BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a technique to drive a liquid jet unit for jetting
droplets from a liquid surface, and more particularly to a technique to jet droplets
by providing vibration to a liquid having a surface limitedly exposed by an opening.
In particular the invention relates to a device according to the preamble of claim
1, which is known from US-A-5 798 779.
Description of the Background Art
[0002] There has been a technique of depositing ink onto printing paper in a form of droplet
to draw images and characters, i.e., ink jet printing. In this description, for distinction,
a term "jet" is used when a plurality of ink droplets are simultaneously generated
and a term "discharge" is used when droplets are sequentially generated one by one.
[0003] To faithfully reproduce tone of an image proposed is a technique of ultrasonically
providing ink with vibration to obtain droplets from an ink surface. For example,
JP-A-2-303849 discloses a technique of controlling the length of period for continuously
providing ink with ultrasound to control the amount of ink to be discharged. JP-A-10-128968
discloses a technique of providing ink with ultrasonic burst including a certain number
of pulses repeatedly a plurality of times to control the amount of ink to be jetted
with the repeat number.
[0004] In the technique of controlling the length of period for continuously providing ink
with ultrasound to control the amount of ink to be discharged, however, in order to
discharge a large amount of ink, it is necessary to provide ink with continuous ultrasonic
supply for a long time. That causes a marked rise of the level of ink surface from
the opening with a radiation pressure as discussed later, resulting in an unstable
discharge of droplets.
[0005] The technique of providing ink with ultrasonic burst repeatedly a plurality of times
is superior in not providing continuous radiation pressure. Since the ink jet printing,
however, requires depositing of ink on printing paper with different tones for dots,
it is necessary to adjust the timing of sending the printing paper and that of disposing
ink for every dot. The length of period while no continuous ultrasonic is supplied
varies from dot to dot in the technique of providing ink with ultrasonic burst repeatedly
a plurality of times as well as in the technique of controlling the length of period
for continuously providing ink with ultrasound. Since the length of period while a
radiation pressure is provided therefore varies from dot to dot, the rise of ink level
from the opening varies from dot to dot. This variation causes an unstable jetting
of droplets, and further causes deterioration in graininess in terms of printing quality
and difficulty in controlling gradation.
SUMMARY OF THE INVENTION
[0006] The document US-A-5 798 779, forming the preamble of claim 1, discloses an ultrasonic
printing method by which recording of a high resolution can be achieved. In the conventional
method, some or all of a plurality of ultrasonic oscillators are selectively driven
in such phases that a difference in phase at a predetermined point between a reference
ultrasonic wave from one of the selected ultrasonic oscillators and another ultrasonic
wave from any other one of the selected ultrasonic oscillators is equal to or less
than one-fourth a wavelength of the ultrasonic waves in a transmission medium for
the ultrasonic waves from the selected ultrasonic waves to the predetermined point.
The ultrasonic printing method can be applied to various printing apparatus for which
recording of a high resolution is required.
[0007] The object underlying the present invention is to provide a liquid jet driving device
as specified in the preamble of claim 1 using a technique for particularly stable
jetting of droplets by improving the shape of the liquid surface from which the droplets
are generated.
[0008] This object is solved in an advantageous manner by a liquid jet driving device comprising
the features of claim 1. Advantageous further developments of the device according
to the invention are set forth in the subclaims.
[0009] By means of the liquid jet driving device according to the invention it is possible
to suppress the variation in shape of the liquid surface in the opening in order to
ensure an easy control of a liquid jet. Since the second vibration exciter is supplied
with the suppressing signal, the first vibration exciter can be designed most suitably
to the first frequency.
[0010] These and other objects, features, aspects and advantages of the present invention
will become more apparent from the following detailed description of the present invention
when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011]
Fig. 1 is a block diagram showing a configuration of a liquid jet driving device which
may be applied to the present invention;
Fig. 2 is a cross section schematically showing a structure of an ink head;
Figs. 3A to 3C are waveform views for description of a jet burst signal;
Fig. 4 is a cross section schematically showing a concept of a sound pressure;
Fig. 5 is a cross section schematically showing a concept of a radiation pressure;
Figs. 6A to 6D are waveform views showing a relation among the jet burst signal, the
sound pressure and the radiation pressure;
Figs. 7 to 14 are cross sections schematically showing states of meniscus of ink in
an opening;
Figs. 15A and 15B are waveform views showing a relation between the jet burst signal
and meniscus motion of ink;
Figs. 16A and 16B are waveform views showing a liquid jet driving technique;
Figs. 17A and 17B are waveform views showing a further liquid jet driving technique;
Fig. 18 is a cross section schematically showing a structure in accordance with an
embodiment of the present invention;
Figs. 19A to 19C are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention;
Figs. 20A to 20C are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention;
Figs. 21A to 21C are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention;
Figs. 22A and 22B are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention;
Figs. 23 to 31 are cross sections showing a liquid jet driving technique in accordance
with the present invention;
Fig. 32 is a cross section schematically showing an exemplary structure of a device
usable in the present invention;
Figs. 33 to 35 are cross sections showing a further liquid jet driving technique;
and
Fig. 36 is a cross section showing a further liquid jet driving technique.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A. Background of The Invention
[0012] Before a detailed discussion on preferred embodiments of the present invention, findings
on a radiation pressure as the background of the present invention will be discussed.
