[0001] The present invention relates to controlling the volume of ink droplets ejected from
a drop on demand ink jet apparatus, and more specifically though not exclusively to
a method for operating an ink jet apparatus for providing selective control within
a range of either the volume of the ink droplets ejected by the apparatus and/or the
amount of ink striking a desired location on a recording medium.
[0002] The design of practical ink jet devices and apparatus for producing a single droplet
of ink on demand is relatively new in the art. In prior drop on demand ink jet apparatus,
the volume of each individual ink droplet is typically dependent upon the geometry
of the ink jet apparatus, the type of ink used, and the magnitude of the pressure
force developed within the ink chamber of the ink jet rejecting an ink droplet from
an associated orifice. The effective diameter and design of the orifice, the volume
and configuration of the ink chamber associated with the orifice, the transducer design,
and the method of coupling the transducer to the ink chamber, are all factors determining
the volume of individual ink droplets ejected from the orifice. Typically, once the
mechanical design of an ink jet apparatus is frozen, control over the volume of the
ejected ink droplets can only be obtained from a narrow range by varying the amplitude
of the electrical pulses or dry voltage applied to the individual transducers of the
ink jet apparatus or array.
[0003] According to the invention from one aspect there is provided a method for controlling
the volume of ink droplets ejected from a drop on demand ink jet apparatus including
transducer means operable for producing a pressure disturbance within an associated
ink chamber, for ejecting an ink droplet from an associated orifice, the method being
characterised in that it comprises operating said transducer means in an iterative
manner, for producing a plurality of successive pressure disturbances within said
ink chamber, for causing a plurality of ink droplets to be ejected from said orifice
within a time period permitting said droplets to merge either while air-borne or upon
striking a recording medium.
[0004] According to the invention from another aspect there is provided apparatus for controlling
the volume of ink droplets ejected from a drop on demand ink jet apparatus characterised
in that it comprises transducer means operable for producing a pressure disturbance
within an associated ink chamber, for ejecting an ink droplet from an associated orifice,
and means operable for operating said transducer means in an iterative manner, for
producing a plurality of successive pressure disturbances within said ink chamber,
for causing a plurality of ink droplets to be ejected from said orifice within a time
period permitting said droplets to merge either while air-borne or upon striking a
recording medium.
[0005] The transducer can be operated for causing a plurality of successively higher, lower,
or equal velocity ink droplets, or some combination thereof, to be ejected from the
orifice of the ink jet. It has been found that when putting the invention into effect,
broader control of the boldness and toning of printing can be obtained. The volume
of ink striking a recording medium at a given point is thereby partly determined by
the number of ink droplets merged prior to striking or at the point of striking.
[0006] The invention will be better understood by referring, by way of example, to the accompanying
drawings, in which :
Figure 1 is a sectional view of one form of ink jet apparatus in accordance with the
invention;
Figure 2 is an enlarged view of a portion of the section shown in Figure 1;
Figure 3 is an exploded perspective view of the ink jet apparatus shown in Figures
1 and 2;
Figure 4 is a partial sectional/schematic diagram view of the transducer shown in
Figure 1 and 3, with the transducer in the de-energised state;
Figure 5 is a partial sectional/schematic diagram or view of the transducer of Figure
4 in the energised state;
Figure 6 shows the wave shapes for electrical pulses of one embodiment of the invention;
Figure 7 shows a typical ejection of an ink droplet from an orifice;
Figure 8 shows the ejection of an ink droplet form an orifice at a time when the previously
ejected ink droplet is still in flight;
Figure 9 shows the merging of two ink droplets while in flight;
Figure 10 shows a typical ink droplet formed after the merger of a number of ink droplets
just prior to striking a recording medium;
Figure 11 shows the waveshapes for electrical pulses for another embodiment of the
invention;
Figure 12 shows the waveshapes for electrical pulses for yet another embodiment of
the invention;
Figure 13 shows a waveshape for another embodiment; and
Figures 14 and 15 show waveshapes which in themselves do not fall within the scope
of the present invention but in some combination of waveshapes selected from Figures
13 to 15 to produce a plurality of ink droplets constitute further embodiments of
the invention.
