200' by means of a second sensing amplifier switch
21'. According to such a configuration, the ensemble of signal generator
22 and sensing amplifier
23 can independently activate the first or the second printing lines, which thus plays
the role of a sensing line. When the first nozzle bank
10 is activated, the second nozzle bank
10' remains inactivated so that it can be used to simulate a virtual ideal nozzle. The
second printing line
200' comprises a second first bypass
201' and a second first shunt
203'. The mean to compare the two currents passing through the first printing line
200 and through the second printing line
200', in particular the difference amplifier
25 or the alternative high input common mode difference amplifier, can thus detect and
characterize the signal difference at the sensing step, just after the ejection of
an ink drop by the first nozzle bank
10. The reverse configuration is also possible, saying that when the second printing
line
200' is activated to initiate ejection of an ink drop by the second nozzle bank
10', the first printing line
200 is used as a sensing line.
[0031] Figure 4 represents a similar arrangement wherein the shunts are combined with shunt
amplifiers
203, 203'. In particular the first shunt
202 is combined with a first shunt amplifier
203. The second first shunt
202' is combined with the second first shunt amplifier
203'.
[0032] For better performances, one or several printing lines can be provided with compensation
networks
24, 24', each comprising a variable capacitor and a resistor. In this case, there is no need
for an inductive element
28 since the printing lines are actually connected to nozzle banks through a connection
cable. The compensation networks
24, 24' have the same role and effect as above-described.
[0033] For all the above-described arrangements, the mean to compare the two currents passing
through the printing line
200 and through the sensing line, is preferably a difference amplifier
25 when the shunt amplifiers
203, 203', 213 are present. Alternatively, it corresponds to a high input common mode difference
amplifier when the circuit does not comprise the shunt amplifiers
203, 203', 213 above described.
[0034] Figure 5 illustrates a configuration wherein the printhead comprises an even number
of nozzle banks, such as 4, 8, 16 nozzle banks. In this particular case, the feature
of the sensing unit
20 above described for two nozzle banks are duplicated. All the variations above-mentioned
for the sensing unit are of course applicable. It is for example possible to include
compensation networks or not. It is further possible to include shunt amplifiers or
not.
[0035] The system according to the present description can comprise a mean to determine
the phase and/or the magnitude of the pressure waves during the sensing step. Figure
8 shows an example of such an arrangement. The sensing unit
20 comprises a phase comparator
260, a logarithmic amplifier
270 or a combination of both. The phase comparator
260 is combined to an analogue to digital conversion unit (ADC) for phase response
261. The logarithmic amplifier
270 is combined to an analogue to digital conversion unit (ADC) for magnitude response
271. According to a possible arrangement, the phase comparator
260 is connected to the assembly of a signal generator
22 and a sensing amplifier
23 through a first phase line
263 and just after the means for comparing the two currents passing through the printing
line
200 and through the sensing line, by means of a second phase line
264. The phase difference between the original signal delivered by the signal generator
22 and the sensed signal can thus be determined. The logarithmic amplifier
270 can be connected through the second phase line
264 to the same point as the phase comparator
260, that is to say after the means for comparing the two currents passing through the
printing line
200 and through the sensing line. One or both of the phase difference and the amplitude
of the signal can be detected during the sensing step, providing a more accurate analysis.
This circuits can especially be used by exciting the piezo with a standing wave, such
as a sinusoidal or other repetitive excitation, during a non-printing sensing step,
wherein no ink drop is ejected. In this specific case, excitation and sensing can
be done simultaneously. Although the phase and the magnitude detection system is here
described in combination with a specific sensing unit
20, all the above-mentioned variations can be considered. For example, compensation networks,
and or shunt amplifiers, can be present or absent.
[0036] Alternatively or in addition, the system according to the present description can
comprise a capacitance measurement system
CC, such as shown in figure 9. A capacitance measurement system
CC can be connected to the first printing line
200 to a second printing line
200' or a sensing line
210 through a first capacitance switch
C1, connecting the capacitance system
CC to the first printing line
200, and a second capacitance switch
C2, connecting the capacitance system
CC to the second printing line
200'. The capacitor measurement system
CC can comprise any suitable element. It can for example be based on constant current
charging, on charge transfer or on an oscillator circuit. Although the capacitor measurement
system is here described in combination with a specific sensing unit
20, all the above-mentioned variations can be considered. For example, compensation networks
and/or shunt amplifiers can be present or absent. It is also applicable to the configuration
based on one printing line and a sensing line
210.