Fig. 1 is a block diagram showing a configuration of a liquid jet driving device which
is also applied to the present invention. The liquid jet driving device comprises
an input-amount conversion circuit 1 for converting an image signal 20 having information
on, for example, a tone value of an image to be drawn, into a jet signal-21, a basic-signal
generation circuit 2 for generating a basic signal 28 consisting of continuous pulses
of frequency f0, a driving circuit 3 adopting the basic signal 28 for a predetermined
period based on the jet signal 21 to generate a jet burst signal 26 consisting of
continuous pulses of frequency f0 in number k and an ink head 9 including a vibration
exciter 9a driven with the jet burst signal 26. The ink head 9 has ink 30 therein.
The vibration exciter 9a is driven with the jet burst signal 26 to jet the ink 30
as a droplet 31 from the ink head 9.
[0013] The input-amount conversion circuit 1 performs conversion in consideration of conversion
of dynamic range (e.g., converting tone values of 256 levels into 32 bits) and a non-linear
relation between the amount of ink to be jetted from the ink head 9 and the repeat
number of jet burst signal 26.
[0014] Further, the driving circuit 3 generates a suppressing signal 27 and the vibration
exciter 9a is also driven with the suppressing signal 27.
[0015] Fig. 2 is a cross section schematically showing a structure of the ink head 9. The
ink head 9 comprises a body 94 having, for example, rotary paraboloid as an inner
wall to store the ink 30, a nozzle plate 93 having an opening 95 to limitedly expose
a surface of the ink 30 and being communicated with the body 94 through the opening
95 and the vibration exciter 9a for providing ink 30 with vibration. Without the nozzle
plate 93, the opening 95 may be disposed in the body 94.
[0016] The vibration exciter 9a has an ultrasonic vibration exciter, for example, a piezoelectric
vibrator 92, and further a protection sheet 91 between the body 94 and the piezoelectric
vibrator 92, for protecting the piezoelectric vibrator 92 so that it should not be
wetted by the ink 30.
(A-1) Description on Jet Burst Signal
[0017] Figs. 3A to 3C are waveform views of the basic signal 28, the jet burst signal 26
and a dot recording signal, respectively, which show a relation among the three signals.
The basic signal 28 has a cycle T0 = 1/f0, assuming herein that, for example, f0 =
10 MHz (T0 = 100 ns). Each jet burst signal 26 is generated by collecting k pulses
of the basic signals 28. It this case, k = 6.
[0018] The jet burst signal 26 is repeated a predetermined number of times every predetermined
burst cycle T2. The jet burst signal 26 is repeated burst number dj of times at the
j-th dot #j, depending on a value of the jet signal 21 corresponding to a tone value
required of one dot. A period corresponding to one dot is defined as a dot cycle T3,
and for example, it is shown that burst numbers d1 = 25, d2 = 5 and d3 = 17 at the
first dot #1, the second dot #2 and the third dot #3, respectively. Since T3-dj·T2
varies from dot to dot, an unstable level of ink surface is caused as above discussed.
(A-2) Description on Emission Pressure
[0019] Figs. 4 and 5 are cross sections schematically showing concepts of a sound pressure
Ps and a radiation pressure Pi. Though a discussion will be made taking a case where
the piezoelectric vibrator 92 makes thickness longitudinal vibration as an example,
these two pressures may be applied to the ink 30 even by other-mode vibration.
[0020] When the piezoelectric vibrator 92 provides the ink 30 with vibration with a frequency
of f0, the sound pressure Ps having a frequency f0 drives the surface of the ink 30
exposed in the opening 95 almost vertically in two direction, as shown in Fig. 4.
Since a rim of the opening 95 serves as a fixed end of the surface of the ink 30,
a first surface wave is created from the rim of the opening 95. The first surface
wave jets the fine droplet 31 from its loop. The first surface wave goes towards the
center of the opening 95 at a velocity Vc, and is attenuated to vanish immediately
after vibration excitement of the piezoelectric vibrator 92 is completed.
[0021] On the other hand, since the ink 30 has an interface with air near the opening 95
and the vibration of the ink 30 is entirely reflected on the interface, the radiation
pressure Pi drives the surface of the ink 30 almost vertically as shown in Fig. 5
in one direction from the piezoelectric vibrator 92 to the opening 93. Since the radiation
pressure Pi vanishes after vibration excitement of the piezoelectric vibrator 92 is
completed, a second surface wave is created from the rim of the opening 95. The second
surface wave goes towards the center of the opening 95 at a velocity Vr.
[0022] Figs. 6A to 6D are waveform views showing a relation among the jet burst signal 26,
the sound pressure Ps and the radiation pressure Pi. The pulse train included in the
jet burst signal 26 has a cycle T0 = 1/f0 and each jet burst signal 26 consists of
pulses in a burst in number k. The burst cycle T2 is set at not less than k·T0, and
therefore there is a relation of T2-k·T0≧0 between adjacent jet bust signals 26.