[0007] Preferred ways of performing the present invention will now be described in the context
of the ink jet apparatus of Figs. 1 to 5. However, the invention may be performed
using a broad range of ink jet apparatus (especially drop on demand ink jet apparatus).
Accordingly, the ink jet apparatus to be discussed herein is presented for purposes
of illustration and example only, and is not meant to be limiting. Also, only the
basic mechanical features and operation of this apparatus are discussed in the following
paragraphs, and reference is made to co-pending U.K. patent application 2094233A for
further details concerning this apparatus. The reference designations used in Figures
1 to 5 are the same as used in Figures 7 to 11 the copending application, in order
to facilitate any referencing back to that application or the patent that may issue
therefrom.
[0008] With reference to Figures 1 to 3, the illustrative ink jet apparatus includes a chamber
200 having an orifice 202 for ejecting droplets of ink in response to the state of
energization of a transducer 204 for each jet in an array of such jets (see Fig. 3).
The transducer 204 expands-and contracts (in directions indicated by the arrows in
Fig. 2) along its axis of elongation, and the movement is coupled to the chamber 200
by coupling means 206 which includes a foot 207, a visco-elastic material 208 juxtaposed
to the foot 207, and a diaphragm 210 which is preloaded to the position shown in Figures
1 and 2.
[0009] Ink flows into the chamber 200 from an unpressurized reservoir 212 through restricted
inlet means provided by a restricted opening 214. The inlet 214 comprises an opening
in a restrictor plate 216 (see Fig. 3). As shown in Figure 2, the reservoir 212 which
is formed in a chamber plate 220 includes a tapered edge 222 leading into the inlet
214. As shown in Fig. 3, the reservoir 212 is supplied with a feed tube 223 and a
vent tube 225. The reservoir 212 is complient by virtue of the diaphragm 210, which
is in communication with the ink through a large opening 227 in the restrictor plate
216 which is juxtaposed to an area of relief 229 in the plate 226.
[0010] One extremity of each one of the transducers 204 is guided by the cooperation of
a foot 207 with a hole 224 in a plate 226. As shown, the feet 207 are slideably retained
within the holes 224. The other extremities of each one of the transducers 204 are
compliantly mounted in a block 228 by means of a compliant or elastic material 230
such as silicon rubber. The compliant material 230 is located in slots 232 (see Fig.
3) so as to provide support for the other extremities of the transducers 204. Electrical
contact with the transducers 204 is also made in a compliant manner by means of a
compliant printed circuit 234, which is electrically coupled by suitable means such
as solder 236 to an electrode 260 of the transducers 204. Conductive patterns 238
are provided on the printed circuit 234.
[0011] The plate 226 (see Figures 1 and 3) includes wholes 224 at the base of a slot 237
which receive the feet 207 of the transducers 204, as previously mentioned. The plate
226 also includes a receptacle 239 for a heater sandwich 240, the latter including
a heater selement 242 with coils 244, a hold down plate 246, a spring 248. associated
with the plate 246, and a support plate 250 located immediately beneath the heater
240. The slot 253 is for receiving a thermistor 252, the latter being used to provide
monitoring of the temperature of the heater element 242. The entire heater 240 is
maintained within the receptacle in the plate 226 by a cover plate 254.
[0012] As shown in Fig. 3, the variously described components of the ink jet apparatus are
held together by means of screws 256 which extend upwardly through openings 257, and
screws 258 which extend downwardly through openings 259, the latter to hold a printed
circuit board 234 in place on the plate 228. The dashed lines in Fig. 1 depict connections
263 to the printed circuits 238 on the printed circuit board 234. The connections
263 connect a controller 261 to the ink jet apparatus, for controlling the operation
of the latter.