[0037] In the above-described embodiments, illustrated by figures 1, 2, 3, 4, 5, 8 and 9,
a sensing unit
20 is provided for several nozzles. In particular, a sensing unit
20 is provided for at least one nozzle bank
10, 10'. In this configuration, only one nozzle of a given nozzle bank is activated so that
the sensing activity can be performed.
[0038] According to another embodiment, each nozzle, or some of the nozzles, can be individually
sensed, meaning that each nozzle, or some of them are connected with a sensing unit
20 as above described. Figures 6 and 7 illustrate some examples of such a configuration.
[0039] In figure 6, The system comprises a printing power amplifier
30 adapted to activate several nozzles
11a, 11b, 11c...11n. The nozzles are individually commanded by means of a suitable nozzle command unit
12. Each one of the nozzles is connected to a dedicated sensing unit
20a, 20b, 20c...20n. The sensing units can be integrated with the corresponding nozzle, inside the printhead.
Alternatively, the sensing units
20a, 20b, 20c...20n can be arranged remote the corresponding nozzles. According to an embodiment, each
sensing unit comprises a printing line and a dedicated sensing line, as described
above in combination with figures 1 or 2. With such a configuration, all the nozzles
can be tested simultaneously. Alternatively, a sensing unit can be connected to 2
nozzles, with 2 distinct printing lines, one of which can be used as a sensing line,
as above-described in combination with figures 3 and 4. In this specific case, half
of the nozzles can be simultaneously tested. In figure 6, the sensing units
20a, 20b, 20c...20n are arranged between the printing power amplifier
30 and the corresponding nozzles
11a, 11b, 11c...11n. Suitable switches such as the printing amplifier switch
30, 30' above described can be provided. Each one of the sensing units
20a, 20b, 20c...20n comprises an assembly of a signal generator
22, a sensing amplifier
23 and a sensing amplifier switch
21, such as those described above.
[0040] Figure 7 illustrates a configuration wherein the sensing units
20a, 20b, 20c...20n are arranged downstream the corresponding nozzles
11a, 11b, 11c...11n. Any other suitable arrangement can be considered.
[0041] Independently of the arrangements above described, a given nozzle can be activated
by one piezo actuator or by several piezo actuators. For example, each nozzle, or
some of them, can be activated by 2, 3, 5, 7 or more piezo actuators. An accurate
sensing system is thus necessary, such as the sensing system here described. According
to a configuration, each piezo actuator, or a part of the several piezo actuator dedicated
to a given nozzle, can be connected to a dedicated sensing unit. A given nozzle is
driven by the corresponding printing power amplifier.
[0042] Figures 10, 10a, 10b, 10c, 10d show examples of different arrangements of the sensing
unit withing the global system comprising the printhead
1. In particular, the sensing system according to the present description comprises
the sensing unit
20 above-described. It is connected to a command unit
U1 of the printhead. Such a command unit
U1 comprises at least the electronic means adapted to pilot the nozzles during a printing
operation or a maintenance operation. Printhead connection cables
PH allow to connect the different units of the system. A signal processing unit
U2 is provided downstream the sensing unit
20 so as to analyse the sensed signal transmitted by the sensing unit
20. The signal processing unit
U2 comprises all the necessary computational means, adapted to detect and identify a
printing default at a given nozzle or nozzle bank. A default can also be characterized
so as to identify the type of default. It can be for example a bubble air, a problem
of viscosity, a complete or partial obstruction of a nozzle, an inkjet deviation or
any other parameter. A given type of defect can be identified based on predetermined
signal responses in the signal processing unit
U2. The computation of the sensed data in the signal processing unit
U2 allows to obtain a response signal
RS. The response signal
RS can be limited to the detection of a default or a failure at a given nozzle and an
alert, so that a user can take suitable actions to remedy to the default. Alternatively,
the response signal
RS can comprise predetermined instructions corresponding to the identified defects so
that the actions of the user actually corresponds to the type of defect. Alternatively
or in addition, the response signal
RS can comprise a command loop adapted to automatically compensate the defect. In case
a certain type of defect is identified on a given nozzle or nozzle bank, the command
unit
U1 can receive adequate instructions based on the response signal
RS so as to automatically compensate the defect. This does not exclude that an alert
signal is also sent to a user.