[0023] The piezoelectric vibrator 92 has a shape of a thin plate perpendicular to a direction
from the vibration exciter 9a to the opening 95 as shown in Fig. 3, and receives the
jet burst signal 26 to excite thickness longitudinal vibration almost in a form of
sine wave. If it is now considered that the ink 30 has a viscosity low enough to ideally
follow the vibration applied by the piezoelectric vibrator 92, assuming that the velocity
of sound propagated in the ink 30 (sound velocity), the maximum value of the velocity
of the ink moving by vibration excitement (maximum velocity) and the density of the
ink 30 are c, u and ρ, respectively, the sound pressure Ps has a cycle T0 and shows
a sine wave with an amplitude ρ cu. When a pressure in a case of no vibration excitement
for the ink 30 is 0, the sound pressure Ps varies within a range of ± ρ cu and drives
the ink 30 in the two directions as shown in Fig. 4. It is assumed herein that a positive
sign is taken in a direction from the piezoelectric vibrator 92 to the opening 95.
[0024] On the other hand, the vibration of the ink 30 is entirely reflected on the interface
with air near the opening 95, and that causes a base radiation pressure Bi showing
a sine wave with a cycle T0/2 and an amplitude ρ u
2/2. The base radiation pressure Bi varies from 0 to ρ u
2. The maximum velocity u of the ink 30 is lower than the sound velocity c, and therefore
the amplitude of the base radiation pressure Bi produces little effect on the motion
of the ink when the sound pressure Ps is applied to the ink 30.. The base radiation
pressure Bi, however, has an average value ρ u
2/2 in a period T1 while the jet burst signal 26 exists. Since the radiation pressure
Pi, as a pulse having the average value, is applied in one direction, when the vibration
excitement continues to perform on the ink 30 for a long time, the meniscus of the
ink 30 in the opening 95 greatly rises.
[0025] Further, when the sound pressure Ps is not applied between the adjacent jet burst
signals 26, the base radiation pressure Bi also vanishes. Therefore, the radiation
pressure Pi produces a considerable effect, i.e., the second surface wave on the ink
30 entirely at the dot cycle T3.
[0026] Further, JP-A-9-57963 discloses an aspect where the piezoelectric vibrator is driven
with a triangular waveform and surface waves going from the rim to the center of the
opening interfere with one another to discharge a droplet and another aspect where
a plurality of fine droplets are jetted from an end of the surface wave. In both aspects,
however, the triangular wave has to make a great drive on the liquid and is not a
technique in consideration of radiation pressure accompanying the jet burst signal
like the present invention.
(A-3) Correction of Burst Cycle
[0027] In a state where the jet burst signal 26 is repeatedly supplied, i.e., the period
dj· T2 in the dot cycle T3 of the j-th dot #j referring to Fig. 3C, the burst cycle
T2 is corrected on the basis of the velocity Vr of the second surface wave, to keep
the state of the meniscus of the ink 30 in a common shape at the point of time when
the jet burst signal 26 is supplied. This correction of the burst cycle is adopted
in the preferred embodiments discussed later as the present invention.
[0028] Figs. 7 to 14 are cross sections schematically showing states of meniscus of the
ink 30 in the opening 95 at time t = 0 µs, 0.6 µs, 1µs, 2µs, 4µs, 8µs, 10µs and 15.6
µs, respectively, assuming that the sound pressure Ps of f0 = 10 MHz and k = 6 begins
to be supplied at the time t = 0 µs.
[0029] At the point of time when the sound pressure Ps begins to be supplied, as shown in
Fig. 7, the meniscus of the ink 30 has a small rise from the opening 95 due to surface
tension. By applying the sound pressure Ps, the droplets 31 are jetted from near the
rim of the opening 95 until k/f0 = 0.6 µs. Since the meniscus has a small rise from
the opening 95 as shown in Fig. 7, the droplets 31 are jetted radiantly from the opening
95.
[0030] With the sound pressure Ps applied, the radiation pressure Pi is applied in a period
from t = 0 to t = 0.6 µs. Considering that the radiation pressure Pi has one-half
wavelength of fi = f0/2k, the velocity Vr of the second surface wave is obtained as
fi·(2π σ / ρ fi
2)
1/3. Assuming that the opening 95 is a circle with a diameter
D, a time required for a to-and-fro movement of the second surface wave in the opening
95 is obtained as 2D/Vr. For example, assuming that the surface tension σ of the ink
30 is 5 × 10
-2 N/m, the density ρ is 1 × 10
3 kg/m
3 and D = 50 µm, the required time can be obtained as 2D/Vr = 15.6 µs.
[0031] Therefore, as shown in Figs. 9 to 13, the state of meniscus has a complicate shape
until t = 15. 6 µs due to propagation of the second surface wave, and returns to a
state almost like that of t = 0, as shown in Fig. 14, when t = 15.6 µS. Setting the
burst cycle T2 at 2D/Vr, the state of meniscus at the point of time when the jet burst
signal 26 is supplied can be kept equally.