[0013] The controller 261 is programmed to at an appropriate time, via its connection to
the printed circuits 238, apply a voltage to a selected one or ones of the hot electrodes
260 of the transducers 204. The applied voltage causes an electric field to be produced
transverse to the axis of elongation of the selected transducers 204, causing the
transducers 204 to contract along their elongated axis. When a particular transducer
204 so contracts upon energization (see Fig. 5), the portion of the diaphram 210 located
below the foot 207 of the transducer 204 moves in the direction of the contracting
transducer 204, thereby effectively expanding the volume of the associated chamber
200. As the volume of the particular chamber 200 is so expanded, a negative pressure
is initially created within the chamber, causing ink therein to tend to move away
from the associated orifice 202, while simultaneously permitting ink from the resevoir
212 to flow through the associated restricted opening or inlet 214 into the chamber
200. Given sufficient time, the newly supplied ink completely fills the expanded chamber
and orifice, providing a "fill before fire" cycle. Shortly thereafter, the controller
261 is programmed to remove the voltage or drive signal from the particular one or
ones of the selected transducers 204, causing the transducer 204 or transducers 204
to return to their deenergized states as shown in Fig. 4. Specifically, the drive
signals are terminated in a step like fashion, causing the transducers 204 to very
rapidly expand along their elongated axis, whereby via the visco-elastic material
208 the feet 207 of the transducers 204 push against the area of the diaphram 210
beneath them, causing a rapid contraction or reduction of the volume of the associated
chamber or chambers 200. In turn, this rapid reduction in the volume of the associated
chambers 200, creates a pressure pulse or positive pressure disturbance within the
chambers 200, causing an ink droplet to be ejected from the associated orifices 202.
Note that as shown in Figure 5, when a given transducer 204 is so energized, it both
contracts or reduces its length and increases its thickness. However, the increase
in thickness is of no consequence to the illustrated ink jet apparatus, in that the
changes in length of the transducer control the operation of the individual ink jets
of the array. Also note, that with present technology, by energizing the transducers
for contraction along their elongated axis, accelerated aging of the transducers 204
is avoided, and in extreme cases, depolarization is also avoided.
[0014] For purposes of illustration, assume that the pulses shown in Figure 6 are applied
via controller 261 to one of the transducers204. As shown, the first and second pulses
1 and 3 respectively each have an exponential leading edge and a substantially linear
trailing edge, respectively, peak amplitudes + V
1, + V
2 volts respectively, and pulse widths of T
l, T
2, respectively. Note that the shapes of the pulses 1,3, respectively, may be other
than as illustrated herein, depending upon the particular ink jet device being driven
and the particular application. In this example, the peak amplitude plus + V
2 of pulse 3 is greater than the peak amplitude V
I of pulse 1, and the fall time for the trailing edge of pulse 3 is less than the fall
time for the trailing edge of pulse 1. Since the degree of contraction of the selected
transducer 204 is directly related within a range to the amplitude of the pulse applied
to the transducer, the greater the amplitude, the greater the degree of contraction.
Accordingly, upon termination of a particular operating or control pulse, the magnitude
of the pressure disturbance produced in the associated chamber 200 will be directly
related within a range to the amplitude of the previously applied control pulse. Also,
the greater the slope or the less the fall time of the trailing edge of the control
pulse, the more rapid the expansion or elongation of the selected transducer 204 to
its rest state upon termination of the control pulse. Correspondingly, the greater
the rate of expansion of the transducer 204, the greater the magnitude of the resulting
pressure disturbance within the associated chamber 200. Assume that the.amplitudes
+ V
1 and + V
2 of pulses 1,3, respectively, are large enough to ensure ejection of an ink droplet
from associated orifice 202 upon termination of these pulses, respectively.
