U.S. patent number 3,832,579 [Application Number 05/330,360] was granted by the patent office on 1974-08-27 for pulsed droplet ejecting system.
This patent grant is currently assigned to Gould Inc.. Invention is credited to John P. Arndt.
United States Patent |
3,832,579 |
Arndt |
August 27, 1974 |
PULSED DROPLET EJECTING SYSTEM
Abstract
A reservoir supplies liquid through a conduit to a nozzle. The
liquid is under small or zero static pressure. Surface tension at
the nozzle prevents liquid flow when the system is not actuated. A
section of the conduit terminating at the nozzle is designed to be
capable of conducting pressure waves in the liquid from end to end
of the section without the occurence of significant reflections
within the section. An electroacoustic transducer is coupled to the
liquid in the reflection-free section. When an electric pulse is
applied to the transducer it applies a pressure pulse to the liquid
sending a pressure wave to the nozzle where it causes ejection of a
droplet. The pressure pulse also sends a pressure wave in the
opposite direction. The system has energy absorbing means coupled
to the liquid and adapted to absorb substantially all of the energy
of the latter wave, thus preventing reflections which could return
to the nozzle and interfere with ejection of a subsequent droplet.
Two classes of energy absorbing means are described: (a) conduit
walls of viscoelastic material which deform under the influence of
the pressure wave and absorb energy therefrom, and (b) several
forms of acoustic resistance elements within the conduit at the
inlet end of the reflection-free section.
Inventors: |
Arndt; John P. (Cleveland,
OH) |
Assignee: |
Gould Inc. (Chicago,
IL)
|
Family
ID: |
23289421 |
Appl.
No.: |
05/330,360 |
Filed: |
February 7, 1973 |
Current U.S.
Class: |
310/326; 347/68;
347/94; 347/47 |
Current CPC
Class: |
B41J
2/055 (20130101); H04R 17/08 (20130101) |
Current International
Class: |
B41J
2/055 (20060101); H04R 17/04 (20060101); H04R
17/08 (20060101); H04r 017/00 () |
Field of
Search: |
;310/8.1,8.3,8.5,8.2
;346/75,140 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Budd; Mark O.
Attorney, Agent or Firm: Hyde; Eber J.
Claims
What is claimed is:
1. A pulsed droplet ejecting system comprising:
a reservoir;
liquid contained in said reservoir;
a conduit communicating with said liquid in said reservoir and
filled with said liquid, said conduit comprising a first section
having an inlet end and an outlet end;
a nozzle terminating said outlet end of said section and filled
with said liquid; and
an electroacoustic transducer coupled to said liquid in said
section;
said section, with said transducer coupled to the liquid therein,
being dimensioned to conduct pressure waves in said liquid between
said ends substantially free of internal reflections and being
dimensioned relative to the properties of said liquid to have a
given characteristic acoustic impedance;
said transducer being adapted to apply a pressure pulse to said
liquid whereby a first pressure wave travels in said liquid in said
first section to said nozzle and causes ejection of a droplet
therefrom, and whereby a second pressure wave travels in said
liquid in said first section toward said inlet end of said
section;
said conduit comprising a second section which is attached to the
inlet end of said first section and is comprised of viscoelastic
material and is dimensioned relative to the properties of said
liquid and to the properties of said viscoelastic material to have
characteristic acoustic impedance substantially matching said
characteristic acoustic impedance of said first section so that the
said second pressure wave travels in said liquid from said first
section into said second section without deleterious reflection at
said inlet end of said first section and which is dimensioned
relative to properties of said viscoelastic material so that said
second wave is substantially fully absorbed by said viscoelastic
material.
2. A pulsed droplet ejecting system as described in claim 1 in
which said transducer is a piezoelectric transducer.
3. A pulsed droplet ejecting system as described in claim 2 in
which said first conduit section is cylindrical and in which said
piezoelectric transducer surrounds said section and is in stress
transmitting engagement therewith.
4. A pulsed droplet ejecting system as described in claim 3 in
which said second section is a viscoelastic tube having inside
diameter smaller than the inside diameter of the inlet end of said
first section, except that said second section is enlarged where it
is attached to said inlet end with the inner surface of said
enlarged portion engaging the outer surface of said inlet end.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a system for ejecting droplets of liquid
on command suitable for use in apparatus such as ink jet printers
and facsimile recorders.
