Note: Descriptions are shown in the official language in which they were submitted.
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OPERATION OF DROPLET DEPOSITION APPARATUS
The present invention relates to methods of operation of droplet
deposition apparatus, particularly inkjet printheads, comprising a chamber
supplied with droplet fluid and communicating with a nozzle for ejection of
droplets therefrom; and means actuable by electrical signals to vary the
volume of said chamber, volume variation sufficient to effect droplet ejection
being effected in accordance with droplet ejection input data.
Apparatus of this kind is well known in the art. EP-A-0 364 136
shows a printhead formed with a number of ink channels bounded on both
sides by piezoelectric side walls which deflect in the direction of an
electric
field applied by electrodes on the wall surfaces, thereby to reduce the
volume of the ink channel and eject a droplet from an associated nozzle.
Unlike 'thermal' printheads in which each ink channel is provided with
a heater that can be actuated so as to generate a bubble of vapour which
pushes ink out of the channel via an associated nozzle, there is no need for
'variable volume chamber' printheads of the kind described above to heat the
ink in the channel.
However, the present inventors have discovered that heating of ink in
the chambers of a'variable volume chamber' printhead can take place,
particularly when it is operated at high frequency. Figure 1 of the
accompanying drawings is a plot of droplet ejection velocity U against the
amplitude V of the electrical signal applied to the piezoelectric side walls
of a
channel in a printhead of the kind shown in the aforementioned
EP-A-0 364 136. Plot A corresponds to a droplet ejection rate of one drop
every droplet ejection period, with each droplet ejection period lasting 0.25
milliseconds, whilst plot B corresponds to a droplet ejection rate of one drop
every 66 droplet ejection periods. It will be seen that for a given amplitude
V
of the electrical signal, a significantly faster droplet will be ejected by
the
printhead when operating at the higher ejection rate than at the lower
ejection rate. Such a velocity increase is attributable to a decrease in
viscous losses during the droplet ejection process due to a reduction in the
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viscosity of the ink, This in turn is the result of an increase in the
temperature of the ink between the two operating conditions A and B caused
by the heating of ink in the channel which, it is believed, is due to
inefficiencies in the printhead.
It will be appreciated that droplet ejection velocity has to be taken into
account when synchronising droplet ejection from the printhead with the
movement of the substrate relative to the printhead and that any variation in
velocity will manifest itself as droplet placement errors in the final print.
For
example, the drop placement tolerance is frequently specified as one quarter
of a drop pitch. Thus for a print matrix density of 360 dots per inch, the
drop
placement tolerance will be aX=18 pm. The variation in droplet ejection
velocity, DU is related to the dot placement tolerance by the formula
AU = UdZ.OX/h. U,,
where h is the flight path length (typically 1.0mm), Uh is the printhead
velocity relative to the print substrate (typically 0.7 ms'') and Ud is the
mean
droplet ejection velocity.
For mean droplet ejection velocities of 5,10 and 15 ms'', the
maximum acceptable variation in droplet ejection velocity is 0.65, 2.6 and
5.8 ms ' respectively. Thus there is a substantially greater allowable
tolerance in the drop velocity when the mean droplet ejection velocity takes
a value greater than 5ms1,
On the other hand, there is maximum droplet ejection velocity
('threshold velocity'), Uw, which corresponds to the onset of capillary
instability. In variable-volume (piezoelectric) printers, the inventors have
found Uthr to be usually in the range 12-15 ms ' when continuous high
frequency droplet ejection is sustained, although higher droplet ejection
velocities can be obtained during short bursts of drop formation.
It will also be appreciated that the rate at which a particular
chamber in a printhead is actuated will depend on the incoming droplet
ejection input data (which will be determined. by the image to be printed and
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generally vary from high to low). Thus in a printhead having a chamber
operating in accordance with Figure 1 and at a given amplitude - for
example 35 - of electrical signal V, droplet ejection input data causing the
chamber to eject droplets frequently (equivalent to plot A) will result in a
droplet velocity of 15 m/s whilst subsequent input data may only cause the
chamber to eject droplets at a lower rate (equivalent to plot B) and
consequently at a much reduced velocity of 2 m/s. Such a large (750%)
variation in ejection velocity will clearly lead to inaccuracies in the
placement
of the droplets and a reduction in the quality of the printed image. Such an
error may occur for every chamber in a multi-chamber printhead. The
degree of difference between these two conditions increases with ink
viscosity and also with operating frequency, making the control of this effect
particularly important in high speed printers.
It will also be evident from Figure 1 that there is only a narrow range
of magnitude V of actuation waveform - denoted W - over which droplet
ejection at both high and low rates can be guaranteed, This in turn severely
inhibits the operational flexibility of the printhead.
According to one aspect of the present invention, these problems are
solved at least in preferred embodiments by a method of operation of droplet
deposition apparatus comprising a chamber supplied with droplet fluid and
communicating with a nozzle for ejection of droplets therefrom; and actuator
means actuable by electrical signals to vary the volume of said chamber;
volume variation sufficient to effect droplet ejection being effected in
accordance with droplet ejection input data; the method comprising the steps
of controlling said electrical signals such that the temperature of the
droplet
fluid in said chamber remains substantially independent of variations in the
droplet ejection input data.
Such a method can avoid velocity variations between enabled
channels due to variations in ink viscosity which in turn are attributable to
temperature variants caused by differential actuation rates. Differential
actuation rates are of course a result of differences in the droplet ejection
input data between enabled channels.