[0032] There may be a case, naturally, where it is found from an actual measurement that
an optimum result can be obtained by setting the burst cycle T2 at a value slightly
shifted from the time required for the to-and-fro movement of the second surface wave
in the opening 95. It falls within correction of burst cycle of this description,
however, to adjust the cycle of variation in level of ink 30 due to the second surface
wave and the burst cycle T2 for keeping equally the shape of meniscus at the point
of time when the jet burst signal 26 is supplied.
(A-4) Problem caused by Non-Continuous Jet Burst Signal
[0033] Figs. 15A and 15B are waveform views showing a relation between the jet burst signal
26 and meniscus motion of the ink 30 at the dot cycle T3. Fig. 15A shows a timing
for supplying the jet burst signal 26 while Fig. 15B shows the center position in
the surface of the ink 30 which is exposed in the opening 95. In Figs. 15A and 15B,
horizontally-extended lines are base lines, indicating that the pressure is 0 in the
figure on the jet burst signal 26 and the center position of the liquid surface in
the state of Fig. 7 in the figure on the position of the liquid surface.
[0034] In a period while the jet burst signal 26 is stopped (T3-dj·T2 in Fig. 3), since
no radiation pressure Pi is applied, the surface of the ink 30 which is driven with
the radiation pressure Pi applied in a period while the jet burst signal 26 is supplied
(dj ·T2 in Fig. 3) makes free vibration in a mode specific to the opening 95. The
free vibration has a wavelength about several times as long as the burst period T2.
As discussed in (A-1), when the tone level required of dot is different, dj becomes
different and T3-dj·T2 also becomes different, and hence the first jet burst signal
26 in the next dot cycle T3 is not necessarily given to the state of meniscus shown
in Fig. 7 or 14.
[0035] Therefore, with control over the free vibration specific to the opening 95 which
may occur in the period while the jet burst signal 26 is stopped, the surface of the
ink 30 is controlled so that the jet burst signal 26 may be given in an appropriate
state of meniscus.
B. SPECIFIC EMBODIMENTS
[0036] Figs. 16A and 16B are waveform views showing a liquid jet driving technique.
[0037] Fig. 16A shows a timing for supplying the jet burst signal 26 and a dummy burst signal
which is adopted as the suppressing signal 27 while Fig. 16B shows the center position
in the surface of the ink 30. The base lines of Figs. 16A and 16B show the same as
those of Fig. 15A and 15B.
[0038] The dummy burst signal refers to a burst signal with which the vibration exciter
9a is driven to provide the ink 30 with vibration not sufficient to jet out from the
opening 95. The dummy burst signal is generated as a burst signal consisting of a
pulse train of a frequency f0, of which pulse number k is smaller than that of the
jet burst signal 26. For example, when the jet burst signal 26 has pulses in number
k = 6, the dummy burst signal has pulses in number k = 4. Alternatively, the dummy
burst signal is generated as a burst signal having pulses in the same number k and
the same frequency f0 as the jet burst signal 26 and a smaller amplitude than the
jet burst signal 26.
[0039] This dummy burst signal drives the vibration exciter 9a in the period while the jet
burst signal 26 is not applied in the dot cycle T3, i.e., at a period T3-dj·T2. Therefore,
over almost entire dot cycle T3, the jet burst signal 26 or the dummy burst signal
is applied to the vibration exciter 9a almost at the burst cycle T2.
[0040] Since the dummy burst signal does not jet the ink 30 unlike the jet burst signal
26 as discussed above, the tone level of the dot is not deteriorated. Since the dummy
burst signal also provide the ink 30 with the radiation pressure, however, it is possible
to control the free vibration as shown in Fig. 15B that the ink 30 can make in the
period T3-dj·T2 to be almost the same as that in the period dj·T2 while the jet burst
signal 26 is applied. Since the dummy burst signal is applied repeatedly at the same
cycle as the jet burst signal 26 is applied, especially, the motion of the ink 30
can be kept in an almost stationary state.
[0041] The dummy burst signal can be generated easily in the driving circuit 3 shown in
Fig. 1. The driving circuit 3 continuously generates dj jet burst signals 26 with
respect to the j-th dot on the basis of the information from the jet signal 21. After
that, the driving circuit 3 continuously generates the dummy burst signals until the
end of the dot cycle T3 to give the signals to the vibration exciter 9a as the suppressing
signal 27. As discussed earlier, since the dummy burst signal has a frequency f0,
the dummy burst signal can be easily generated from the basic signal 28 by using different
number k of pulses or different amplitude, like the jet burst signal 26.
[0042] In the dot cycle T3, the dummy burst signal does not necessarily have to be supplied
after the jet burst signal 26. For example, earlier in the dot cycle T3, some dummy
burst signals are supplied to the vibration exciter 9a, then the jet burst signals
26 are supplied and thereafter the dummy burst signals are supplied until the end
of the dot cycle T3.
[0043] Figs. 17A and 17B are waveform views showing a further liquid jet driving technique.
Fig. 17A shows a timing for supplying the jet burst signal 26 and the dummy burst
signal while Fig. 17B shows the center position in the surface of the ink 30 which
is exposed in the opening 95. The base lines of Figs. 17A and 17B show the same as
those of Fig. 15A and 15B.