2. Description of the Prior Art
This invention is an improvement on the system described in U.S.
Pat. No. 3,683,212, issued to Steven I. Zoltan on Aug. 8, 1972,
assigned to the same assignee as the present invention.
A system constructed as described in the Zoltan patent having the
dimensions cited by way of example works very well when the pulse
rate is less than about one kiloHertz. If the pulsing is continuous
and the pulse rate is gradually increased above about one
kiloHertz, alternate increases and decreases in droplet velocity
may be observed.
When a burst of pulses equally spaced in time is applied to the
system, and the time interval between pulses exceeds about one
millisecond, the resulting droplets are ejected with uniform
spacing. However, when the time between pulses is decreased to a
fraction of a millisecond, the first several droplets which are
ejected generally have irregular spacing.
The above described irregularities are undesirable in many
applications. An experimental and theoretical investigation has
shown that they are caused by acoustic resonances, reflections, and
interference phenomena in the liquid in the system.
OBJECT AND SUMMARY OF THE INVENTION
The object of this invention is to provide a droplet on command
system generally similar to the system described in U.S. Pat. No.
3,683,212 but which is substantially free of the irregular
performance at high pulse rates observed in systems constructed as
described in that patent.
According to the invention a reservoir supplies liquid through a
conduit to a nozzle. A section of the conduit terminating at the
nozzle is adapted to conduct pressure waves in the enclosed liquid
without the occurence of significant reflection within the section.
An electroacoustic transducer is coupled to the liquid in the
reflection-free section of the conduit and is adapted to apply a
pressure pulse to the liquid whereby a first pressure wave travels
in the liquid to the nozzle and causes ejection of a droplet
therefrom, and a second pressure wave travels in the liquid toward
the inlet end of the reflection-free section. An energy absorbing
means is coupled to the liquid in the conduit and is adapted to
absorb substantially all of the energy of the second pressure
wave.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, together with
other and further objects thereof, reference is had to the
following description taken in connection with the accompanying
drawings, and its scope will be pointed out in the appended
claims.
In the drawings:
FIG. 1 shows a system according to the invention partly in section
and partly schematic;
FIG. 2 shows a test set up for selecting certain system
parameters;
FIG. 3 shows graphs obtained with the set up of FIG. 2;
FIGS. 4 to 11 inclusive show modifications of the system of FIG.
1;
FIG. 12 is an exploded view of a system according to the invention
which differs substantially in mechanical detail from the system
shown in FIG. 1; and
FIG. 13 is a conventional sectional view along lines 13--13 of FIG.
12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a reservoir shown schematically at 1 contains
ink or other liquid 2. A conduit indicated generally by reference
character 4 communicates with liquid 2 in the reservoir and is
filled with the liquid. Conduit 4 terminates in a nozzle 10 which
also is filled with liquid 2. Droplets 13 of the liquid may be
ejected on command through orifice 11 of the nozzle as will be
explained in later paragraphs.
Conduit 4 comprises a section 5 having an inlet end 7. Section 5 is
formed of a material such as glass which provides a smooth internal
surface and relatively stiff walls. The internal cross-sectional
area is substantially constant along the length of section 5. At
dashed line 8 the cros-sectional area begins a gradual reduction,
forming nozzle 10 having exit orifice 11. Therefore section 5 may
be regarded as having an outlet end at 8, and this end is
terminated by nozzle 10.
Conduit 4 also comprises a liquid supply section 14 formed of
viscoelastic material such as a plasticized polyvinyl chloride.
The internal diameter of supply section 14 is smaller than the
internal diameter of section 5. Section 14 is expanded at one end
and forced over the outside of section 5 at inlet end 7 thereof.
Supply section 14 may continue to reservoir 1 where it may
terminate below the surface of liquid 2, or it may be coupled to an
additional section 16 leading from reservoir 1.