[0015] With reference to Figure 7, assume that pulse 1 is applied to a selected one of transducers
204. Upon termination of pulse 1, a typical ink droplet 5 will be ejected from the
associated orifice 202. Substantially upon the termination of pulse 1, assume that
pulse 3 is applied to the selected transducer 204. Shortly after the termination of
pulse 3, a second ink droplet 7 will be ejected from the associated orifice 202 as
shown in Figure 8, for example. Ink droplet 7 will have a substantially greater velocity
than the air-borne ink droplet 5 because the amplitude of pulse 3 is greater of that
than pulse 1 and the fall time of pulse 3 is less than that of pulse 1. Note that
as previously explained though, the velocity of the second ink droplet 7 will be greater
than that of ink droplet 5 so long as at least one of either the amplitude of pulse
3 is greater than that of pulse 1 even if the fall times of these pulses are equal,
or the fall time of pulse 3 is less than that of pulse 1 even if their amplitudes
are equal. Accordingly, either amplitude control of the control pulses, or trailing
edge fall time control of the control pulses or a combination of the two can be used
to produce a higher velocity second droplet 7 as illustrated in Figure 8, for example.
By properly controlling the pulse parameters, the velocity of the second ink droplet
7 can be made high enough to cause droplet 7 to catch up with droplet 5 while each
is air-borne, causing these droplets to begin to merge together as shown in Figure
9. Assuming sufficient flight time, the merger of droplets 5 and 7 may result in a
droplet shape as shown in Figure 10 prior to the merged droplets striking a recording
medium. Alternatively, depending upon the relative speeds (successively higher or
lower) of the droplets and movement of the recording media, the droplets can be made
to strike the recording media at the same point or spot, without merging while air-borne,
thereby obtaining the same result. In this manner, the size of the ink droplet or
volume of ink striking a recording media at a particular point is substantially increased
relative to using only a single droplet, and such control of the volume of ink directly
provides control of the boldness of printing. Typical values for the parameters of
pulses 1,3 used by the inventor in conducting his experiments, were 28 volts and 30
volts for + V
1, + V
2, respectively; 60 microseconds for each one of the pulse widths T
1 and T
2; and fall times of 2 microseconds and 1 microsecond for pulses 1,3, respectively.
The viscosity of the ink in this example was 12 centipoise. For the particular ink
jet device operated by the present inventor, the approximate diameter of droplet 5
was 1.8 mils, for the second ink droplet 7 was 2.2 mils, and for the merged ink droplet
9 was 4.0 mils. Other ink droplet diameters or volumes may be obtained within a range
via control of the amplitudes and fall times of pulses 1 and 3, as previously mentioned.
[0016] Within a range, control of the size of ink droplets ejected from the ink jet device
can be controlled by adjusting the amplitudes and fall times of the control pulses
applied to the ink jet device. The range of control of the volume of ink or ultimate
ink droplet size striking a recording media is substantially extended via another
embodiment of the present invention for merging a plurality of ink droplets in flight
or at the point of striking a recording media.
[0017] In Figure 11, the amplitudes + V
1, + V
2 of pulses 11, 13, respectively, are shown to be equal (typically 30 volts, for example).
In this example, the trailing edge of pulse 11 is about 10 microseconds in fall time,
whereas the trailing edge of pulse 13 has a fall time of about 1 microsecond. Accordingly,
the ink droplet resulting from the application of pulse 11 to a selected transducer
204 will have a velocity that is substantially slower than the velocity of the following
ink droplet resulting from the application of pulse 3 to the transducer 204. Accordingly,
only fall time control is being used to adjust the velocities of the ink droplets
resulting from the application of pulses 1 and 3. In this example, it is assumed that
the second ejected higher velocity ink droplet will merge with the first ejected ink
droplet while air-borne or at the point of striking a recording media, as previously
described.