[0044] The dummy burst signal is supplied at a cycle different from the burst cycle T2.
Since the wavelength of the free vibration of the ink 30 in a mode specific to the
opening 95 can be obtained to be several times as long as the burst cycle T2 through
calculation or actual measurement, the dummy burst signal, being adjusted to the wavelength,
is supplied for the vibration exciter 9a.
[0045] In this case, it is desirable to supply the dummy burst signal when the meniscus
is displaced closest to the piezoelectric vibrator 92. Supplying the dummy burst signal
at this point of time, the piezoelectric vibrator 92 provides the ink 30 with the
radiation pressure in a direction from the piezoelectric vibrator 92 to the opening
95, ensuring an easy control of the free vibration. Though there may be a case, naturally,
where it is found from an actual measurement that an optimum result can be obtained
by supplying the dummy burst signal at a timing slightly shifted from the time when
the meniscus is displaced closest to the piezoelectric vibrator 92, it is also possible
to supply the dummy burst signals intermittently in the direction to suppress the
free vibration as discussed above.
[0046] In the embodiments described above, the dummy burst signal is used to generate the
radiation pressure Pi. Instead of using the radiation pressure Pi accompanying the
sound pressure Ps, however, a pressure independent of the sound pressure Ps can be
used and by applying the pressure to the ink 30, the free vibration in the surface
of the ink 30 in the opening 95 can be suppressed.
[0047] Fig. 18 is a cross section schematically showing a structure of the ink head 9 additionally
provided with a structure to apply that pressure. The body 94 and the vibration exciter
9a of Fig. 2 are replaced by a body 97 and a vibration exciter 9b, respectively.
[0048] The vibration exciter 9b comprises a pressure pulse generator 96 as well as the protection
sheet 91 and the piezoelectric vibrator 92. The body 97 is opened on a side of the
vibration exciter 9b more widely than the body 94. The ink 30 is given pressure by
the pressure pulse generator 96 as well as the piezoelectric vibrator 92 with the
protection sheet 91 interposed.
[0049] Since the pressure pulse generator 96 has no need of generating a sound pressure
having a frequency f0, the ink 30 may be given a pressure by generating bubbles with
a heating device, instead of being given vibration by a piezoelectric device. This
pressure may be applied, for example, for the same period as the radiation pressure,
i.e., the period T1 while the jet burst signal 26 exists, or may be applied for a
shorter period. Further, the pressure does not necessarily have the same magnitude
as that of the radiation pressure Pi by the jet burst signal 26. In other words, this
is advantageous in designing flexibility which is greater than a case of using the
dummy burst signal as the suppressing signal 27.
[0050] In the following discussion, a pressure pulse signal is adopted as the suppressing
signal 27, and a positive pressure is applied to the ink 30 in a period while the
pressure pulse signal is "H". This pressure pulse signal can be generated easily by
using a well-known technique in the driving circuit 3 of Fig. 1.
[0051] Figs. 19A to 19C are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention. Fig. 19A shows a timing for supplying
the jet burst signal 26, Fig. 19B shows a timing for supplying the pressure pulse
signal and Fig. 19C shows the center position in the surface of the ink 30 which is
exposed in the opening 95. The base lines of Figs. 19A and 19B indicate that the pressure
is 0 and that of Fig. 19C indicates the center position of the liquid surface in the
state of Fig. 7. This pressure pulse signal can be generated easily by using a well-known
technique in the driving circuit 3.
[0052] In this embodiment the dummy burst signal is replaced by the pressure pulse signal,
but this embodiment can produce the same effect.
(C-2) The Fourth Preferred Embodiment
[0053] Figs. 20A to 20C are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention. Fig. 20A shows a timing for supplying
the jet burst signal 26, Fig. 20B shows a timing for supplying the pressure pulse
signal and Fig. 20C shows the center position in the surface of the ink 30 which is
exposed in the opening 95. The base lines of Figs. 20A to 20C show the same as those
of Figs. 19A to 19C. This pressure pulse signal can be generated easily by using a
well-known technique in the driving circuit 3.
[0054] In this embodiment the dummy burst signal is replaced by the pressure pulse signal,
but this embodiment can produce the same effect.
(C-3) The Fifth Preferred Embodiment
[0055] By additionally providing the pressure pulse generator 96 as shown in Fig.18, a negative
pressure pulse can be also applied to the ink 30. Only at a desired timing, the pressure
pulse signal is made "L". This can be achieved in a case of using a heating device
as the pressure pulse generator 96, by stopping heating only at a desired timing,
as well as the case of using the piezoelectric device. The pressure pulse signal can
be generated easily by using a well-known technique in the driving circuit 3.
[0056] Figs. 21A to 21C are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention. Fig. 21A shows a timing for supplying
the jet burst signal 26, Fig. 21B shows a timing for supplying the pressure pulse
signal and Fig. 21C shows the center position in the surface of the ink 30 which is
exposed in the opening 95. The base lines of Figs. 21A to 21C show the same as those
of Figs. 19A to 19C. For convenience, it is shown that the negative pressure pulse
signal is supplied at the point of time when the pressure pulse signal becomes "L".
