Note: Descriptions are shown in the official language in which they were submitted.
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~Q~__ELESS LIQUID DROpl,ET EJECTQRS
FIELD OF THE INVENTION
This invention relates to leaky Rayleigh wave focused
acoustic generators for ejecting liquid droplets from
liquid filled reservoirs and, more particularly, to
relatively reliable print heads for ink jet printexs and
the like.
BACKGROUND OF THE I NVENTI ON
Substantial effort and expense have been devoted to the
development of ink jet printers, especially during the
past couple of decades. As is known, ink jet printing
has the inherent advantage of being a plain paper
compatible, direct marking technology, but the printers
which have been developed to capitalize on that advantage
have had limited commercial success. Although the
reasons for the disappointing commercial performance of
these printers are not completely understood~ it is
apparent that the persistent problems which have impeded
the development of low cost, reliable print heads for
them have been a contributing factor. Print heads have
been provided for low speed ink jet printers, but they
have not been fully satisfactory from a cost or a
reliability point of view. Moreover, higher speed ink
jet printing has not been practical due to the
performance limitations of the available print heads.
; "Continuous stream" and "drop on demand" print heads have
been developed for ink jet printers. There are
functional and structural differences which distinguish
those two basic print head types from one another, but
print heads of both types customarily include nozzles
which have small ejection orifices for defining the size
of the liquid ink droplets emitted thereby. They,
therefore, suffer from many of the same drawbacks,
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including unscheduled maintenance requirements because of
clogged nozzle6 and a fundamental cost barrier due to the
expense of manufacturing the nozzles.
Others have proposed noz71eless print heads for ink jet
printing. For example, Lovelady et al. United States
Patent No. 4,308,547, which issued December 24, 1981 on a
"Liquid Drop Emitter," pertains to acoustic print heads
for such printers. This patent is especially noteworthy
because one of its emhodiments relates to a print head in
which a hemispherically shaped piezoelectric transducer
i6 submerged in a reservoir or pool of liquid ink for
launching acoustic energy into the reservoir and for
bringing that energy to focus at or near the surface of
the reservoir, so that individual droplets of in~ are
15 propelled therefro~As will be seen, the patent also
proposes an alternative embodiment which utiliæes a
planar piezoelectric crystal for generating the acoustic
energy, a conical or wedged shaped horn for bringing the
acoustic energy to focus, and a moving belt or web for
transporting the ink into position to be propelled by the
focused acoustic energy. However, the additional
complexity of this alternatiYe proposal is contrary to
the principal purpose of the present invention.
A substantial body of prior art is available on the
subject of acou~tic liquid droplet ejectors in ~eneral.
Some of the earliest work in the field related to fog
generators. See Wood R./W. and Loomis, A.L., "The
Physical and Biological Effects of High Frequency Sound-
Waves of Great Intensity," Phil. Maq. A Ser. 7, Vol. 4,
No. 22, Sept. 1927, pp. 417-436 and Sollnar, K., ~'The
Mechanism of the Formation of Fogs by Ultrasonic Waves,"
Trans. Faraday ~oa. . Vol. 32, 1936, pp. 1532-1536. Now,
however, the ph~vsics of such ejectors are sufficiently
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well understood to configure them for ink jet printing
and other applications where i.t is necessary to control
both the timing of the droplet ejection and the size of
the droplets that are ejected. Indeed, an inexpensive,
reliabIe, readily manufacturable liquid droplet ejector
providing ~uch control is clearly needed for nozzleless
ink jet printing and the like.
SUMMARY OF THE INVENTION
In response to the above-identified need, the present
invention provides a nozzleless droplet ejector
comprising a surface acoustic wave transducer which is
submerged at a predetermined depth in a liquid filled
reservoir for launching a converging cone of coherent
acoustic waves into the reservoir, thereby producing an
acoustic beam which comes to a focus at or near the
surface of the reservoir (i.e., the liquid/air
interface). The acoustic beam may be intensity modulated
or focused/defocused to control the ejection timing, or
an external source may be used to extract droplets from
the acoustically excited liquid on the surface of the
pool on demand. Regardless of the timing mechanism
employed, the size of the ejected droplets is determined
by the waist diameter of the focused acoustic beam.