[0043] Figure 10a illustrate a standard configuration without any sensing unit as described
here. Figure 10b corresponds to a configuration wherein the sensing unit
20 represents an independent module arranged between the printhead, which comprises
the nozzles, and the command unit of the printhead. This configuration allows the
retrofit of existing systems so that they can be upgraded with a sensing unit as described
here. External printhead connection cable
PH can be used to associate the sensing unit to the other modules of the systems, in
particular to the printhead
1 and its command unit
U1.
[0044] Figure 10c represents an arrangement wherein the sensing unit
20 is integrated into the command unit
U1 of the printhead
1. A more compact arrangement is here possible. It also allows to combine or merge some
functions with some of the command functions, for example by using common features
already present in the command unit
U1. For example the signal processing unit
U2 can share some functions with the command unit
U1. A printhead connection cable
PH can be used to connect the printhead
1 to the assembly of sensing unit
20 and command unit
U1.
[0045] Figure 10d represents an arrangement wherein the sensing unit
20, as well as the corresponding signal processing unit
U2 is integrated in the printhead
1, meaning that it remains close to the nozzles. A printhead connection cable
PH can be used to connect the command unit U1 to the assembly of the sensing unit
20 and the printhead
1.
[0046] All the above-described circuits are adapted for providing an activation of one or
more nozzles or nozzle banks and their sensing. The activation and the sensing are
successive. This is in particular the case when the activation corresponds to an ink
drop ejection, meaning that the corresponding printing power amplifier
30 is activated. In this specific case, the sensing of the corresponding nozzle or nozzle
bank occurs just after the activation step, as better described below. The present
description also covers a method for sensing at least one nozzle or at least one nozzle
bank of an inkjet printhead 1 described here. The method is based on the comparison
of a signal provided by a piezo actuator of a nozzle and an ideal virtual signal provided
by a signal generator
22 as above described. The signal comparison is done through a mean to compare the two
currents passing through a printing line
200 and through a corresponding sensing line, being either a dedicated sensing line
210 or another printing line
200', which is inactivated and used for a virtual ideal signal. Depending on the configuration,
the mean to compare the two currents can be a difference amplifier
25 or a high input common mode difference amplifier or any other suitable circuit.
[0047] The method comprises an activation step
51. The activation step of at least one nozzle or one nozzle bank includes the excitation
of the corresponding piezo actuator by mean of a suitable electrical signal. The activation
step
S1 corresponds to a printing step wherein at least one ink drop is ejected from the
corresponding nozzle or nozzle bank.. Figure 11a represents the circuit configuration
under such an activation step. One printing power amplifier
30 is activated and connected to the corresponding printing line
200. This means that the corresponding printing power amplifier switch
31 is closed so that a drop of ink is ejected from a nozzle
11 according to a predetermined program, which can correspond to a printing program
or only to a sensing program wherein some drop of ink are ejected. Preferably, only
one printing power amplifier
30 of a pair of printing power amplifier
30, 30' is activated so that only one nozzle bank
10 of a pair of nozzle banks
10, 10' is activated during the activation step. This is applicable to arrangements wherein
a second printing line is used as a sensing line, as described above. Preferably,
only one nozzle
11 of the corresponding nozzle bank
10 is activated where applicable. Alternatively, only one nozzle of a pair of nozzles
is activated where applicable. The activation step
S1 applies in the same way in case of a single printing power amplifier
30 as described for example in figures 1 and 2 above, wherein the sensing line is a
dedicated line. During the activation step
S1, the signal generator 22 is disconnected from all the lines including the printing
lines and the sensing lines. In other words, all the corresponding sensing amplifier
switches
21, 21' are open. Also, if applicable, the compensation networks
24, 24', when present, are not active. Also, the sensing circuit is bypassed, through the
suitable first
201, 201' and second
211, 211' bypass. A pair of nozzles or a pair of nozzle banks define a pair of nozzles or nozzles
banks which are both connected to a given sensing circuit.