[0057] In this embodiment, it is desirable to supply the negative pressure pulse signal
when the meniscus is displaced farthest from the piezoelectric vibrator 92. Supplying
the negative pressure pulse signal at this point of time, the piezoelectric vibrator
92 provides the ink 30 with a pressure in a direction from the opening 95 to the piezoelectric
vibrator 92, ensuring an easy control of the free vibration. Though there may be a
case, naturally, where it is found from an actual measurement that an optimum result
can be obtained by supplying the negative pressure pulse signal at a timing slightly
shifted from the time when the meniscus is displaced farthest from the piezoelectric
vibrator 92, it falls within this embodiment to supply the negative pressure pulse
signals intermittently in the direction to suppress the free vibration.
[0058] Thus, this embodiment can produce the same effect as the previous embodiment Since
the pressure pulse generator 96 applies a pressure to the ink 30 also when the droplets
31 are jetted on the basis of the jet burst signal 26, the meniscus at that time is
likely to have a shape protruding outside from the opening 95 as compared with the
state of Fig. 7. Since the meniscus has the same shape when the jet burst signal 26
is supplied in the dot cycles T3, however, it is advantageously possible to avoid
different controls of jetting for dots.
[0059] Further, the embodiments described above can be also achieved with the ink head 9
having the structure of Fig. 2 by driving the piezoelectric vibrator 92 with the pressure
pulse signal, instead of using the ink head 9 having the structure of Fig. 18. Since
the piezoelectric vibrator 92, however, is designed to provide the ink 30 with larger
vibration with near the frequency f0 in order to effectively generate the sound pressure
Ps, there may be a case where the piezoelectric vibrator 92 does not effectively work,
for example, when the pressure pulse is applied in the period T1 = k/f0. In other
words, in the case of applying the pressure pulse, providing the pressure pulse generator
96 additionally to the piezoelectric vibrator 92 driven with the jet burst signal
26 is better for easy designing.
[0060] The ink head 9 having the structure of Fig. 18 can apply the negative pressure pulse
at the same timing as supply of the jet burst signal 26. The pressure pulse signal
can be generated easily by using a well-known technique in the driving circuit 3.
[0061] Figs. 22A and 22B are waveform views showing a liquid jet driving technique in accordance
with a further embodiment of the present invention. Fig. 22A shows a timing for supplying
the jet burst signal 26 and Fig. 22B shows a timing for supplying the pressure pulse
signal. The base lines of Figs. 22A and 22B show the same as those of Figs. 19A and
19B. For convenience, it is shown that the negative pressure pulse signal is applied
at the point of time when the pressure pulse signal becomes "L".
[0062] This embodiment can dilute the radiation pressure Pi based on supply of the jet burst
signal 26 by applying the negative pressure pulse at the same timing and can thereby
suppress generation of the second surface wave. Therefore, even without correction
of the burst cycle T2 which can be applied to the present invention in (A-3), a considerable
effect can be achieved in preventing variation of meniscus.
[0063] Fig. 23 is a cross section schematically showing the state of meniscus of the ink
30 in a waiting state of the ink head. The "waiting state of the ink head" refers
to a stationary state where only atmospheric pressure is applied to the ink 30 for
a long time. In the waiting state of the ink head, the ink 30 is retreated towards
the piezoelectric vibrator 92 while keeping wetting against the inner wall of the
nozzle plate 93 with surface tension, and the meniscus is out of contact with the
rim of the opening 95.
[0064] Fig. 24 is a cross section schematically showing the state of meniscus of the ink
30 when the jet burst signal 26 is applied to the piezoelectric vibrator 92 immediately
after the waiting state of the ink 30. Since the liquid surface is out of contact
with the rim of the opening 95 serving as a fixed end in the state immediately before
the ink is vibrated, the first surface wave becomes hard to create and it becomes
difficult to jet the droplets 31. When the jet burst signal 26 is supplied for the
piezoelectric vibrator 92 several times, the surface level of the ink 30 gradually
rises and the ink 30 reaches the rim of the opening 95 to appropriately jet the droplets
31.
[0065] In consideration of this phenomenon, at least when the jet burst signal 26 is applied
from the waiting state of the ink head, it is desirable to provide the ink 30 with
hydrostatic pressure, which overwhelms the surface tension to push the liquid level
up. On the other hand, once the liquid level of the ink 30 rises, it takes considerable
time to reach a stationary state because at least one jet burst signal 26, dummy burst
signal or pressure pulse signal is applied to the vibration exciters 9a and 9b every
dot cycle T3.
[0066] Fig. 26 is a cross section schematically showing the state of meniscus of the ink
30 after the jet burst signal 26 is supplied for the piezoelectric vibrator 92 several
times, where the hydrostatic pressure continues to be applied. As shown in this figure,
when the hydrostatic pressure continues to be applied to the liquid surface which
once reaches the rim of the opening 95, the shape of the liquid surface is deformed.