A tubular electroacoustic transducer 17 surrounds conduit section 5
and is secured thereto in stress transmitting engagement by epoxy
cement 19. Preferably transducer 17 comprises a piezoelectric lead
zirconate-lead titanate ceramic tube 20, having electrodes 22, 23
on the cylindrical surfaces, and is radially polarized. A metal
foil strip 25 is inserted to provide electrical contact with
electrode 22 prior to introduction of cement 19.
Terminal wire 26 is wrapped around conduit section 5 in contact
with foil strip 25 and is secured in electrical contact therewith
by conductive epoxy 28. Terminal wire 29 is wrapped around
electrode 23 and secured in electrical contact therewith by
conductive epoxy 31.
Due to the well known piezoelectric effect, the inside diameter of
transducer 17 decreases almost instantaneously when a voltage of
suitable polarity is connected between terminal wires 26 and 29.
This diameter decrease forces decrease in diameter of the portion
of conduit member 5 which is surrounded by transducer 17. Liquid 2
within that portion of section 5 must therefore either be
compressed, or experience some displacement. As the voltage between
terminals 26 and 29 is reduced to zero, transducer 17 and conduit
member 5 return to their original dimensions, again causing
pressure change in liquid 2, or displacement thereof. Thus
transducer 17 is coupled to the liquid in conduit section 5.
Reservoir 1 is maintained at an elevation which applies little or
no pressure to the liquid 2 in nozzle 10. A slight negative
pressure, on the order of two to three centimeters of head seems to
be advantageous. Under quiescent conditions, the surface tension of
the liquid in orifice 11 prevents flow of liquid 2 in either
direction.
When it is desired to have a droplet ejected from nozzle 10, a
voltage pulse of the polarity which causes contraction of the
transducer is applied between terminals 26 and 29. The transducer
contracts in response to the pulse causing slight decrease in the
internal volume of conduit member 5. This momentarily compresses
that portion of liquid 2 which is within transducer 17 and causes
pressure waves to travel in the liquid toward outlet 8 and nozzle
10 and also toward inlet 7 and reservoir 1.
Conduit section 5, surrounded over part of its length by transducer
17, may be regarded as an acoustic transmission line. By virtue of
the relatively stiff walls, and the uniform cross-sectional area of
the enclosed liquid along the length of the section, it conducts
pressure waves in liquid 2 substantially without the occurrence of
reflections within the section.
The pressure wave which travels in the liquid toward outlet end 8
of section 5 causes ejection of a droplet from nozzle 10.
When supply section 14 of conduit 4 is formed of appropriate
material and is suitably porportioned as hereinafter described, the
characteristic acoustic impedance looking into the liquid in
section 14 from inlet end 7 of section 5 is approximately matched
to the characteristic acoustic impedance of section 5. Thus the
pressure wave which travels in the liquid from transducer 17 toward
inlet end 7 of conduit section 5, passes into the liquid within
section 14 without deleterious reflection. The wave therefore
continues in the liquid in section 14 toward reservoir 1. As the
wave progresses it causes elastic deformation of the viscoelastic
material of supply section 14, progressively along the length
thereof. Since the material is viscoelastic, part of the energy
transferred from the liquid to the material to cause deformation is
converted to heat and is not returned to the liquid as potentialor
kinetic energy as the wave passes. Thus, as the wave progresses
toward reservoir 1, the energy of the wave is progressively
absorbed by conduit section 14.
When the attenuated wave reaches the reservoir end of conduit
section 14 it encounters an impedance discontinuity with consequent
reflection back toward inlet end 7 of conduit section 5. As the
reflected wave travels through section 14 toward inlet 7 of section
5 it is attenuated by absorption in the viscoelastic material in
the manner above described. Conduit supply section 14 is made long
enough so that the energy of the reflected wave, when it reaches
nozzle 10, is too low to have substantial influence on the ejection
of a new droplet when a new voltage pulse is applied to terminals
26 and 29. Thus the viscoelastic material of supply section 14 may
be regarded as energy absorbing means coupled to the liquid in
conduit 4 which absorbs substantially all of the energy of the wave
which travels from transducer 17 toward inlet end 7 of conduit
section 5.
