[0001] This invention relates to acoustic printers and, more particularly, to sparse arrays
of acoustic droplet ejectors for such printers.
[0002] Substantial effort and expense have been devoted to the development of plain paper
compatible direct marking technologies. Drop-on-demand and continuous stream ink jet
printing account for a significant portion of this investment, but these conventional
ink jet systems suffer from the fundamental disadvantage of requiring nozzles with
small ejection orifices, which easily clog.
[0003] Acoustic printing is a potentially important, alternative direct marking technology.
It is still in an early stage of development, but the available evidence indicates
that it is likely to compare favorably with ordinary ink jet systems for printing
either on plain paper or on specialized recording media, while providing significant
advantages of its own. More particularly, acoustic printing has increased intrinsic
reliability because there are no nozzles to clog. As will be appreciated, the elimination
of clogged nozzle failure is especially relevant to the reliability of arrays comprising
a large number of individual printing devices. Furthermore, small ejection orifices
are unnecessary, so acoustic printing is compatible with a greater variety of inks
than conventional ink jet printing, including inks having higher viscosities, and
inks containing pigments and other particulate components.
[0004] When an acoustic beam impinges on a free surface (i.e.. a liquid/air interface) of
a pool of liquid from beneath, the radiation pressure which the beam exerts may reach
a sufficiently high level to release individual droplets of liquid from the surface
of the pool, despite the restraining force of surface tension. To control the droplet
ejection process spatially , the acoustic beam advantageously is brought to focus
on or near the surface of the pool, thereby intensifying its radiation pressure for
a given amount of input power.
[0005] Ultrasonic (rf) acoustic beams have been employed in known ink jet and acoustic printing
systems for releasing small droplets of ink from pools of ink. For example, K. A.
Krause, "Focusing Ink Jet Head,"
IBM Technical Disclosure Bulletin, Vol 16, No. 4, September 1973, pp. 1168-1170 described an ink jet in which an acoustic
beam emanating from a concave surface and confined by a conical aperture was used
to propel ink droplets out through a small ejection orifice. US-A- 4,308,547 showed
that the small ejection orifice of the conventional ink jet is unnecessary. To that
end, it discloses providing spherical piezoelectric shells as transducers for supplying
focused acoustic beams to eject droplets of ink from the free surface of a pool of
ink, and acoustic horns driven by planar transducers to eject droplets of ink from
an ink-coated belt.
[0006] Classical line printing requires a separate acoustic droplet ejector for each of
the picture elements ("pixels") that are needed to define a line of the printed image.
This means that a very large number of densely-packed droplet ejectors must be provided
to perform line printing at an acceptably high resolution. Acoustic lens arrays for
printing at resolutions of 500 s.p.i. or even higher are believed to be feasible,
but care has to be taken to avoid crosstalk among the lenses of such an array. The
other known acoustic droplet ejection technologies, such as piezoelectric shell transducers
and IDTs, are believed to be inferior to the acoustic lens for high resolution line
printing, although they are fundamentally acceptable technologies. Unfortunately,
raster scan printing with a single acoustic droplet ejector is too slow for many of
the applications which are of interest. Accordingly, alternative printer design approaches
are still needed to provide increased design flexibility for moderate speed acoustic
printers, without sacrificing the pixel placement accuracy or the pixel size control
capabilities which make acoustic printing such an attractive direct marking technology.
[0007] This invention responds to that need by providing sparse arrays of droplet ejectors
for acoustic printing. Sparse droplet ejector arrays are capable of performing at
higher print rates than single ejector printheads because of their parallelism, and
they avoid many of the design limitations of ordinary page width arrays. For example
the increased center-to center spacing of the droplet ejectors in these sparse arrays
significantly simplifies the design of acoustic-lens-type arrays because larger lenses
and transducers may be employed, thereby enabling the transducers to operate at lower
power densities, permitting the use of thicker substrates for the lenses while maintaining
the diffraction of the acoustic power at a negligibly low level, and increasing the
permissible focal length of the lenses so as to permit the use of thicker layers of
ink.
[0008] Accordingly the present invention provides an acoustic printer which is as claimed
in the appended claims.
[0009] This invention will now be described, by way of example, with reference to the accompanying
drawings, in which:
Figure 1 is a fragmentary, simplified isometric view of an acoustic printer having
a printhead comprising a sparse array of acoustic lenses in accordance with this invention
;
Fig. 2 a more-detailed cross-sectional view of the printer shown in Fig. 1;
Fig. 3 is a fragmentary, isometric view of an acoustic printer having printhead comprising
a sparse array of piezoelectric spherical transducer shells, and
Fig. 4 is a fragmentary, schematic plan view of a printhead having a two-dimensional
sparse array of droplet ejectors in accordance with this invention.