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This aspect of the present invention also comprises the method of
operation of droplet deposition apparatus comprising first and second
chambers each supplied with droplet fluid and communicating with a nozzle
for ejection of droplets therefrom and having actuator means actuable by
electrical signals to effect droplet ejection selectively from said chambers
in
accordance with droplet ejection input data; the method comprising operating
said actuator means to effect droplet ejection from the first chamber but not
from the second chamber, and selectively electrically heating the fluid in the
second chamber to reduce the difference in temperature between fluid in the
second chamber and fluid in the first chamber.
Again, by reducing variation in the temperature of the droplet fluid
between first and second chamber, viscosity-related droplet ejection speed
differences can be reduced.
Thus again according to the invention there is provided a method of
operation of droplet deposition apparatus comprising a chamber supplied
with droplet fluid and communicating with a nozzle for ejection of droplets
therefrom; and actuator means actuable by electrical signals to effect droplet
ejection from the chamber in accordance with droplet ejection input data; the
method comprising controlling said electrical signals such that the maximum
droplet ejection velocity lies just below a threshold velocity (U,n,), as
hereinbefore defined and the variation in the droplet ejection velocity due to
variations in the temperature of the droplet fluid in said chamber lies within
a
range determined by constraints in drop landing position.
According to another aspect of the present invention there is provided
a method of operation of droplet deposition apparatus comprising a chamber
supplied with droplet fluid, a nozzle communicating with the channel for
ejection of droplets therefrom and actuator means having first and second
electrodes and actuable by a potential difference applied across first and
second electrodes to effect droplet ejection from the chamber via the nozzle;
the method comprising the steps of applying to the first electrode a first non-
zero voltage signal for a first duration, applying to the second electrode a
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second non-zero voltage signal for a second duration, the first and second
voltage signals b'eing applied simultaneousiy for a length of time less than
at
least one of said first and second durations.
This second aspect allows short potential pulses to be generated
using voltage waveforms that are of longer duration and thus simpler to
generate, not requiring complex and expensive circuitry. Such short pulses,
whilst generally applicable in printhead operation, are of particular use when
implementing the other aspects of the invention described above.
The novel principie of selectively electrically heating non-firing (drop
ejecting) chambers in a droplet deposition apparatus to reduce temperature
variations between the fluid in different chambers is applicable to any such
apparatus regardless of the mechanism by which the chambers are fired.
Thus in another aspect the invention provides a method of operation
of droplet deposition apparatus comprising a chamber supplied with droplet
fluid and communicating with a nozzle for ejection of droplets therefrom; and
actuator means actuable by electrical signals to effect droplet ejection in
accordance with droplet ejection input data; the method comprising the steps
of controlling said electrical signals such that the temperature of the
droplet
fluid in said chamber remains substantially independent of variations in the
droplet ejection input data.
According to another aspect of the invention there is provided a
method of operation of droplet deposition apparatus comprising a chamber
supplied with droplet fluid and communicating with a nozzle for ejection of
droplets therefrom; and actuator means actuable by electrical signals to vary
the volume of said chamber, volume variation sufficient to effect droplet
ejection being effected in accordance with droplet ejection input data; the
method comprising applying electrical signals so as to actuate said actuator
means without effecting droplet ejection from said nozzle, the electrical
signals being controlled in dependence on a further signal representative of
temperature.
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According to another aspect of the invention,
there is provided method of operating droplet deposition
apparatus comprising first and second chambers each supplied
with droplet fluid and communicating with a nozzle for
ejection of droplets therefrom and having actuator means
actuable by electrical signals to vary the volume of the
said chambers to effect droplet ejection selectively from
said chambers in accordance with droplet ejection input
data; the method comprising operating said actuator means to
effect droplet ejection from the first chamber but not from
the second chamber, and to selectively electrically heat the
fluid in the second chamber to reduce the difference in
temperature between fluid in the second chamber and fluid in
the first chamber.
According to a further aspect of the invention,
there is provided signal processing means configured for
operating a droplet deposition apparatus, the droplet
deposition apparatus comprising first and second chambers
each supplied with droplet fluid and communicating with a
nozzle for ejection of droplets therefrom and having
actuator means actuable by electrical signals to vary the
volume of the said chambers to effect droplet ejection
selectively from said chambers in accordance with droplet
ejection input data; the signal processing means comprising
means for operating said actuator means to effect droplet
ejection from the first chamber but not from the second
chamber, and means for operating said actuator means to
selectively electrically heat the fluid in the second
chamber to reduce the difference in temperature between
fluid in the second chamber and fluid in the first chamber.
Such a method in preferred embodiments may
facilitate more sophisticated control of the temperature of
the droplet deposition fluid.
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The present invention also comprises signal processing means
configured for carrying out the aforementioned methods and droplet
deposition apparatus incorporating such signal processing means.
Preferred features and embodiments of the present invention are set
out in the subordinate claims and the description that follows.
The invention will now be described by way of example only by
reference to the remainder of the accompanying drawings, in which:
Figure 2 illustrates a perspective exploded view of one form of ink jet
printhead incorporating piezoelectric wall actuators operating in shear mode
and comprising a printhead base, a cover and a nozzle plate;
Figure 3 illustrates the printhead of Figure 2 in perspective after
assembly;
Figure 4 illustrates a drive circuit connected via connection tracks to
the printhead to which are applied a drive voltage waveform, timing signals
and droplet ejection input data for the selection of ink channels, so that on
application of the waveform, drops are ejected from the channels selected;
Figure 5(a) and (b) show waveforms according to one embodiment of
the present invention;
Figure 6 illustrates the response of a piezoelectric actuator to a step
voltage input;
Figure 7 illustrates the variation in droplet ejection velocity U with
amplitude V of electrical signal applied to eject a droplet from a printhead
operated in accordance with the present invention;
Figure 8 shows the relationship between droplet ejection velocity U
and actuation pulse magnitude for a typical printhead of the type shown in
Figures 2 to 4;
Figure 9 is an embodiment of a non-droplet-ejecting actuating
waveform in accordance with the present invention;
Figure 10 is a further embodiment of a non-droplet-ejecting actuating
waveform;
Figure 11 shows the actuating voltage waveforms applied to six
adjacent channels operating in "multi-cycle" mode in accordance with the
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present invention.