To carry out this invention, the transducer has a pair of
multi-element, ring-shaped electrodes which are
concentrically deposited in interdigitated relationship
on the upper surface of an essentially planar
piezoelectric substrate, whereby radially propagating,
coherent Rayleigh waves are piezoelectrically generated
on that surface (the "active surface" of the transducer)
when an ac. power supply is coupled across the
electrodes. Due to the incompressibility of the liquid
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and the relatlvely low velocity o~ sound through it,
these surace acoustic waves cause a generally circular
pattern of coherent lon~itudinal acoustic waves to leak
into the reservoir at a predetermined acute angle with
S respect to the active surface of the transducer, thereby
producing the focused acoustic beam. Electrically
independent interdigitated outrigger electrodes may be
deposited on the transducer substrate radially outwardly
~rom the ring-shaped electrodes to allow for acoustic
steering of the focused acoustic beam in a plane parallel
to the surface of the re~ervoir. For example, two
orthogonal sets of outrigger electrodes may be provided
to perform the acoustic steering required for matrix
printing and the like. Alternatively, a beam steering
capability may be built into the transducer by
circumferentially segmenting its interdigitated ring-
shaped electrodes, thereby permitting them to be
diffsrentially excited.
In view of the planar geometry of the transducer,
standard fabrication processes, such as photolithography,
may be employed to manufacture precisely aligned,
integrated linear and areal arrays of such transducers,
6 o i nexpensive and reliable multiple droplet ejector
arrays can be produced.
2 5 BRI EF DESCRI PTI ON OF THE DRAWI NGS
Other objects and advantages of this invention will
becon~e apparent when the following detailed description
is read in conjunction with the attached drawings, in
which:
Figure 1 is a sectional elevational view of a li~uid
droplet ejector constructed in accordance with the
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present invention;
Figure 2 is a plan view of a sur~ace acoustic wave
transducer for the ejector shown in Figure 1;
Figure 3 is a plan view of a linear array of surface
acoustic wave transducers which have orthogonal steering
electrodes for performing matrix printing and the like;
Figure 4 is a sectional elevational view taken along the
line 4-4 in Figure 3 to illustrate the beam steering
mechanism in further detail;
Figure 5 is a plan view of a surface acoustic wave
transducer having circumferentially segmented, ring-like
interdigitated electrodes for beam steering; and
Figure 6 is a sectional elevational view taken along the
line 6-6 in Figure 5 to illustrate the alternative beam
steering mechanism in further detail.
DETAI LED DESCRI PTI ON O~ THE I LLUSTR~TED EMBODI MENTS
While the invention is described in some detail
hereinbelow with reference to certain illustrated
embodiment, it is to be understooà that there is no
intent to limit it to those embodiments. On the
contrary, the aim is to cover all modifications,
alternatives and equivalents falling within the spirit
and scope of the invention as defined by the appended
claims.
Turning now to the drawings, and at this point especially
to Figures 1 and 2, there is a nozzleless droplet ejector
11 comprising a surface acoustic wave transducer 12 which
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is submerged at a predetermined depth in a liquid filled
reservoir 13 for ejecting individual droplets of liquid
14 therefrom on demand. Provision (not shown) may be
made for replenishing the liquid level of the reservoir
13 duri~ng operation to ensure that the submersion depth
of the transducer 12 remains essentially constant.
For ink jet printing, the ejector 11 emits a time
sequenced series of liquid ink droplets 14 from the
reservoir 13 to print an image on a suitable recording
medium 16, such as plain paper. The recording medium 16
is located a short distance above the liquid/air
interface (i.e., the surface) 17 of the reser~oir 13, so
the velocity at which the droplets 14 are ejected from
the reservoir 13 is selected to cause them to traverse
that air gap with substantial directional stability. In
practice, baffles (not shown) may be provided for
supprsssing at least some of the ambient air currents
which might otherwise cause unwanted deflection of the
droplets 14.
The recording medium 16 typically is advanced in a cross-
line direction, as indicated by the arrow 18, while an
image i6 being printed. The ejector 11, on the other
hand, may be mounted on a carriage (not shown) for
reciprocating movement in an orthogonal direction
parallel to ~he plane of the recording medium 16, thereby
permitting ~he image to be printed in accordance with a
rastex scanning pattern. Alternatively, a line length,
linear array of ejectors 11 (see ~igure 3) may be
provided for printing the image on a line-by-line basis.