[0010] Turning now to the drawings, and at this point especially to Figs. 1 and 2, there
is shown an acoustic printer 11 (shown only in relevant part) having a printhead 12
comprising a sparse linear array 13 of acoustic lenses 14a - 14i for ejecting individual
droplets of ink 15 (Fig. 1) on demand from a free surface 16 of an ink supply, such
as a pool of ink 17, to print an image on a suitable record medium 18, such as a sheet
of plain paper. A transport 19 supports the record medium 18 at a small gap distance
(e.g., 1 - 2 mm) from the free surface 16 of the ink 17, and the transport 19 advances
the record medium 18 crosswise and lengthwise of the array 13 at preselected rates
as more fully described below. The speed at which the ink droplets 15 are ejected
from the free surface 16 of the pool of ink 17 is sufficiently high to propel them
onto the record medium 18 with a well-defined and repeatable trajectory, so the picture
elements ("pixels") of the image are printed on accurately-located centers. Indeed,
the droplet placement accuracy is sufficiently precise and repeatable that multiple
droplets can be effectively deposited on top of each other in rapid succession, before
the ink has time to dry, whereby the number of ink droplets 15 per pixel may be controlled
to vary the size of the printed pixels, thereby imparting a gray scale shading to
the printed image.
[0011] The lenses 14a - 14i are defined by small spherical indentations or cavities which
are formed in a surface of a solid substrate 21 on relatively widely separated centers,
with the center-to-center spacing of the lenses 14a - 14i being selected to ensure
that they are effectively acoustically isolated from each other. A piezoelectric transducer
22 is intimately mechanically coupled to the opposite surface of the substrate 21,
and a controller 23 (Fig 2) is coupled across the transducer 22 to apply independently
controlled rf drive voltages across spatially-separated sites along the transducer
22 during operation. These drive voltages locally excite the transducer 22 into oscillation,
thereby causing it to generate spatially separated acoustic waves in the substrate
21, such as shown at 24a in Fig. 1, for illuminating the lenses 14a - 14i, respectively.
[0012] The substrate 21 is composed of a material, such as silicon, silicon nitride, silicon
carbide, alumina, sapphire, fused quartz, and certain glasses, having an acoustic
speed (i. e., the speed of sound in the substrate 21) which is much higher than the
speed of sound in the ink 17. Thus, the acoustic waves, such as the wave 24a (Fig.
1), propagate through the substrate 21 at a relatively high speed until they impinge
upon the lenses 14a - 14i, respectively, where they exit into a lower acoustic speed
medium (i.e., the ink 17 or an intermediate medium) as converging acoustic beams,
such as shown in Fig. 1 at 26a. The focal lengths of the lenses 14a - 14i, which
typically are approximately equal to their radius of curvature, are selected so that
these acoustic beams come to focus approximately at the free surface 16 of the ink
supply 17. In practice, the acoustic speed of the substrate 21 preferably is at least
about 2.5 times higher than the acoustic speed of the ink 17, which is sufficient
to ensure that the aberrations of the focused acoustic beams, such as 26a, are small.
Indeed, an acoustic speed ratio of 4:1, or even higher, is readily achievable should
it be necessary or desirable to reduce the aberrations to a negligibly-low level.
[0013] The printhead 11 may be immersed in the pool of ink 17 as shown, or provision may
be made for acoustically coupling the lenses 14a - 14i to an ink supply which is carried
by a suitable transport, such as a thin film of 'Mylar' or like plastics material.
As a general rule, the piezoelectric transducer 22 has a relatively narrow band resonant
frequency response characteristic, so the radiation pressures which the individual
acoustic beams, such as the beam 26a, exert against the free surface 16 of the ink
17 may be controlled with respect to a predetermined threshold level as required for
drop-on-demand printing by designing the controller 23 (Fig. 2) to modulate independently
the amplitude, frequency or duration of the rf voltages it applies to the transducer
22. Amplitude control, frequency control and pulse width modulation also can be employed
to control the size of the droplets of ink 15 which are ejected.
[0014] The threshold pressure for ejecting a droplet of ink 15 from the free surface 16
of the ink supply 17 is dependent on the particular ink that is employed, and can
be determined empirically. To stabilize the droplet ejection control process, the
free surface 16 of the ink supply 17 advantageously is maintained at a substantially
constant distance of about one focal length from the lenses 14a - 14i. Various techniques
may be employed to accomplish that. For example, as shown in Fig. 2, the illustrated
embodiment utilizes a closed-loop control system 31 comprising a laser 32 for supplying
a light beam 33 which strikes the free surface 16 of the ink supply 17 at a grazing
angle of incidence, together with a split photodetector 34 which intercepts the light
beam 33 after it reflects from the surface 16. The photodetector 34 is optically aligned
so that the light beam 33 centers on it only if the free surface 16 of the ink supply
17 is at its desired set point level. Thus, any significant deviation of the surface
16 from that level imbalances the outputs of the photodetector 34, thereby causing
a differential amplifier 35 to supply an error signal for energizing a motor 36. The
motor 36, in turn, drives a plunger 37 of an ink-filled pump 38 to add or drain ink
from the ink supply 17
via a supply line 39 as required to restore its free surface 16 to the desired set point
level. Alternatively, a knife-edge liquid level control technique, such as shown in
US-A- 4,580,148 could be utilized.