[0048] The method comprises a sensing step
S2. The sensing step
S2 occurs just after an activation step S1 above described. The sensing step
S2 allows to sense the piezo actuator, or some or all of the piezo actuators, of the
nozzle which has been activated. In particular, the sensing step allows to sense the
acoustic response of the piezo actuator or piezo actuators after activation of the
corresponding nozzle. It allows to sense the response of the corresponding piezo actuator
or piezo actuators so as to determine the status of the corresponding nozzle. Figure
11b illustrates the circuit configuration under the conditions of the sensing step
S2 after an activation step The printing power amplifier
30 is deactivated, for example by opening the corresponding printing power amplifier
switch
31, 31'. The signal generator
22 is connected to all the lines, including the printing line
200 which as been used during the activation step
S1 and the other printing line
200' or the sensing line
210. This means that all the corresponding sensing amplifier switches
21, 21' are closed. The nozzle or the nozzle bank, where applicable, which has not been activated
during the activation step
S1, are compensated by means of the corresponding compensation network
24, 24'. In particular, variable capacitor and resistor comprised in the compensation network
are tuned in a way to simulate the characteristics of the nozzle, or nozzle bank,
which as been used during the activation step
S1. With an ideal compensation, the current difference between the nozzle or nozzle bank
which has been activated during the activation step
S1 and the non active nozzle or nozzle bank only corresponds to the response of the
corresponding piezo actuator or piezo actuators. The current difference is determined
by a mean to compare the two currents passing through the printing line
200 and through the sensing line, whether it is a difference amplifier
25 or a high input common mode difference amplifier or an equivalent circuit. The pressure
waveform, resulting from the activation can thus be reconstructed, wherein the measure
current is proportional to the derivation in respect of time of the pressure exerted
onto the piezo. The sensing step
S2 applies similarly in case only one printing line
200 and a dedicated sensing line
210 is provided. During the sensing step
S2, the suitable first
202, 202' and second
212 shunt are activated so that the current can be measured by the mean to compare the
two currents passing through the printing line
200 and through the sensing line.
[0049] The sensing step
S2, although it occurs short after a printing activation step
S1 for a given nozzle, can be activated during a printing program, wherein several nozzles
are successively activated and deactivated. There is thus no need to stop the printing
program so as to apply a specific maintenance program. The switches of the system,
in particular the sensing amplifier switches
21, 21' and the printing power amplifier switches
30, 30', are designed so as to have a very low leakage and parasitic capacitance so as to
not disturb the system. The shunt and shunt amplifiers
203, 303', 213 are advantageous elements allowing a very accurate current measurement.
[0050] Figure 12b represents the sensing signal after an activation step
S1. The piezo drive signal
PS is represented by the first diagram. It comprises an inactivation period
T1 wherein the measured piezo remains at idle. The following driving period
T2 corresponds to the activation of a piezo actuator, which is optimized to properly
eject a drop of ink. Just after the activation of the piezo starts the observation
period
T3, wherein the activated piezo is no longer driven by the corresponding printing power
amplifier
30, 30' while still being excited by effects resulting from the drop ejection. The sensed
signal
SS is measured during the observation period
T3.
[0051] The signal sensed during the observation period
T3 is stored in a memory for further signal processing, for example through a signal
processing unit
U2, as above described, so that a response signal
RS is generated.
[0052] The present method further comprises a computing step
S3 of computing the sensed parameters so as to provide a response signal
RS.
[0053] Depending an on signal provided by the signal generator
22 and the sensed signal, it is possible to discriminate the sensed defects. For examples
defects such as angle jetting, non-jetting, wetting, air bubbles can be identified.
[0054] The sensing step
S2 can further comprise a step of determining one or both of the phase and the magnitude
of the pressure waves resulting from the activation step
S1. The phase can be determined by means of a phase comparator
260 and the corresponding arrangement above-described. The amplitude can be defined by
means of logarithmic amplifier
270 and the corresponding arrangement above-described.
[0055] The sensing step
S2 can alternatively or in addition comprise a capacitance measurement. Such a capacitance
measurement can be performed by means of the capacitance measurement system
CC and the corresponding arrangement above-described. According to embodiment, the
CC system does the activation of the nozzle itself. This means that the sensing amplifier
switches
21, 21', and the printing amplifier switches
31 and
31' are open and the capacitance switches
C1 and
C2 closed.
[0056] The present method further comprises a calibration step
S0 adapted to determine the electrical characteristics of the virtual ideal nozzle.
The calibrating step
S0 comprises an excitation of the nozzle through the corresponding signal generator
22 and a concomitant sensing of the response of the piezo-actuator or piezo actuators.