Such an array may be shifted (by means not shown~ back
and ~orth in the orthogonal direction while the image is
being printed to more completely ~ill the spaces between
the ejectors 11, without having to reduce their center-
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to-center spacing. See K.A. Fishbeck's commonly assigned
United States Patent No. 4,509,058, which issued April 2,
1985 on "Ink Jet Printing ~sing Horizontal Interlace."
Areal ejector arrays (not shown) also may be co~lstructed
in accordance with this invention, but an areal array
having the large number of ejectors 11 which would be
required for printing a standard page size image at an
acceptable resolution without any relative movement of
the recording medium 16 is likely to be too expensive for
most applications.
In keeping with the present invention, the transducer 12
comprises a pair of ring-shaped electrodes 21 and 22,
each of which is radially patterned to have a plurality
of electrically interconnected, ring-like elements 23a-
23l and 24a-2~l, respectively. As will be seen, the
electrode elements 23a-23l and 24a-24i are concentrically
deposited in interdigitated relationship on a generally
planar surface 26 of a piezoelectric substrate 27.
Furthermore, in the illustrated embodiment, the electrode
elements 23~-23l and 24a-24l are of essentially uniform
width and have a fixed radial pitch, but it is to be
understood that their width and/or pitch may be varied
without departing from this invention. For example, the
width of the electrodes elements 23a-23l and 24a-24l may
be varied radially of the transducer 12 to acoustically
apodize it. Li~ewise, the pitch of the electrodes
elements 23a-23l and 24a-2~l may be varied radially of
the transducer 12 to permit its acoustic focal length to
be increased or decreased under electrical control. The
substrate 27 may be~ a piezoelectric crystal, such as
LiNBO3, or a piezoelectric polymer, such as PVF2.
Indeed, it will be apparent to persons of ordinary skill
in the art that the substrate 27 may be a composite
composed o~ a piezoelectric film, such as ZnO2, deposited
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upon a passi.ve insulating support, such as a glass. It
is important to note, however, that its electrode bearing
or "active~ surface 26 is planar because that ~acilitates
the use of standard metallization patterning processes,
such as- photolithography, for fabricating the electrodes
21 and 22. As will be understood, the relatively simple
and straightforward construction of the transducer 12 is
a significant advantage, especially for the cost
effective production of the linear and areal transducer
arrays which may be needed for applications requiring
precisely aligned arrays of droplet ejectors 11.
In operation, the transducer 12 is oriented with its
active surface 26 facing and in generally parallel
alignment with the surface 17 of the reservoir 13.
Furthermore, an ac. power supply 31 having a
predetermined, output frequency of between approximately
1 MHz. and 500 MHz. is coupled across its electrodes 21
and 22, whereby its pie~oelectric substrate 27 is excited
to generate a Rayleigh-type acoustic wave which travels
along the surface 26. Due to the ring-like shape of the
electrodes 21 and 22, the Rayleigh wave has a pair of
nearly circular, opposed wavefronts which propagate
radially inwardly and outwardly, respectively, with
respect to the electrodes 21 and 22. The outwardly
propagating or expanding wavefront is gradually
attenuated as it expands away from ~he electrodes 21 and
22, but the inwardly propagating or contracting wavefront
create~ a constructive interference centrally of the
electrodes 21 and 22. The output fre~uency of the power
supply 31 is matched with the radial pitch (or one of the
radial pitches) of the interdigitated electrode elements
23a-23i and 24a-24i to efficiently transform the
electrical energy into acoustic energy.
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Coherent acoustic waves are induced into the
incompressible li~uid 32 within the reservoir 13 in
response to the Rayleigh waves generated by the
transducer 12. More particularly, a converging cone of
S longitudinal acoustic waves are launched into the liquid
32 in response to the contracting wavefront of surface
acoustic waves, and a diverging, doughnut-like cone of
longitudinal acoustic waves are launched into the liquid
32 in response to the expanding wavefront of surface
acoustic waves. As will be seen, the coverging cone of
induced or acoustic waves from an acoustic beam 33 for
ejecting the droplets 14 from the reservoir 13 on demand.