[0018] In Figure 12, a third control or firing pulse 15 has been added following the termination
of pulse 13. In one experiment with a given ink jet device, the present inventor set
the amplitude of pulses 11, 13, 15 all at 30 volts (+ V
1, + V
2 and + V
3 all equal 30 volts), with pulses 11, 13 and 15 typically having exponential fall
times of 10 microseconds, 5 microseconds and 1 microsecond, respectively; and pulse
widths of 60 microseconds, 40 microseconds and 30 microseconds, respectively, for
example. When applied to a selected transducer 204 of the given ink jet device, pulse
11 caused a first ink droplet to be ejected, pulse 13 caused a second ink droplet
of greater velocity than the first to be ejected, and pulse 15 caused a third ink
droplet of even greater velocity to be ejected, whereby all of these ink droplets
were of such relative velocities that they merged in flight prior to striking a recording
media. In this manner, an even greater range of control can be obtained for adjusting
the size of an ink droplet in an ink jet system. Depending upon the distance of the
selected ink jet orifice 202 from the recording medium, the relative speeds of movement
of the recording medium and/or the ink jet head, and the design of the particular
ink jet device, it is possible that an even greater number of ink droplets can be
ejected at correspondingly greater velocities in order to permit merger in flight
or at the point of striking, providing even greater control of ink droplet size from
one marking position to another on a recording medium.
[0019] Note that in practice, an ink droplet is not ejected immediately after the termination
of a particular firing pulse. For example, if the pulses 1,3 of Figure 6 are applied
to a transducer 204 of the ink jet device used by the present inventor in his experiments,
an ink droplet 5 is ejected 4 microseconds after the termination of pulse 1, and the
second ink droplet is ejected 3 microseconds after the termination of pulse 3. The
velocity of the first ejected ink droplet was measured to be 3.5 meters per second
and of the second ejected ink droplet 5.0 meters per second.
[0020] With reference to Figure 13, the combination of waveshapes shown cause the ink jet
apparatus to emit two droplets, which merge at a common point of striking on a print
medium to produce dots varying in diameter from 5.3 to 5.6 milliinches, for producing
very bold print. Typically, T
l, T
2, T
3, and T
4 are 80, 4, 18 and 6 microseconds, respectively, with the amplitudes of pulses 17
and 19 at 110 volts, and pulse 21 at about 73 volts, for producing the previous dot
diameter range on a particular type of paper (Hammermill XEROCOPY, manufactured by
Hammermill Papers Co., Inc., Erie, PA), using an ink having a wax base. The type of
paper and ink formulation affects the dot diameter in a given application. Typically,
the fall time of pulses 17 and 19 are 9 microseconds and 1.0 microseconds, respectively.
Under the conditions indicated above, shortly after termination of pulse 17, a first
droplet having a velocity ranging from 8 to 10 meters per second was produced. Also,
the combination of pulses 19 and 21, caused a second droplet to be produced about
2 microseconds after the termination of pulse 19. Pulse 21 is not of sufficient amplitude
to cause a third droplet to be produced, but does cause the second droplet to breakoff
earlier from the orifice of the ink jet relative to operating without pulse 21. Also,
pulse 21 permits higher frequency operation of the ink jet apparatus, and reduced
ink blobbing problems at the orifice. Using the pulse time periods and amplitudes
mentioned above, the velocity of the second droplet is typically 6 to 8 meters per
second. The slower velocity of the second droplet relative to the first droplet is
caused by the presence of pulse 21. In this example, by increasing the amplitude of
pulse 19, the velocity of the second droplet can be increased. Also, by varying the
delay time T
2 between the termination of pulse 17 and initiation of pulse 19, the boldness can
be modulated within a range.
[0021] By using various combinations of the waveforms of Figures 13, 14 and 15, desired
shading can be accomplished. Such shading is known as half-toning. Note that with
respect to Figure 13, that although the second droplet is lower in velocity than the
first droplet, they are merged at a common point of impact as the point medium. In
Figure 14, by using only pulse 17 to operate the ink jet apparatus; a dot having a
diameter range of 3.3 to 3.5 milliinches can be obtained. Such dot diameters produce
much less bold print relative to operating the ink jet apparatus via the combination
of pulses 17, 19 and 21. With reference to Figure 15, the combination of pulses 17
and 21, as shown, operated the ink jet for producing an ink droplet having diameters
ranging from 2.9 to 3.0 milliinches. This combination produces a very light print.
It is stressed that the waveforms of Figure 14 or 15 on their own, which produce only
a single droplet, do not lie within the scope of the invention.