Fig. 27 is a cross section schematically showing the state of meniscus of the ink
30 when the jet burst signal 26 is supplied for the piezoelectric vibrator 92 in the
state of Fig. 26. In a state immediately before the ink 30 is vibrated, since the
liquid level greatly rises from the opening 95 or the liquid spills over, it becomes
difficult in some cases to control jetting of the droplets 31. Further, in some cases,
a large droplet is discharged from the liquid surface of the ink 30, to deteriorate
the tone of dot.
[0067] Therefore, when the jet burst signal 26 is supplied from the waiting state of the
ink head, it is desirable to provide the ink 30 with the hydrostatic pressure, to
push the liquid level of the ink 30 up to the rim of the opening 95, and to reduce
the hydrostatic pressure or provide a negative hydrostatic pressure after the jet
burst signal 26 is supplied for the piezoelectric vibrator 92.
[0068] Fig. 29 is a cross section schematically showing the state where a positive hydrostatic
pressure Pp is applied in the waiting state of the ink head. Thus, by applying the
positive hydrostatic pressure Pp, the liquid surface of the ink 30 reaches the rim
of the opening 95. Fig. 30 is a cross section schematically showing the state where
the jet burst signal 26 is supplied for the piezoelectric vibrator 92 and a negative
hydrostatic pressure Pn is applied. As discussed above, instead of applying the negative
hydrostatic pressure, there may be a case where the positive hydrostatic pressure
Pp is reduced to the degree smaller than that in the waiting state of the ink head.
That makes the meniscus stable and avoids jet failure of the droplet 31 and discharge
of large droplet.
[0069] Fig. 32 is a cross section schematically showing an exemplary structure to achieve
this aim. The ink 30 is supplied from an ink tank 34 through an ink supply tube 33
to the inside of the ink head 9. Applying the positive hydrostatic pressure Pp as
shown in Fig. 29 is achieved by moving the ink tank 34 in the upward direction P and
applying the negative hydrostatic pressure Pn or reducing the positive hydrostatic
pressure Pp as shown in Fig. 30 is achieved by moving the ink tank 34 in a downward
direction N. The moving operation of the ink tank 34 is achieved by using a well-known
technique.
[0070] The embodiment described above proposes a control to avoid the state where the meniscus
of the ink 30 is out of contact with the rim of the opening 95 while preventing the
ink 30 from spilling over from the opening 95. It is possible, however, to control
the hydrostatic pressure while keeping the liquid surface of the ink 30 in contact
with the rim of the opening 95 also in the state where the ink head is driven at the
dot cycle T3, instead of the waiting state of the ink head.
[0071] Figs. 33 to 35 are cross sections schematically showing the state of meniscus of
the ink 30 in the period while the jet burst signal 26 is supplied for the piezoelectric
vibrator 92. The negative hydrostatic pressure Pn is higher in Fig. 34 than in Fig.
33, and higher in Fig. 35 than in Fig. 34. As the negative hydrostatic pressure Pn
becomes higher, the liquid surface of the ink 30 moves in a direction from the opening
95 towards the piezoelectric vibrator 92. Since the first surface wave is created
when the liquid surface of the ink 30 is in contact with the rim of the opening 95,
the more converged droplets 31 can be jetted as the negative hydrostatic pressure
becomes higher, and narrowing the width of jetting, the droplets 31 responding to
more delicate image can be jetted.
[0072] The control of the shape of the meniscus shown in Figs. 33 to 35 is achieved by controlling
the ratio of the burst cycle T2 to the period k·T0 for supplying the jet burst signal
26 to change the length of period to apply the radiation pressure Pi to the ink 30.
Specifically, if the period k·T0 for supplying the jet burst signal 26 is fixed, the
burst cycle T2 is made longer to retreat the shape of meniscus towards the piezoelectric
vibrator 92 and that makes the width of jetting narrower. Further, in order to keep
the shape of meniscus stable, it is desirable to supply the piezoelectric vibrator
92 with the dummy burst signal and the pressure pulse signal as discussed in the first
and third preferred embodiments.
[0073] As discussed earlier, the piezoelectric vibrator 92 supplied with the jet burst signal
26 provides the ink 30 with the radiation pressure Pi as well as the sound pressure
Ps. Therefore, adjusting the timing of applying the radiation pressure Pi, i.e., the
burst cycle T2 to the cycle of free vibration specific to the opening 95 allows great
displacement of the meniscus.
[0074] Fig. 36 is a cross section schematically showing the motion of the meniscus in accordance
with this preferred embodiment Although the second surface wave is created from the
rim of the opening 95 by the radiation pressure Pi, since the jet burst signal 26
is supplied at the cycle of the free vibration specific to the opening 95, the meniscus
is greatly displaced to jet the large droplets 31.
[0075] Thus, in this embodiment, by controlling the burst cycle T2, the small droplets 31
are jetted from the first surface wave to obtain an excellent graininess when the
image data has a great number of tone levels, and the large droplets 31 are jetted
from the second surface wave to obtain a sharp image when the image data has a small
number of tone levels (e.g., binary image such as character information). Depending
on whether the number of tone levels is large or small, the suitable droplets 31 can
be deposited on printed paper.