The diverging acoustic waves, on the other hand, can be
ignored because they are suppressed, such as by sizing or
otherwise constructing the reservoir 13 to prevent any
significant reflection of them.
In accordance with the present invention, provision is
made for bringing the acoustic beam 33 to a generally
spherical focus approximately at the surface 17 of the
reservoir 13. The speed (Sp) at which sound travels
through the piezoelectric substrate 27 of the transducer
12 characteristically is much greater than the speed (Sl)
at which it travels through the liquid 32. Thus, the
generally circular wavefront of the converging leaky
Rayleigh waves propagates into the liquid 32 at an acute
angle, , with respect to the surface 26 of the
transducer substrate 27, where
~ =sin~1(SI/S~
Accordingly, the depth at which the transducer 12 should
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be submerged in the reservoir 13 to cause the acoustic
beam 33 to come to a focus at the surface 17 of the
reservoir, using the outermost electrode of the
transducer 12 as a reference, is less than or equal to:
d
D = -- (2)
2 tan lsin l (Sj/Sp)l
where: D = lhe subme~ion dcpu17 cf the Lr3nsducer 12 as mcasured to
i~s electrode bc~ring surface 26;
d = the outside diamc~cr of the electrodes 21, 22~
Equation (2) assumes a diffraction limited focus of the
acoustic beam 33, such that the wavelength , of the
transducer generated Rayleigh waves determines the waist
diameter of the acoustic beam 33 at focus. As will be
understood:
A = Sp~f (3)
where: f = the output frequcncy of ~he power supply 31
The surface tension and the mass density of the liquid 32
determine the minimum threshold energy level for ejecting
droplets 14 from the reservoir 13. Moreover, additional
energy is required to eject the droplets 14 at the
desired e~ection velocity. To meet these energy
requirements, suitable provision may be made for
controlling the ac. power supply 31 so that it intensity
modulates the acoustic beam 33 to acoustically propel the
droplets 14 from the reservoir 13 on damand.
Alternatively, as indicated by the arrow 36, provision
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may be made for selectively focusing and defocusin~ the
acoustic beam 33 on the surface 17 of the reservoir 13,
such as by mechanically moving the transducer 12 up and
Aown in the reservoir 13 or by modulating the frequency
at which it is being driven. Still another option is to
provide an external source 34 for controlling the
ejection timing of the droplets 14.
For external timing control, the intensity of the
acoustic beam 33 advantageously is selected to
acoustically excite the liquid 32 within the beam waist
to a sub-threshold, incipient droplet formation energy
state (i.e., an energy level just slightly below the
threshold level for forming a droplet 14 at room
temperature), whereby the external source 34 need only
supply a small amount of supplemental energy to cause the
ejection of the droplet 14. As will be appreciated, the
supplemental energy supplied by the external source 34
may in any suitable form, such as thermal energy for
heating the acoustically excited liquid 32 to reduce its
surface tension, or electrostatic or magnetic energy for
attracting an acoustically excited, electrostatically
charged or a magnetically responsive, respectively,
liquid 32. RegardIess of the techni~ue employed to
~ontrol the ejection timing, the size of the ejected
droplets 14 is primarily determined by the waist diameter
of the acoustic beam 33 as measured at the surface 17 of
the reservoir 13.
Referrin~ to Figures 3 and 4, there is an array 41 of
surface acoustic wave transducers 12aa-12a to form an
array of droplst ejectors 11a (only one of which can be
seen in Fig. 4). As shown, the transducers 12aa-12~' are
linearly aligned on uniformly separated centers, so they
are suitably configured to enable the droplet ejectors
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1la to function as a multi-element print head for ink jet
1ine printing. Preferably, the transducer 12~-12~ are
integrated on and share a single or common piezoelectric
substrate 27~, thereby permitting the alignment of the
transducers 12aa-12a to be performed while they are
being manufactured.
The transducers 12aa-12a are identical to each other and
are similar in construction and operation to the above-
described transducer 12, except that the transducers
12~a-12~ further include provision for acoustically
steering their focused acoustic beams 33a. As a result
of the beam steering capability of the transducers 12aa-
12ai, the droplet ejectors 11a have greater flexibility
than the ejector 11 (for instance, the ejectors 1la may
be used for dot matrix ink jet printing or they may
perform solid line printing without the need for any
mechanical motion of the transducers 12aa-12al, but they
otherwise are related closely to the ejector 11.