Figures 12 to 15 show alternative embodiments of actuation waveform
to be applied to non-ejecting/enabled channel (e) and its neighbours,
together with the resulting potential difference across the wails bonding
channel (e);
Figure 16 illustrates the actuating voltage waveforms applied to four
adjacent channels in a "shared-wall" printhead when operating according to
another embodiment of the invention;
Figure 17 represents conventional greyscale operation in three
channels;
Figure 18 corresponds to the operation of Figure 17 when
incorporating the present invention;
Figure 19 illustrates the actuating voltage waveforms applied to four
adjacent channels when operating according to a second aspect of the
present invention;
Figure 20 illustrates the potential differences generated across the
walls of enabled channels when actuated by the waveforms of Figure 19;
Figures 21 and 22 correspond to the left-hand portions of Figures 19
and 20 when utilising a first aspect of the present invention; and
Figures 23 and 24 illustrate an alternative embodiment of the manner
of operation shown in Figures 19 and 20.
Figure 2 shows an exploded view in perspective of a typical ink jet
printhead 8 incorporating piezoelectric wall actuators operating in shear
mode. It comprises a base 10 of piezoelectric material mounted on a circuit
board 12 of which only a section showing connection tracks 14 is illustrated.
A cover 16, which is bonded during assembly to the base 10, is shown
above its assembled location. A nozzle plate 17 is also shown adjacent the
printhead base.
A multiplicity of parallel grooves 18 are formed in the base 10
extending into the layer of piezoelectric material. The grooves are formed as
described, for example, in the aforementioned EP-A-0 364 136 and
comprise a forward part in which the grooves are comparatively deep to
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provide ink channels 20 separated by opposing actuator walls 22. The
grooves in the rearward part are comparatively shallow to provide locations
for connection tracks. After forming the grooves 18, metallized plating is
deposited in the forward part providing electrodes 26 on the opposing faces
of the ink channels 20 where it extends approximately one half of the
channel height from the tops of the walls and in the rearward part is
deposited providing connection tracks 24 connected to the electrodes in
each channel 20. The tops of the walls are kept free of plating metal so that
the track 24 and the electrodes 26 form isolated actuating electrodes for
each channel. The base 10 may thereafter be coated with a passivant layer
for electrical isolation of the electrode parts from the ink.
Subsequently, the base 10 is mounted as shown in Figure 2 on the
circuit board 12 and bonded wire connections are made connecting the
connection tracks 24 on the base 10 to the connection tracks 14 on the
circuit board 12.
The ink jet printhead 8 is illustrated after assembly in Figure 3. In the
assembled printhead, the cover 16 is secured by bonding to the tops of the
actuator walls 22 thereby forming a multiplicity of closed channels 20 having
access at one end to the window 27 in the cover 16 which provides a
manifold 28 for the supply of replenishment ink. The nozzle plate 17 is
attached by bonding at the other end of the ink channels. The nozzles 30
are formed by UV excimer laser ablation at locations in the nozzle plate
corresponding with each channel.
The printhead is operated by delivering ink from an ink cartridge via
the ink manifold 28, from where it is drawn into the ink channels to the
nozzles 30. The drive circuit 32 connected to the printhead is illustrated in
Figure 4. In one form it is an external circuit connected to the connection
tracks 14, but in an alternative embodiment (not shown) an integrated circuit
chip may be mounted on the printhead. The drive circuit 32 is operated by
applying (via a data link 34) input data 35 defining locations in each print
line
at which printing - i.e. droplet ejection - is to take place as the printhead
is
scanned over a print surface 36. Further, a voltage waveform signal 38 for
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channel actuation is applied via the signal link 37. Finally, a clock pulse 42
is applied via a timing link 44.
As is known, e.g. from EP-A-O 277 703, appropriate application of
voltage waveforms to the electrodes on either side of a channel wall will
result in a potential difference being set up across the wall which in turn
will
cause the poled piezoelectric material of the channel walls to deform in
shear mode and the wall to deflect transversely relative to the respective
channel. One or both of the walls bounding an ink channel can be thus
deflected - movement into the channel decreasing the channel volume,
movement out of the channel increasing the channel volume - thereby to
establish pressure waves in the ink along the closed length of each channel,
also known as the 'active length' of the channel and denoted in Figure 2 by
'AL'.The pressure waves cause a droplet of ink to be expelled from the
nozzle.
It should be noted that in constructions of the type shown in Figures 2
to 4, it is usually convenient for connections to be made between the wall
electrodes internally to provide one electrode per channel: when a voltage
waveform signal is applied to the electrode corresponding to a channel and
a datum voltage waveform is applied to the electrodes of the neighbouring
channels (both controlled by the drive circuit 32 in response to droplet
ejection input data), the resulting potential differences across the walls
adjacent the channel then effect displacements of each wall causing the
volume and pressure in the ink in each channel to be either increased or
decreased. Regardless of whether the connections are made internally or
externally of the printhead, it is then convenient to describe the actuating
waveform as being applied "to a selected channel". In the waveform
representations in the Figures that follow, a positive signal would result in
the walls bounding a channel moving outwardly from the channel i.e. to
cause an increase in the voiume of the channel.