[0015] The acoustic waves, such as the wave 24(a), are diffracted as they propagate through
the substrate 21. In keeping with this invention, however, the center-to-center spacing
of the lenses 14a - 14i may be sufficiently great that the diffracted acoustic power
has a negligible effect on adjacent lenses, either by reason of the substrate 21 having
a thickness on the order of a Rayleigh length, or by virtue of the dissipation that
occurs over the distance between adjacent lenses. Acoustic mismatch regions (not shown)
may be built into the substrate 21, but sufficient acoustic isolation of the lenses
14a - 14i can also be achieved in a sparse array by merely locating the lenses 14a
- 14i on widely-separated centers. For example, a 1mm center-to-center spacing of
the lenses 14a - 14i may be employed to print a 500 s.p.i. image. The pixel diameter
at 500 s.p.i. is 50 µm, so if the lenses 14a - 14i are on 1mm centers, each of them
needs print only 20 pixels to form a solid line image lengthwise of the array 13.
That compares favorably to a single ejector scanner which would have to print approximately
4000 - 5000 pixels to form a solid line image across a normal 8˝ - 10˝ imaging field
at a resolution of 500 s.p.i. There is a print rate penalty associated with selecting
the center-to-center spacing of the lenses 14a - 14i so that it is much greater (typically,
an order of magnitude or more) than the pixel diameter, but the wider separation of
the lenses 14a - 14i greatly simplifies the fabrication of the printhead 12.
[0016] As best shown in Fig. 1, the transport 19 may comprise, for example, a drum 41 for
supporting the record medium 18, and an actuator 42 for rotating and translating the
drum 41 on an axis which is generally parallel to the longitudinal axis of the array
13. The drum 41 is rotated at a relatively high rate to advance the record medium
18 crosswise of the array 13, and is continuously or incrementally translated lengthwise
of the array 13 at a much slower rate, so that the record medium 18 is longitudinally
shifted approximately one pixel diameter with respect to the array per revolution
of the drum 41. As a result, the image is composed by printing full lines crosswise
of the array 13 and successive lines lengthwise of the array 13. Other transports
will, of course, suggest themselves.
[0017] Other types of acoustic droplet ejectors may be employed to provide a sparse array
in accordance with the present invention. For example, as shown, in Fig. 3, a printhead
51 comprising a suitable frame 52 for supporting a sparse array of piezoelectric spherical
shell transducers 53a - 53i may be utilized to carry out this invention. Heretofore,
piezoelectric shell transducers have been unattractive for use in arrays because their
diameters are too great to achieve the packing density needed for the simultaneous
printing of abutting pixels. This invention, however, overcomes that limitation and,
therefore extends the utility of such droplet ejectors. Apart from its printhead 51,
the embodiment of Fig. 3 is generally the same as the embodiment shown in Figs 1 and
2, so like reference numerals have been employed to identify like features.
[0018] As will be appreciated, the principles of the present invention may be extended to
printheads comprising two or more rows of droplet ejectors. See, for example, the
printhead 61 of Fig. 4.
[0019] In view of the foregoing, it will now be understood that the present invention simplifies
the design of acoustic printheads having arrays of droplet ejectors, while increasing
the design options relating to the choice of droplet ejectors for use in such arrays.
Moreover, it will be appreciated that the foregoing has been achieved, without significantly
compromising either the pixel placement accuracy or the pixel size control which make
acoustic printing such an attractive direct marking technology.
1. An acoustic printer, including means for moving a record medium (18) along a path
which is close to the intended free surface of ink of which droplets are intended
to mark the medium, and of which a supply is to be positioned between the record medium
and a printhead (21) having an array of acoustic ejectors (14) for ejecting individual
droplets of ink on demand from the free surface to print an image on the record medium,
the image being composed of a plurality of individual pixels on a predetermined center-to-center
spacing, the droplet ejectors having a center-to-center spacing which is approximately
at least an order of magnitude greater than the center-to-center spacing of the pixels,
and wherein the record medium is able to be moved lengthwise and crosswise of the
array during the printing process.
2. The printer as claimed in Claim 1, wherein the droplet ejectors comprise
respective spherical acoustic lenses (53) which are substantially acoustically
isolated from each other, and
piezoelectric transducers for supplying separate acoustic waves to the lenses.
3. The printer as claimed in Claim 2, wherein the center-to-center spacing of the
droplet ejectors is sufficient to isolate the acoustic lenses acoustically from each
other.
4. The printer as claimed in any preceding Claim, wherein the droplet ejectors are
part-spherical piezoelectric transducer shells.
5. The printer as claimed in any preceding Claim, wherein the droplet ejectors are
acoustically coupled to the free surface of the ink to direct focused acoustic beams
at the surface.
6. The printer as claimed in any preceding Claim, wherein the droplet ejectors are
aligned longitudinally of the printhead.
7. The printer as claimed in any preceding Claim, wherein
the record medium is able to be advanced crosswise of the array at a relatively
high speed and synchronously translated lengthwise of the array at a much slower speed,
with the speeds being selected so that the translation of the record medium per cycle
is approximately equal to the center-to-center spacing of the pixels.
8. The printer as claimed in any preceding Claim, wherein the printhead is adapted
to be immersed in ink