[0022] As previously mentioned, depending upon the relative speeds of the ink droplets,
the ink jet head, and the recording medium, the droplets can be made to strike the
recording medium at substantially the same spot or point, and are thereby merged at
that point for producing a desired dot size. Accordingly, the shapes of the waveforms
used to drive the ink jet apparatus can be designed to cause successively produced
ink droplets to have successively higher or lower relative velocities, or some combination
thereof, so long as system timing permits the droplets to strike the recording medium
at substantially the same point. In this manner, one droplet or a plurality of ink
droplets can be selectively chosen for printing a dot of desired boldness at a point
on a recording medium.
[0023] The controller 261 can be provided via hardwired logic, or by a microprocessor programmed
for providing the necessary control functions, or by some combination of the two,
for example. Note that a Wavetek Model 175 waveshape generator, manufactured by Wavetek,
San Diego, California, was used by the present inventor to obtain the waveshapes shown
in Figures 6, 11, 12, 13, 14 and 15. In a practical system, a controller 261 would
typically be designed for providing the necessary waveshapes and functions, as previously
mentioned, for each particular application.
1. A method for controlling the volume of ink droplets ejected from a drop on demand
ink jet apparatus including transducer means (204) operable for producing a pressure
disturbance within an associated ink chamber (200), for ejecting an ink droplet from
an associated orifice (202), the method being characterised in that it comprises operating
said transducer means (204) in an iterative manner, for producing a plurality of successive
pressure disturbances within said ink chamber (200), for causing a plurality of ink
droplets to be ejected from said orifice (202) within a time period permitting said
droplets to merge either while air-borne or upon striking a recording medium.
2. Apparatus for controlling the volume of ink droplets ejected from a drop on demand
ink jet apparatus characterised in that it comprises transducer means (204) operable
for producing a pressure disturbance within an associated ink chamber (200), for ejecting
an ink droplet from an associated orifice (202), and means operable for operating
said transducer means (204) in an iterative manner, for producing a plurality of successive
pressure disturbances within said ink chamber (200), for causing a plurality of ink
droplets to be ejected from said orifice (202) within a time period permitting said
droplets to merge either while air-borne or upon striking a recording medium.
3. A method according to claim 1 or apparatus according to claim 2, characterised
in that said transducer means (204) is responsive to an electrical pulse for producing
the pressure disturbance within an associated ink chamber (200), the magnitude of
the pressure disturbance being directly proportional to the slope of the trailing
edge of said electrical pulse, and the transducer means (204) is operated by applying
successive electrical pulses having either one of successively greater or reduced
or equal trailing edge slopes, or some combination thereof, to said transducer means
(204).
4. A method or apparatus according to claim 3, characterised in that said electrical
pulses are shaped to have exponential leading edges.
5. A method or apparatus according to claim 3 or 4, characterised in that the trailing
edges of said electrical pulses are shaped to be exponential.
6. A method or apparatus according to claim 5, characterised in that the amplitude
of each one of said electrical pulses is adjusted for obtaining a desired velocity
for an associated ink droplet, whereby the magnitudes of the pressure disturbances
produced by said transducer means (204) are directly proportional to the amplitudes
of said electrical pulses, respectively.
7. A method or apparatus according to claims 3 or 4, characterised in that the trailing
edges of said electrical pulses are shaped to be substantially linear.
8. A method or apparatus according to claim 7, characterised in that the amplitudes
of said electrical pulses are adjusted, whereby the magnitudes of said pressure disturbances
are directly proportional to the amplitudes of said pulses, respectively.
9. A method or apparatus according to any one of claims 3 to 8, characterised in that
a secondary pulse is applied immediately after given ones of said electrical pulses
for causing earlier breakoff from said orifice (202) of the ink droplets associated
with said given ones of said electrical pulses, relative to the time of breakoff of
said droplets in the absence of said secondary pulses.
10. A method or apparatus according to any one of claims 3 to 9, characterised in
that the delay time is controlled between said successive electrical pulses for controlling
the boldness of printing.