Figure 5 shows actuation waveforms for operating an inkjet printhead
in accordance with the present invention. Figure 5(a) shows a voltage
waveform of the 'draw-release-reinforce' type: part 50 of the signal causes
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an initial increase in the volume of the channel for a period of approximately
AL/c (AL being the active length of the channel, c being the speed of
pressure waves in the ink, 2AL/c being the period of oscillation of pressure
waves in the ink in the channel), with subsequent part 55 decreasing the
volume of the channel for a period of approximately 2AL/c to eject of a
droplet from the nozzle. Waveforms of this genre have already been
discussed in WO 95/25011. After completion of a droplet ejection period L,
the length of which will be determined by a number of factors including the
time taken for pressure waves in the chamber to die down, the actuation
waveform can be applied again to effect ejection of another droplet.
In printheads of the kind described above, it is believed that a
significant cause of heating of the ink is the transmission to the ink of heat
generated by hysteresis in the piezoelectric material when subjected to step
changes in the applied potential difference. Print data requiring frequent
firing of a channel will result in greater number of hysteresis cycles in the
respective actuators, resulting in the generation of significant amounts of
heat, much of which will be transferred to the ink, raising its temperature
and
reducing its viscosity. In contrast, in those channels which - due to the
incoming print data - are fired less frequently, there will be less heat
generation, less warming of the ink and therefore less reduction in ink
viscosity. Heat will of course be carried away from the channel by the drops
that are ejected, with frequently firing channels losing a greater amount of
heat than less frequently firing channels. Heat will also be lost from the
printhead as a whole due to convection and radiation. Nevertheless, it has
been found that the net energy input is greater in frequently firing channels
than in less frequently firing channels, giving rise to a variation in droplet
ejection velocity between channels which may manifest itself as droplet
placement errors on the printed page.
A solution to this problem according to one embodiment of the
invention involves the application of a first drop-ejecting actuation waveform
- which may well be known in the art per se - to the selected channel when
required to fire in accordance with the print data, and appiying a second
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waveform to the channel when required not to fire by the print data, one or
both of the waveforms being chosen such that the temperature change of
the droplet fluid in said chamber when actuated with said first drop-ejecting
actuation waveform is substantially equal to the temperature change of the
droplet fluid in said chamber when actuated with said second drop-ejecting
actuating waveform.
An example of a drop-ejecting waveform is illustrated in Figure 5(a).
An example of a corresponding, non-droplet ejecting waveform is shown in
Figure 5(b) and comprises a number n of square wave pulses of magnitude
A and duration d spread over the same droplet ejection period of duration L
as the drop-ejecting waveform. A combination of A, d and n are chosen so
as (a) to cause a change in the temperature of the droplet fluid substantially
equal to that caused by the drop-ejecting waveform, and (b) not to cause
drop ejection.
A waveform meeting conditions (a) and (b) may be established by a
simple process of trial and error, with parameters A, d and n being modified
until a consistent drop ejection speed (and ink temperature) is achieved
independent of the density of the firing signals applied to the chamber and
actuation means.
Figure 7 illustrates the improvement in performance obtained with the
present invention. Plot A is taken from Figure 1 and shows the variation in
droplet ejection velocity U with the magnitude V of the actuation waveform
for a printhead of the kind shown in Figures 2 to 4 operating with the
waveform of Figure 5(a) and at a droplet ejection rate of one drop every
droplet ejection period (0.25 milliseconds). Plot B' is the corresponding
characteristic for the printhead operating at a droplet ejection rate of one
drop every 66 droplet ejection periods but actuated with a non-ejecting
waveform of the kind shown in Figure 5(b) for each of the 65 intervening
droplet ejection periods.
The two characteristics, A and B', are practically the same, indicating
that the temperature of the ink in the channel is the same in both cases.
There will consequently be negligible variation in droplet ejection velocity
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with droplet ejection rate i.e. with droplet ejection input data. It will also
be
clear that droplet ejection at both high and low rates is possible over
practically the entire range of magnitudes V of the actuation waveform,
enhancing the operational flexibility of the printhead.
Alternatively, approximate vaiues for the parameters can be obtained
by consideration of the piezoelectric actuator itself. As has been explained
above, application of a voltage "to a selected channel" together with
application of voltages to neighbouring channels results in changes in the
potential difference across each of the walls bounding the selected channel.
Each potential difference change induces a current flow that in turn is
determined by the resistive and capacitive properties of the channel wall and
driving circuitry. The electrodes on either side of a wall of piezoelectric
material form a capacitor C whilst the electrodes themselves have resistance
R. A loss tangent, tanS, is also associated with the capacitor C, where Ctan6
- which may be regarded as a parallel, non-linear resistor - represents
hysteresis loss in the PZT in response to changes in the potential difference
between the wall electrodes. Further resistance, also usually non-linear, is
also associated with the drive circuit. Together, these can be treated as a
lumped R-C network (although a distributed R-C-L network might be a
more accurate model) and the current flow in response to a potential
difference change calculated using established electrical principles. This is
true not only of printhead of the kind shown in Figures 2 to 4 but of
piezoelectric actuators in general and many other kinds of actuators.
When the actuator is subjected, for example, to a step change in
potential difference as indicated by dashed line V in Figure 6, current will
flow in the circuitry associated with the actuator in an exponentially
decaying
manner (line i in Figure 6) with the initial magnitude lo of the induced
current
being proportional to the magnitude Vo of the voltage step and the decay
rate being determined by the RC time constant of the circuit. The energy
dissipated will be proportional to the integral of the square of the current
flow
which can be shown to be equal to an ohmic loss 0.5(CVo2) occurring in the
resistive elements of the circuit. In addition, a hysteresis loss of
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0.25.rr.(CVoZ1. tanS per step change is generated, where tanS takes a value
corresponding to the electric field in the piezoelectric wall. Therefore, a
doubling of Va will result in a quadrupling of the area under the curve i,
equating to a quadrupling of the energy dissipated, and if, for example, the
magnitude of a voltage step in a non-drop ejecting actuation waveform were
half that of an equivalent step of a drop ejecting actuation waveform, the
energy dissipated by the former would be one quarter that of the latter.