The currents passing through the printing line
200 and the sensing line are compared. The characteristics of the compensation network
24, 24' are tuned so as to mimic the real nozzle.- The calibration step
S0 can be performed according to different activation profiles depending on the parameters
to be sensed. For example, the characteristics of a nozzle can be determined by exciting
the corresponding nozzle according to a predetermined voltage and/or frequency so
as to obtain a reference waveform of the measure response. The corresponding sensing
can then involve a damped oscillator function fitting with the activation parameters
in real time. Other methods such as Fourrier transformation, wavelet transformation
can be considered. In case of standing wave sinusoidal excitation a frequency sweep
with phase and magnitude mapping can be considered. For example, the nozzle can be
excited with a sinusoidal sweep from 60KHz to 600KHz so that the magnitude and phase
response can be mapped and resonance and/or antiresonance frequencies are determined.
Alternatively, a transient mode is used wherein a pulse waveform having defined low
and high level, pulse width and slew rate is applied to one nozzle pf the printhead,
which is preferably empty, but which can also be filed with some ink. After charging
an new ink and/or when the temperature varies, the sensing system can be tuned. To
this end, the set resistance can be tuned to zero, the sweep capacitive can be tuned,
the capacitor value and/or the resistor value can be selected so that the sensed signal
is the lowest, the sweep resistive can be tuned. Once the sensing system is tuned,
the active nozzle or nozzle bank of a given pair of nozzle or nozzle bank as the same
impedance that the corresponding non active nozzle or nozzle bank.
[0057] The calibration step
S0 is performed in absence of any printing operation. In particular, the printing power
amplifier 30, 30' are inactive.
[0058] The response of a given nozzle under standard conditions can then be determined.
The standard conditions can vary with the type of ink, the temperature and/or any
other parameters. The characteristics of a nozzle can thus be determined even with
an ink having unknown characteristics, and in particular rheological characteristics.
This allows to adapt the calculation step to do correct sensing.
[0059] According to an embodiment, the rheological properties of an ink can be determined.
For example, a nozzle can be excited with a sinusoidal signal over a large frequency
range while preventing any drop ejection. The nozzle excitation step can corresponds
to the calibration activation step
S0a above-described. The acoustic frequency and the phase response can be mapped during
concomitant sensing step, such as the calibration sensing step
S0b. The rheological properties of the ink can be deduced from the magnitude and phase
of the responding signal. In this case, the necessary phase comparator, logarithmic
amplifier and the corresponding arrangements are necessary.
[0060] Figure 12a represents an example of sensed signals measured during a calibration
step
S0. The piezo drive signal
PS is represented by the first diagram. It comprises an inactivation period
T1 wherein the measured piezo remains at idle. It comprises a driving period
T2 wherein the measure piezo is activated by means of the corresponding signal generator
23. . The driving period
T2 can correspond to a calibration activation step
S0a.. The driving period
T2 is followed by an observation period
T3 wherein the considered piezo actuator is still under excitation. It can be considered
as a calibration sensing step
S0b, concomitant to the excitation of the nozzle. The sensed signal
SS corresponds to the response over time of the excited piezo. The observation period
T3 allows to properly determine the electrical characteristics of an ideal virtual piezo
and to tune the compensation network accordingly. It is followed by a relaxation period
T4. Another period after the relaxation period
T4 can be determined as a monitoring period
T5. A second calibrating sensing step
S0c can be performed after the relaxation period
T4, during a monitoring time
T5, so as to monitor the piezo.
[0061] According to an embodiment, the meniscus pressure can be sensed. The resonance frequency
of all the nozzles of the printhead can be determined and averaged. It is possible
to determine the mean resonance frequencies for different known meniscus pressures
so as to built a calibration table. Based on such a calibration, an unknown pressure
condition can be quantified and characterized. This measurement can be improved by
the rheological measurement and the capacitance measurement to compensate for ink
changes (temperature, viscosity...).
[0062] According to an embodiment, the flow rate can be determined. To this end, a piezo
actuator can be heated by electrical excitation. The cooling effect of the flow rate
can be sensed for each nozzle so as to determine the corresponding flow rate. The
temperature can be determined based on the piezo capacitance and the leakage current
sensed by the system. Such a piezo capacitance and leakage current can be determined
by mean of the capacitance measurement system and the corresponding arrangement above
described. The flow rate distribution over the different nozzles can thus be determined.