[0076] In this embodiment, in order to keep the shape of meniscus stable, it is desirable
to supply the piezoelectric vibrator 92 with the dummy burst signal as discussed above
and the pressure pulse signal as discussed above.
[0077] Though there may be a case, naturally, where it is found from an actual measurement
that an optimum result can be obtained when there is a slight difference between the
burst cycle T2 and the cycle of free vibration specific to the opening 95, it it is
also possible to control the burst cycle T2 to excite the above free vibration by
utilizing resonance.
1. Flüssigkeitsstrahl-Treibervorrichtung, die folgendes aufweist:
- eine Flüssigkeitsstrahleinheit (9), die eine Strahlaustrittsfläche (93) aufweist,
die mit einer Öffnung (95) versehen ist, die dazu ausgebildet ist, eine Oberfläche
einer abzugebenden Flüssigkeit (30) begrenzt freizulegen, und die einen Schwingungserreger
(9a, 9b) aufweist, der auf der der Strahlaustrittsfläche (30) gegenüberliegenden Seite
mit der Flüssigkeit (30) dazwischen vorgesehen ist, um die Flüssigkeit (30) mit Schwingungen
zu beaufschlagen; und
- eine Treibereinheit (3), die dazu ausgebildet ist, den Schwingungserreger (9a, 9b)
mit einem Unterdrückungssignal (27) und mit einem Strahlausstoßsignal (26) zu beaufschlagen,
das aus einem Impulszug mit einer ersten Frequenz besteht, um den Schwingungserreger
(9a, 9b) anzusteuern;
- wobei die Schwingung, die für die Flüssigkeit (30) durch den mit dem Strahlausstoßsignal
(26) angesteuerten Schwingungserreger (9a, 9b) erzeugt wird, ausreichend ist, damit
die Flüssigkeit (30) von der Strahlaustrittsfläche (93) ausgestoßen wird, und wobei
Schwingung, die für die Flüssigkeit (30) durch den mit dem Unterdrückungssignal (27)
angesteuerten Schwingungserreger (9a, 9b) erzeugt wird, nicht ausreichend ist, um
die Flüssigkeit (30) aus der Strahlaustrittsfläche (93) auszustoßen,
dadurch gekennzeichnet,
daß der Schwingungserreger (9b) einen ersten Schwingungserreger (92), dem das Strahlausstoßsignal
zugeführt wird, und einen zweiten Schwingungserreger (96) aufweist, dem das Unterdrückungssignal
zugeführt wird.
2. Vorrichtung nach Anspruch 1,
wobei das Unterdrückungssignal (27) ein Unterdrückungsstoßsignal ist, das aus einem
Impulszug mit der ersten Frequenz besteht.
3. Vorrichtung nach Anspruch 1,
wobei das Unterdrückungssignal (27) eine geringere Anzahl von Impulsen als das Strahlausstoßsignal
(26) hat.
4. Vorrichtung nach Anspruch 1,
wobei das Unterdrückungssignal (27) ein Unterdrückungsstoßsignal ist, das aus einem
Impulszug mit der ersten Frequenz besteht, der eine geringere Amplitude als die des
Strahlausstoßsignals (26) hat.
5. Vorrichtung nach Anspruch 1,
wobei der erste Schwingungserreger (92) mit dem Strahlausstoßsignal angesteuert wird,
um die Flüssigkeit (30) mit einem Strahlungsdruck zu beaufschlagen,
wobei die Flüssigkeit (30) bedingt durch den Strahlungsdruck freie Schwingungen in
einem Modus ausführt, der für die Formgebung und die Größe der Öffnung (95) spezifisch
ist,
wobei das Unterdrückungssignal für den zweiten Schwingungserreger (96) in der Nähe
von mindestens einem Zeitpunkt zugeführt wird, in dem die freie Schwingung eine Amplitude
mit extremem Wert annimmt, und
wobei der zweite Schwingungserreger (96) mit dem Unterdrückungssignal angesteuert
wird, um die Flüssigkeit mit einem Druck zu beaufschlagen, der in einer Richtung zum
Unterdrücken der freien Schwingung wirksam ist.
6. Vorrichtung nach Anspruch 1,
wobei das Strahlausstoßsignal (26) und das Unterdrückungssignal (27) für den Schwingungserreger
(9a, 9b) wiederholt mit einer zweiten Frequenz zugeführt werden.
7. Vorrichtung nach Anspruch 6,
wobei die zweite Frequenz eine Frequenz ist, bei der die Flüssigkeit (30) freie Schwingungen
in einem Modus ausführt, der für die Formgebung der Öffnung (95) spezifisch ist.
8. Vorrichtung nach Anspruch 1,
wobei das Strahlausstoßsignal (26) während einer Periode, die für eine hinund hergehende
Bewegung einer Oberflächenwelle in der Öffnung (95) erforderlich ist, für den Schwingungserreger
(9a, 9b) wiederholt zugeführt wird, wobei diese auf der Basis eines Strahlungsdrucks
aktiviert wird, um die Flüssigkeit (30) in einer Periode, während der der Schwingungserreger
(9a, 9b) angesteuert wird, aus der Öffnung (95) auszustoßen.