Hence four steps would be required in the non-drop ejecting actuation
waveform to achieve the same energy dissipation as the drop ejecting
actuation waveform.
In practice, less energy will be required because some heat is taken
from the channel by the ejected drop during firing whereas no such loss
occurs during the non-ejecting pulses. In actuators of the kind described
above, it has been found that over one half (approximately 60%) of the heat
loss from a channel is by conduction through the body of the printhead, with
the remainder (approximately 40%) being lost through droplet ejection. Thus
in a non-ejecting channel, the electrical signal need only generate sufficient
hysteresis loss to balance that energy lost through the body of the printhead.
It will be appreciated that waveforms such as that shown in Figure
5(a) comprise a number of voltage steps (or "edges"), each of which will
induce current flow and energy dissipation. All such steps need to be taken
into account in the calculation for condition (a). It will further be
understood
that the quadratic relationship between dissipated energy and voltage step
magnitude will not hold where current flow does not decay completely
between successive voltage steps. Indeed, control of the time that elapses
between successive steps in such a situation allows accurate control of the
amount of energy dissipated. In such situations the power flow will have to
be calculated by other methods as are well known.
As regards condition (b), the threshold value of pulse magnitude Vt
below which droplet ejection will not occur can be determined empirically for
any particular printhead design. Figure 8 illustrates the relationship between
dropiet ejection velocity U and actuation voltage pulse amplitude for a
typical
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printhead of the type shown in Figures 2 to 4.
Figure 9 shows a second form of non-firing actuating voltage suitable
for use in conjunction with the drop ejecting waveform shown in Figure 5(a).
In contrast to the waveform of Figure 5(b), it is the frequency content -
rather than the amplitude - of the waveform that is chosen so as to avoid
droplet ejection. Fourier analysis of the waveform of Figure 8 incorporating
ramp portions 60 would reveal a frequency spectrum deficient in those
frequencies necessary to excite droplet ejection from the printhead. The
amplitude and duration of such a ramp pulse could nevertheless be chosen
so as to generate the same temperature change in the ink.
The same concept lies behind the waveform illustrated in Figure 10:
whilst the amplitude of the pulses 65 might be greater than the threshold
voltage Vt shown in Figure 8, the overall frequency content of the waveform
is such that it will not excite droplet ejection.
The principles described above are generally applicable to any droplet
deposition apparatus comprising chamber, nozzle and piezoelectric actuator,
particularly where a plurality of such elements are arranged into an array,
the chambers being arranged in an array direction, as is well known in the
art. However, the underlying problems - and thus the need for a solution -
will be more acute in those devices wherein said piezoelectric material
extends over the major part of a wall of said chamber, as described e.g. in
US-A-4 584 590 and US-A-4 825 227, and especially in printheads
of the kind described with reference to Figures 2 to 4 in which the chamber
is one of a plurality of channels formed in a base, walls being defined
between said channels, with each wall comprising piezoelectric materiai
actuable by means of electrical signals to deflect said wall relative to said
channel, thereby to vary the volume of said channel.
Yet further refinements are possible when such methods of operation
are to be applied to a "shared-wall" device of the kind shown, for example,
in Figures 2 to 4 and in which it is not possible to simultaneously fire two
adjacent channels separated by a shared actuating wall. Such devices are
conveniently operated "multi-cycle" mode, whereby successive channels in
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the array are assigned to one of a plurality of groups in a regular manner
and each group of channels is enabled for droplet ejection in successive
droplet ejection periods. EP-A-0 278 590 discloses "two-cycle' operation,
where alternate channels are assigned to one of two groups and each
groups of channels is enabled for droplet ejection in alternate droplet
ejection periods. EP-A-0 376 532 describes the division of channels into
three groups, with each channel of a particular group being separated by
channels belonging to the other two groups, each group being enabled in
turn whilst the other two groups remain disabled. Operation with more than
three cycles is also possible.
In a corresponding embodiment of the present invention, it is only
necessary to apply the droplet ejecting or non-ejecting waveforms in
accordance with the print data to those channels belonging to the group
enabled for droplet ejection at that time. Such waveforms will be referred to
as 'enabled/ejecting' and 'enabled/non-ejecting' hereinafter.
Channels belonging to the remaining, disabled groups (of which there
are two in the case of three-cycle operation) can remain inactive and, in the
case of devices having electrodes in the channels as described above, this
entails applying a common actuating signal to the channel electrodes of the
disabled channels. As a result, no electric field will be set up across the
wall
which separates the two disabled channels and this will remain stationary. A
channel (in this case the disabled channels) will not eject a droplet if one
or
both of its walls does not move. At the end of the period of enablement of
the enabled channel group, one of the other channel groups may be enabled
as is well known in the art. Such operation is disclosed in W095/2501 1.
Figures 11 to 16 illustrate implementations of the above principles.
Lines (a)-(f) of Figure 11 show the voltages applied to the electrodes
of six adjacent channels (a)-(f) in a 'shared-wall' printhead. Successive
channels are assigned to one of three groups in a regular manner such that
channels (a) and (d) belong to a first group, channels (b) and (e) to a
second group and channels (c) and (f) to a third group. In the example of
Figure 11, the second group is enabled (the first and third groups being
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disabled), with the droplet ejection input data being such that channel (b) of
the second group is actuated to eject a droplet whilst channel (e) of the
second group is not.