As the electric drive pulse decays to zero, transducer 17 and
conduit section 5 return to their original dimensions. After a
droplet has been ejected, the liquid in nozzle 10 withdraws from
the end thereof leaving an empty space which is then refilled by
liquid from the conduit under the urging of capillary forces in the
nozzle. Following refill of the nozzle, quiescent conditions
prevail until another electric drive pulse is applied to transducer
17. When a new pulse is applied, the above described process
repeats. Thus droplets may be ejected on command, each command
being given by applying an electric pulse to transducer 17.
The pulse shape requirement is not critical. It has been found
advantageous to have rise time less than two microseconds dwell
time of five to fifty microseconds, and fall time greater than two
microseconds. Good results also have been obtained using a cosine
squared pulse shape with period of ten to one hundred
microseconds.
Many electric circuit arrangements can be devised for generating
and applying suitable electric drive pulses. For examples of such
circuits, reference may be made to U.S. Pat. No. 3,683,212 to
Zoltan.
In order for the system to operate as described it is necessary to
have a suitable inter-relationship between the properties of the
material forming supply conduit section 14, the dimensions of
section 14, the inside diameter of conduit section 5, and
properties of liquid 2. If a proper relationship is not
established, a pressure wave traveling in the liquid from
transducer 17 will be at least partially reflected when it reaches
inlet end 7 of section 5. When that reflected wave reaches nozzle
10 it may cause ejection of an additional, undesired droplet, or it
may interfere with the desired ejection of a new droplet which
happens to be timed to occur as the reflection reaches the nozzle.
When the reflected wave reaches the nozzle it will be at least
partially reflected back toward inlet 7, and upon arrival at inlet
7 this newly reflected wave will be reflected just as the original
wave from transducer 17 was reflected. In severe cases of incorrect
matching of supply section 14 to section 5 a large number of
reflections may thus take place before the energy decreases enough
so as not to interfere with ejection of another droplet initiated
by a new command pulse. Thus, the stronger the reflections, the
longer the time interval before a new droplet can be ejected
without disturbance from the reflecting waves.
Selection of suitable viscoelastic material and dimensions for
conduit section 14 to prevent deleterious reflections at inlet end
7 of conduit member 5 may be accomplished by testing, as
hereinafter described, a series of sample sections constructed of
various materials and with a range of dimensions for each material.
For use in such testing, the assembly of FIG. 1 is provided with an
additional transducer 32 secured to conduit section 5 close to
nozzle 10. Transducer 32 may be identical to transducer 17 except
that preferably it is made much shorter. Foil strip 34 is inserted
and terminal wires 35, 37 are secured by conducting epoxy 38,40
just as in the case of strip 25, terminal wires 26,29 and epoxy
28,31 associated with transducer 17.
The tests may be performed using the dual transducer assembly of
FIG. 1 in a test set-up as shown in FIG. 2. In FIG. 2 the liquid
supply section 14' under test is expanded at one end and forced
over the inlet end 7 of conduit section 5 as in FIG. 1. A
hypodermic syringe 41 fitted with a blunt needle 43 selected for a
snug fit in section 14' serves as a reservoir. Syringe 41 is loaded
with liquid 2 of the kind that will be used with the droplet
ejecting system, and the liquid is forced through the conduit to
eject a stream from nozzle 10 until all air is swept out of the
system. Thereafter, during the test, no pressure is required but
care must be exercised to prevent drawing liquid back out of nozzle
10 into conduit section 5.
A variable frequency sine wave oscillator 44 is connected by
coaxial cable 46 to terminals 26,29. Oscillator 44 preferably has a
continuous frequency range from below 1,000Hz to 50KHz. A
substantially constant output voltage is desirable. A level of
about two volts is satisfactory.
A motor drive mechanism 47 is mechanically coupled to the frequency
control dial 49 of oscillator 44 to sweep the oscillator slowly
over the entire frequency range. A potentiometer 50, supplied with
current from dc source 52 also is coupled to motor drive 47. The
output of potentiometer 50 goes to the x-axis terminals 52 of an
X-Y plotter 53. Thus, as oscillator 44 is swept over its frequency
range, the pen 55 of X-Y plotter 53 is driven across chart 56.