Therefore, to avoid unnecessary repetition, like parts
are identified by like reference numerals using a
convention, whereby the addition of a single or double
letter suffix to a reference numeral used hereinabove
identifies a modified part shown once or more than once,
respectively. Unique references are used to identify
unique parts.
The transducer 12aa is generally representative of the
transducers within the array 41. It has a pair of
radially pattern, interdigitated, ring-shaped electrodes
21 and 22, 60 it may launch an acoustic beam 33a into a
liquid filled reservoir 13a and bring the beam 33a to a
focus approximately at the surface 17 of the reservoir
13a as described hereinabove. Additionally, the
transducer 12aa has interdigitated outrigger electrodes
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43, 44 and ~5, 46, which are deposited on the surface 26a
of the piezoelectric substrate 27a concentrically with
the electrodes 21 and 22 and radially outwardly
therefrom. The outrigger electrodes 43, 44 and 45, 46
are electrically independent of the electrodes 21 and 22,
but they may be fabricated concurrently therewith using
the same metalli~ation patterning process.
To steer the acoustic beam 33a in a plane parallel to the
surface 17 of the reservoir 13a (i.e., a plane parallel
to the recording medium 16), the outrigger electrodes 43,
44 and 45, 46 are of relatively short arc length, so that
they cause circumferentially asymmetrical Rayleigh waves
to propagate along the surface 26a when they are
energized. The electrodes 21 and 22 and the outrigger
electrodes 43, 44 and 45, 46 may be coherently or
incoherently driven. However, if they are coherently
dri~en, it is important that they be suitably phase
synchronized to avoid destructive interference among the
Rayleigh waves they generate.
As will ~e appreciated, the circumferentially
asymmetrical Rayleigh waves that are produced by
energizing the outrigger electrodes 43, 44 and/or 45, 46
cause asymmetrical acoustic waves to leak into the liquid
32, thereby causing the focused beam 33~ to shift
~5 parallel to the surface 17 of the reservoir 13~ until it
reac~es an acoustic equilibrium. Ideally, the outrigger
electrodes 43, 44 and 45, 46 are electrically independent
of one another and are positioned orthogonally with
respect to one another, thereby permitting the beam 33a
to be orthogonally steered for dot matrix ink jet
printing and similar applications.
Turning to Figures 5 and 6, differential phase and/or
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amplitude e~citation of an electrically segmented surface
acoustic wave transducer 12ba al50 may be employed for
beam steexing purposes. To that end, the transducer 12
has a ring-like interdigitated electrode structure which
is circumferentially segmented to form a plurality of
electrically independent sets of electrodes 21kl, 22bl;
21~2, 22k2; and 21b3, 22b3. Three sets of electrodes are
shown, two of which (21bl, 22bl and 21b2, 22b2) span arcs
of approximately 90 each and the third of which (21b3
and 22b3) spans an arc of approximately 180, but it will
be understood that the number of independent electrode
set6 and the arc spanned by each oE them may be selected
as required to best accommodate a given application of
beam steering function. Separate sources 31b1, 31k2 and
a 31b3 are provided for exciting the electrode sets 21kl,
22kl; 21k2, 22~2; and 21b3, 22b3, respectively.
Unidirectional steering of the acoustic beam 33 is
achieved by adjusting the relative amplitudes of the ac.
drive voltages applied across the electrodes 21kl, 22kl;
21b2, 22~2; and 21k3, 22b3, while bidirectional steering
is achieved by adjusting the relative phases of those
voltages. The axes about which such steering occurs are
orthogonal to one another in the illustrated embodiment,
so there is a ~ull 36Co control over the direction in
which the droplet 14~ is sjected from th~ rsservoir~ As
will be ùnderstood, a linear or areal array of
transducers 12ba may be employed to form an array of
droplet ejectors (see Fig. 3), pxeferably on a common
pie7oelectric substrate 27a.
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CONCL~SION
In view of the foregoing, lt will he understood that the
present invention provides relatively inexpensive and
reliabl'e nozzleless liquid droplet ejectors, which may be
appropriately configured for a wide variety of
applications.
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