Application of voltage pulse 72 (the enabled/ejecting waveform) to
enabled channel (b) followed by voltage pulses 70 to disabled channels (a)
and (c) results in a'draw- release-reinforce' potential difference of the kind
shown in Figure 5(a) across each of the walls bounding channel (b), causing
them to move to eject a droplet from channel (b).
An enabled/non-ejecting waveform is applied to enabled channel (e).
This comprises a plurality (three in the example shown) of pulses 74 each
having the same amplitude as pulses 70 and each having a trailing edge 74
synchronous with the trailing edge 70 of the corresponding pulse 70 applied
to the neighbouring channels. Pulses 74 are, however, of greater duration
than pulses 70, resulting in a potential difference 76 of the kind shown in
Figure 11(g) being applied to each of the walls bounding channel (e). Whilst
this potential difference will have the same amplitude as pulses 70,72, its
duration is chosen to be insufficient to effect droplet ejection.
At the end of period T, the second channel group is disabled and one
of the other groups is enabled for droplet ejection, as is well known in the
art. Although the droplet ejection period T for a multi-channel arrangement
should ideally be no longer than the droplet ejection period L of a single
channel as mentioned above with reference to Figure 5(a), T may need to
be longer than the ideal if it is necessary to accommodate several non-
drop-ejection pulses 74.
Figure 12 shows a second version of an enabled/non-ejecting
waveform for use with the enabled/ejecting waveform of Figure 11(b) and in
place of the waveforms of Figure 11 (d)-(f). A first pulse 80 of duration
(and,
optionally, amplitude) insufficient to effect droplet ejection is applied
synchronously with the first pulse 72 of the enabled/ejecting waveform of
Figure 11(b) and thereafter a second pulse 82 is applied to balance the
pulse 70 applied to the adjacent disabled lines . The resulting potential
difference is shown in Figure 12(g).
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A third version of enabled/non-ejecting waveform for use in
combination with the enabled/ejecting waveform of Figure 11(b), is shown in
Figure 13. Pulse 90 is of the same amplitude as pulse 70 but is of shorter
duration and is delayed in time by an amount 'o'. The resulting potential
difference, shown in Figure 13(g), has two pulses each of duration
insufficient to eject a droplet. Such a potential difference has twice the
number of edges (two rising edges 92,94 and two falling edges 96,98) and
thus has the potential to generate twice the current flow of the potential
difference of Figure 12(g).
Figure 14 illustrates a fourth version, namely a pulse 100 applied to
channel (e) and having the same magnitude and duration as pulse 70 but
advanced by an amount 'p' relative to the pulse 70. The resulting potential
difference, illustrated in Figure 14(g), has both positive and negative
elements that generate positive and negative pressure waves in the channel.
Offset 'p' and the duration of pulses 70,100 can be chosen such that the
elements are delayed in time by 2AL/c so that the resulting pressure waves
cancel one another in the channel, thereby reducing the amount of time
taken for pressure waves in the channel to die down and thus the length of
the droplet ejection period. This cancellation principle is known from the
aforementioned W095/2501 1, which also discloses the principle of making
the second pulse of lower amplitude to allow for the fact that the first pulse
is damped before being cancelled. This principle is also applicable in the
present invention.
An enabled/non-ejecting waveform in accordance with Figure 15 has
an advantage over previous embodiments in that both the magnitude and
the duration of the resulting potential difference across the walls bounding
the non-ejecting channel can be controlled. To this end, a first, short pulse
110 is followed by a longer pulse 112 having identical timing, duration and
magnitude as the pulses 70 except for a 'cutout' 114 having the same
amplitude and duration as pulse 36'. The resulting potential difference is as
shown in Figure 14(g). Again, timing and magnitude of pulse 112 and cutout
114 can be chosen so as to reduce the iength of the droplet ejection period
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as explained above.
Many other variations on the embodiments above will be obvious to
the skilled man and are to be considered as comprised in the present
invention.
During the periods when channels are disabled, there will of course
be a reduction in the energy that they receive which could in turn result in a
cooling of the ink therein. However, since all channels are disabled to the
same proportion, such cooling will be the same for all disabled channels and
the temperature of the ink will continue to remain substantially independent
of the nature of the droplet ejection input data.
In an alternative embodiment, "enabled/non-ejecting' waveforms can
be applied to all non-firing channels, be they enabled or disabled. Figure 16
illustrates the waveforms applied to four adjacent channels in a"shared-
wall" printhead and operating in three cycle mode. Channels (a) and (d)
belong to the same, enabled channel group and are supplied with an
enabled/ejecting "draw-release" waveform 120 (of the kind well known in the
art) and three, reduced-width pulses 125, 126, 127 respectively. The
reduced-width pulses are chosen so as to effect substantially the same
temperature change in the ink as enabled/ejecting pulse 120.
Similar non-ejecting waveforms are applied to disabled channels (b)
and (c). As shown, they are identical to those applied to channel (d), albeit
staggered in time (it will be evident from the earlier description relating to
Figures 2 to 4 that application of equal voltages to channels either side of
an
actuator wall would result in zero potential difference across the wall and
therefore zero current flow and wall movement) and will generate the same
temperature change of the ink in the respective channel as ejecting pulse
120.
One result of this additional energy input is that the printhead
operates at a higher overall temperature. The energy input of the non-
ejecting waveforms (dictated by the dimension and number of the pulses) on
the non-enabled lines can advantageously be varied in real time by a
controller so as to maintain the temperature of the head at a constant value.