The swept frequency voltage from oscillator 44 causes transducer 17
and the portion of conduit section 5 surrounded by transducer 17 to
alternately increase and decrease in diameter in synchronism with
the oscillator voltage. These dimensional variations cause
corresponding pressure variations in the liquid 2. The amplitude of
the preseure variations is too small to cause ejection of droplets
from nozzle 10. However, the pressure variations within transducer
32 adjacent to nozzle 10 stress the transducer sufficiently to
produce measurable AC voltage between terminals 35,37,
corresponding to the pressure variations. This voltage may be in
the range of one to ten millivolts when conduit section 14' is
properly matched to section 5, and much higher when there is a
serious mismatch.
The pressure pickup signal which is developed between terminals 35
and 37 is applied to an electronic AC voltmeter 58 via coaxial
cable 59. Meter 58 has output terminals 62 between which appears a
DC signal proportional to meter deflection. Terminals 62 connect to
Y-axis terminals 64 of X-Y plotter 53. Thus, as oscillator 44 is
swept over its frequency range, the X-Y plotter draws a graph of
pressure behind nozzle 10 vs frequency. A pressure calibration of
the system is not required but it is desirable to have a rough
calibration of the X or frequency axis.
Preferably meter 58 is a tuned voltmeter with tuning dial 65
coupled to oscillator dial 49 in a manner which insures accurate
tracking. The use of a tuned meter reduces difficulties that may
otherwise be encountered with pickup of noise and stray signals.
Instruments are commercially available which combine in one unit
the functions of oscillator 44, tracking tuned voltmeter 58, sweep
drive 47, potentiometer 50, and DC supply 52. One such instrument
is the model 302A Wave Analyzer equipped with Model 297A Sweep
Drive, manufactured by the Hewlett Packard Company. One of these
instruments was used in the tests hereinafter described.
In continuing the description of the test procedure reference will
be made to a particular series of tests that resulted in the
selection of material and dimensions for conduit section 14 that
later produced good results in actual pulsed droplet ejection.
Referring to FIG. 1, approximate specifications and dimensions were
as follows:
Conduit section 5 lime glass length 2.5 cm inside diameter 0.051 cm
wall thickness 0.01 cm Transducer 17 Lead Zirconate-Lead Titanate
Ceramic length 1.25 cm inside diameter 0.076 cm wall thickness
0.025 cm Transducer 32 Lead Zirconate-Lead Titanate Ceramic length
0.16 cm inside diameter 0.076 cm wall thickness 0.025 cm Nozzle 10
orifice diameter 0.007 cm Liquid 2 distilled water
FIG. 3 is a copy of X-Y plots for several sample conduit members
14'. Curve 67 was obtained with a conduit section 14' made of soft
vinyl material. The inside diameter was 0.063 cm and the outside
diameter was 0.16 cm. The pressure peak at about 15 kHz occurred
because conduit section 14' was too large in inside diameter, and
was too soft, providing very low acoustic impedance, thus allowing
almost complete reflection at inlet end 7 of conduit section 5.
Nearly complete reflection also took place at nozzle 10 because of
the very high acoustic impedance presented by the nozzle,
representing nearly a blocked condition. Thus the 15 kHz peak may
be regarded as a quarter wave resonance in conduit section 5. The
peak at about 40 kHz may be regarded as a three quarter wave
resonance. The lack of exact 3 to 1 correspondence probably is due
to the impedance of conduit member 14' not being zero and the
impedance of the nozzle not being infinite.
Curve 68 in FIG. 3 was obtained with a conduit section 14' of much
stiffer material. The inside diameter was 0.063 cm and the outside
diameter was 0.18 cm. The reduced height of the peaks at 15 kHz and
40 kHz indicates that a significant part of the energy of the wave
traveling in the liquid from transducer 17 toward conduit section
14' continued into the liquid in section 14' as desired.
Curve 70 in FIG. 3 was obtained with a conduit section 14' made of
the same material and having the same outside diameter involved in
curve 68 but the inside diameter was 0.041 cm. This curve indicates
substantially reflection-free transmission from conduit section 5
to section 14' and represents a satisfactory selection of material
and dimensions. The slight hump in the curve at about 23 kHz
suggests that possibly a slightly larger inside diameter would be
preferable, but pulsed operating tests of a system employing this
conduit section produced substantially uniform drop ejection at
rates up to 10 kHz, which is an order of magnitude improvement over
the results obtained with a droplet on command system constructed
as desribed in U.S. Pat. No. 3,683,212.