[0063] According to an embodiment, the aging of the system is determined. A capacitance
variation can be attributed to a certain aging of the system. When testing all the
nozzles as above described, and determine their capacitance, the corresponding aging
can be deduced. A mean capacitance can also be determined. Alternatively, the capacitance
can be used to identify some electronic failures.
[0064] It results that parameters such as nozzle status, ink rheology characteristics, flow
mapping, capacitance mapping can be sensed according to the present method. They can
be individually analysed or combined so as to identify more specific parameters. The
sensed parameters can be combined to other parameters originating from different sources,
being integrated to the printer or remote. According to an embodiment, the data above-mentioned
can be stored in a memory and computed with an artificial intelligence program.
[0065] Different actions can be initiated based on the above data. For example, one or several
maintenance operations can be initiated as reactive and/or as preventive measure,
use redundant nozzle to compensate failure of a given nozzle, adjust the printing
parameters of nozzles adjacent a non-functional nozzle such as over-printing, replacing
a non functioning nozzle by combination of alternative colours, adapting the ejection
waveform, activate a nozzle with a tickling pulse for recovery or repair, correct
ink system parameters such as pressure, flow rate, temperature reverse flow, alert
an operator, etc..
[0066] The present method comprises an automatic adjusting step
S4 based on the response signal
RS resulting from the sensed signal. Additional data can be combined to the sensed signal,
where appropriate, so that the response signal is more accurate. For example, data
originating from external devices such as ambient temperature, hygrometry, or know
physical parameters of the used ink can be considered and computed by the signal computing
unit
U2 or another dedicated unit.
[0067] The aim of such an automatic adjusting step
S4 is to modulate the driving signal of a given nozzle in response to the sensed signal
and potential additional data, so as to avoid or limit the manual intervention of
an operator.
[0068] Based on the sensed data, when a defect is identified, some or several actions are
automatically initiated during the automatic adjusting step
S4. These actions comprises automatically use redundant nozzle to compensate failure
of a given nozzle, automatically adjust the printing parameters of nozzles adjacent
a non-functional nozzle such as over-printing, automatically replacing a non functioning
nozzle by combination of alternative colours, automatically adapting the ejection
waveform, automatically activate a nozzle with a tickling pulse for recovery or repair,
automatically correct ink system parameters such as pressure, flow rate, temperature
or reverse flow. Other actions can be performed depending on the needs.
[0069] Furthermore, based on the sensed data, the characteristics of the ideal virtual nozzle
can be automatically adjusted. In particular, the compensation network can be automatically
tuned so as to mimic an ideal virtual nozzle.
reference symbols in the figures
[0070]
- 1
- Printhead
- 10, 10'
- Nozzle bank
- 11, 11', 11a, 11b, 11c, 11n
- Nozzle
- 12, 12'
- Nozzle command unit
- 20, 20a, 20b, 20c, 20n
- Sensing unit
- 21, 21'
- Sensing amplifier switch
- 22
- Signal generator
- 23
- Sensing amplifier
- 24, 24'
- Compensation network
- 24a
- Variable capacitor
- 24b
- Resistor
- 25
- Difference amplifier
- 26
- Filtering unit
- 27
- AD converting device
- 28
- Inductive element
- 200
- Printing line
- 201, 201'
- First bypass
- 202, 202'
- First shunt
- 203
- First shunt amplifier
- 210, 210'
- Sensing line
- 211
- Second bypass
- 212
- Second shunt
- 220
- Connection point
- 240
- First connection line
- 241
- Second connection line
- 260
- Phase comparator
- 261
- ADC for phase response
- 263
- First phase line
- 270
- Logarithmic amplifier
- 271
- ADC for magnitude response
- 30, 30'
- Printing power amplifier
- 31, 31'
- Printing power amplifier switch
- C1, C2
- Capacitance switch
- CC
- Capacitor measurement system
- U1
- Command unit of printhead
- U2
- Signal processing unit
- RS
- Response signal
- S1
- Activation step
- S2
- Sensing step
- S3
- Computing step
- S4
- Adjusting step
- S0
- Calibration step
- S0a
- Calibration activation step
- S0b
- Calibration sensing step
- S0c
- Second calibration sensing step
- T1
- inactivation period
- T2
- Driving period
- T3
- Observation period
- T4
- Relaxation period
- T5
- Monitoring period
- PS
- Piezo drive signal
- SS
- Sensed signal