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This technique, namely the actuation of means to vary the volume of
the chamber of an inkjet printhead without ejecting a droplet and with the
express intention of raising the temperature of the ink in the chamber, is not
restricted to situations where the temperature of the ink in a chamber is to
be kept independent of the droplet ejection input data and can be used
wherever it is desired to heat the ink, for example particularly but not
exclusively with the objective of reducing temperature variations (and thus
ejection velocity variations) between channels.
Also by way of example, the printhead may incorporate a temperature
detector and the printhead controller may be arranged to adjust the
magnitude or number of non-ejecting waveforms applied to maintain the
printhead at a constant temperature based on feedback from the sensor.
Alternatively, feedback from both an ambient temperature sensor and a
printhead temperature sensor may be employed. Furthermore, should it be
found that there is a non-uniform heat loss over the extent of a printhead -
for example that there is greater heat loss to ambient non-channels of the
extremities of the array - extra heat may be generated in these channels
using non-droplet ejecting waveforms. It may also be desirable to heat
selected channels to compensate for variations in inks of different colours,
thereby to equalise the colour.
The technique is equally applicable to non-ejecting or ejecting
channels: in the latter case, both a heating pulse and a droplet ejection
pulse may be applied in a single droplet ejection period.
Droplet ejection velocity changes also occur at the commencement of
printhead operation: even in the embodiments outlined above where the
temperature of the ink remains independent of the print data, the heat
generated in a channel will produce a temperature rise in the ink in that
channel until an operating temperature is reached at which the heat
generated in the channels equals the heat dissipated e.g. by convection
from the printhead, by throughflow of ink. In accordance with another
embodiment of the invention, the velocity changes associated with such a
temperature variation can be avoided by applying to the channels of a
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printer which has been long quiescent a series of non-droplet ejection
pulses to heat the ink to the operating temperature. In the case of actuators
of the kind shown by way of example in Figures 2 to 4, the time constants of
heating are 2 to 5 seconds. Conveniently, this time is of the order of the
time
spent by a printer in receiving data and carrying out other preparation and
would not therefore constitute an additional delay.
The present invention is in no way restricted to those embodiments
given by way of example above. In particular, the invention is applicable to
any droplet deposition apparatus comprising a chamber supplied with droplet
fluid and communicating with a nozzle for ejection of droplets therefrom and
actuator means actuable by electrical signals to vary the volume of said
chamber. Such actuation need not be piezoelectric - it may employ
electrostatic means for example. Similarly, control in response to
charge/current rather than electrical potential (as employed in the examples
given) may prove desirable.
The present invention is also appiicable to printheads operating in
"multipulse" mode, i.e the successive ejection of several droplets from a
channel which then merge either in flight or on the printing substrate to form
a single printed dot. By varying the number of droplets ejected, the size of
the printed dot can be controlled. Such operation is described in
EP-A-0 422 870 and is commonly known as "greyscale operation".
As will be evident from Figure 17, which represents a conventional
eight level multipulse operation (seven levels of grey plus white) with the
"draw-release" actuating waveform 130 that might be applied to three - not
necessarily adjacent - channels (a),(b) and (c) in response to print data
specifying print densities of 7/7, 4/7and 1/7 respectively, there will be a
greater increase in the temperature of the ink when a high number of
droplets are ejected than when a low number or zero droplets are ejected.
Thus there is potential for temperature and ink viscosity differences between
the channels, leading to print errors, and indeed these problems have been
found to be more acute in a printhead operated in multipulse mode. This is
attributed to the greater number of waveform edges and the reduced cooling
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effect of the smaller droplets employed.
A solution to this problem in accordance with the present invention is
illustrated by way of example in Figure 18: it will be seen that in those
channels (b) and (c) where less than the maximum possible number (seven
in the example shown) of actuation pulses 130 is applied, further pulses 135
can be applied to make up the deficiency. The amplitude and/or duration of
the further pulses 135 should be chosen such that although droplet ejection
does not occur, the same temperature change is induced in the ink as by
the actuating pulses 130. Thus the total energy dissipated in the period of
enablement T remains independent of the print data. As is also known from
EP-A-0 422 870, greyscale operation can be effected in groups or with
adjacent channels operating in antiphase. In the former case, the methods of
group operation described with regard to "binary" (firing either 1 drop or
zero
drops) operation above are applicable: non-enabled channels can either be
left completely unactuated or fed with non-droplet ejecting waveforms of the
type mentioned above. It may also be possible to actuate non-droplet-
ejecting channels with a lesser number of waveforms having a longer
duration than the droplet ejecting pulses but inducing the same temperature
change in the ink. Note that other drop ejecting waveforms - for example the
"draw-release-reinforce" waveform of Figure 5(a) - may also be used in
greyscale operation together with their non-ejecting counterpart waveforms.
It is believed that hysteresis loss in the piezoelectric material is the
major - but not the sole - cause of heating of the ink in the channels of a
printhead. Actuation of channels will give rise to movement of ink in the
channels which in turn will increase the temperature by fluid friction, with a
high level of channel operation giving rise to a greater increase in ink
temperature than a iow level. Yet another source of heat will be resistance
losses in the actuating electrodes. Empirically-derived non-ejecting
waveforms will take account of such further loss mechanisms. They may
also be incorporated to a greater or lesser extent into the mathematical
model described above.
As mentioned at the beginning of the description, "thermal" printheads
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operate on the principle of heating ink in a chamber to create a vapour
bubble which pushes ink out of the chamber via a nozzle. Such heating is
localised to that section of the channel in which the heater is located,
however, and it has been recognised by the present inventors that, in the ink
in the nozzle and the part of the channel adjacent thereto which is remote
from the heater, problems with variation in droplet ejection speed due to
differences in ink temperature - similar to the problems discussed with
reference to Figure 1 - may occur. It is believed that the solutions outlined
above with regard to "variable volume chamber" devices may also be
applicable to "thermal" printheads. In particular, non-ejecting actuating
signals may be applied to a channel, the signals being chosen so as to
induce the same temperature change in the fluid at the nozzle as droplet-
ejecting signals.
The manner in which the short duration pulses 24,26,30,32,36 of
Figures 11 to 15 are applied comprises a further aspect of the present
invention, namely the method of operation of droplet deposition apparatus
comprising a chamber supplied with droplet fluid, a nozzle communicating
with the channel for ejection of droplets therefrom and actuator means
having first and second electrodes and actuable by a potential difference
applied across first and second electrodes to effect droplet ejection from the
chamber via the nozzle;
the method comprising the steps of applying to the first electrode a first non-
zero voltage for a first duration, applying to the second electrode a second
non-zero voltage for a second duration, the first and second voltages being
applied simultaneously for a length of time less than at least one of said
first
and second durations.
This further aspect is particularly advantageous when applying short
pulses of the kind shown in Figures 11 to 15. For a printhead operating at a
droplet ejection frequency of 100 kHz for example, such pulses could have a
duration as short as 1 us. Circuitry to generate such short pulses can be
complex and consequently expensive. By using the aforementioned second
concept, it is possible to apply short duration pulses using longer duration
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signals which are easier to generate.
The concept is also of use when operating a "shared-wall" printhead
in two-cycle, two-phase mode as discussed in W096/10488. Successive
channels in an array are alternately assigned to one of two groups, with
each group being alternately enabled for droplet ejection in successive
cycles. Within each cycle, successive channels in a group eject droplets in
antiphase. This mode is particularly suited to multipulse operation, with a
number of droplets being ejected from a channel in any one cycle in
accordance with the input data, thereby to form a corresponding printed dot.
Figure 19 illustrates the voltage waveforms to be appiied to four
adjacent channels a,b,c,d of a "shared wall" printhead to implement two
cycle / two phase operation in accordance with the aforementioned concept
of the present invention. The corresponding potential difference variation
across the walls bounding channels a-d is shown in Figure 20.
The left-hand side of Figure 19 corresponds to a first cycle of
operation where the group including channels (a) and (c) are enabled. To
each channel in the disabled group - which includes channels (b) and (d) -
there is applied a common repeating waveform 191 which, in the example
shown, comprises a square pulse of duration AUc followed by a dwell period
also of duration AL/c.
A similar repeating waveform 192, 192' having the same amplitude is
applied to enabled channels, albeit with square pulse and dwell period
durations of 2ALIc and with the waveform 192' applied to channel (c) 180
degrees out of phase with the waveform 192 applied to channel (a). Figure
20 illustrates the resulting potential differences 201,202 across the actuator
walls bounding channels (a) and (c) and which will result in "draw-release-
reinforce" actuation of channel (a) thereby to eject a droplet. Since the
similar actuation of channel (c) takes place 2AL/c later, the droplet ejection
from this channel will be in antiphase with that from channel (a). Both
channels (a) and (c) may be actuated several times in immediate succession
in accordance with the input print data so as to eject several droplets and
form a correspondingiy-sized printed dot.
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The right-hand side of Figures 19 and 20 shows the similar behaviour
when the second group including channels (b) and (d) is enabled and
actuated in accordance with the print data.
Figures 21 and 22 are similar to Figures 16 and 17 in demonstrating
that the temperature of the droplet fluid in a chamber can be maintained
independent of the droplet ejection input data by applying further non-
ejecting pulses - in this case a potential difference 221 of width
insufficient
to induce droplet ejection - in place of the ejecting pulses that might
otherwise be applied. The ampiitude/duration/number of these pulses can be
chosen using either of the empirical or theoretical methods outlined above to
generate losses (particularly hysteresis) and thereby heat such that the
temperature of the ink in the channel remains independent of the number of
ejecting pulses applied in a droplet ejection period.
Figure 23 shows an alternative embodiment of the two cycie/two phase
concept. A repeating "sawtooth" actuating voltage waveform 231 - known
per se in the art - is applied to the disabled channels (b) and (d), whilst to
the enabled channels (a) and (c) there is applied a square wave 232,232' of
the same amplitude but half the repeating frequency, with the waveform 232
applied to channel (a) being in antiphase to the waveform 232' applied to the
neighbouring channel in the same group, namely channel (c). The potential
difference across the channel walls of the enabled channels is shown in
Figure 24: again a sawtooth waveform, it has twice the amplitude of either
the actuating waveforms applied to the channels as per Figure 23 due to the
action of the enabled channel voltage falling whilst the voltage applied to
its
immediate neighbours is rising. The right-hand side of Figures 23 and 24
illustrate the situation when channels (b) and (d) are enabled. It will be
evident that droplet ejection, initiated by the vertical edge of the waveform,
can take place at a higher rate than possible with the embodiment of Figure
19. Droplet ejection between neighbouring channels in the same enabled
group will still be in antiphase, however. Furthermore, this waveform has
been found to reduce pressure crosstalk between channels in a "shared-
wall" printhead which might otherwise cause non-ejecting channels to eject
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accidentally.
Each feature disclosed in this specification (which term includes the
claims) and/or shown in the drawings may be incorporated in the invention
independently of other disclosed and/or illustrated features.
The text of the abstract filed herewith is repeated here as part of the
specification.
In droplet deposition apparatus comprising one or more independently
actuable ink ejection chambers, electrical signals are applied to reduce
variation in the temperature of the droplet fluid between chambers and with
variations in droplet ejection input data. Short potential difference pulses,
suitable for influencing the temperature of the droplet fluid in a chamber,
can
be generated by application of longer duration voltages to ink chamber
actuation means.