Curve 71 in FIG. 3 was obtained with a conduit section 14' made of
the same material and having the same outside diameter involved in
curves 68 and 70 but having inside diameter of 0.025 cm. The low
response at 15kHz and the pronounced hump at about 25kHz indicate
that the inside diameter was too small.
An additional series of tests provides a useful guide to
determining the minimum length for conduit section 14'. After
selecting suitable material and diameters for section 14' as
illustrated by the example referring to FIG. 3, the selected sample
is cut to one-half length and a new curve is run and compared with
the curve for the original length. The section just tested is again
cut to half length and a new curve is run and again compared with
the original curve. This process is repeated until a new curve is
obtained which is significantly different from the original curve.
At this point it may be assumed that the length is too short and
the preceding length should be considered to be approximately the
minimum length.
The conduit section 14' which resulted in curves 68, 70,71 in FIG.
3 were made of a plastisol prepared as follows:
Ingredients and Source Parts by Weight
______________________________________ Resin -- vinyl chloride
homopolymer identified as Geon 121 powder, sup- plied by B. F.
Goodrich Chemical Co., Cleveland, Ohio 70 Plasticizer -- diactyl
phthalate, identified as Good-Rite GP261 Plasticizer, supplied by
B. F. Goodrich Chemical Co., Cleveland, Ohio 30 Stabilizer --
identified as 6-V-6-A Stabilizer, supplied by Ferro Chemical
Division, Ferro Corp., Bedford, Ohio 1
______________________________________
The plasticizer and stabilizer were added to the resin powder in a
beaker and hand stirred for over thirty minutes. This formed a very
stiff mixture which then was placed in a bell jar and evacuated and
held for 24 hours.
The conduit section was moulded by forcing the thick mixture to
fill the space around a smooth wire tensioned and centered within a
glass tube, and then curing at a temperature of 160.degree.C for
about one to three minutes. The wire then was stretched beyond its
elastic limit to reduce the diameter, and withdrawn. The
viscoelastic tube then was withdrawn from the surrounding glass
tube which formed part of the mold.
Satisfactory pulsed droplet ejections over a wide pulse frequency
range has been obtained with a system having the parameters
tabulated above and provided with conduit section 14' formulated as
described and having inside diameter of 0.041 cm, and length of 10
cm. This was the conduit member resulting in curve 70 of FIG.
3.
Similar results have been obtained employing an extruded section of
plasticized polyvinyl chloride tubing supplied by the Norton
Company of Akron, Ohio, under the Trade Mark "TYGON". The outside
diameter was 0.178 cm and the inside diameter was 0.041 cm, and the
length was 10 cm. The particular TYGON composition was identified
as formulation S54-HL.
Preferably conduit section 5 is formed of glass, but metal, and
plastic sections have been used successfully. Preferably nozzle 10
if formed integrally with conduit section 5 in a manner to provide
a smooth contour as illustrated in FIG. 1. However, an abrupt
transition from the relatively large diameter of conduit section 5
to the small diameter of orifice 11 does not mitigate against
satisfactory operation of this invention.
The use of viscoelastic material to form part of conduit 4 is not
the only way to provide energy absorbing means coupled to the
liquid in the conduit. Another way that has produced satisfactory
results is to install a suitable acoustic resistance element at the
inlet end 7 of conduit section 5 through which the liquid 2 flows,
as shown in FIGS. 4 to 11.
Referring to FIG. 4, the construction may be generally similar to
the construction shown in FIG. 1. Conduit section 5, preferably of
glass, is terminated at the outlet end at 8 by nozzle 10.
Transducer 17 is secured to section 5 by epoxy cement 19, and
terminal wires 26,29 are attached as in FIG. 1. A pressure
measuring transducer is not shown in FIG. 4 but if desired one may
be provided by using a longer conduit section 5 and attaching a
transducer 32 as in FIG. 1.
One form of acoustic resistance unit that has given satisfactory
results has been provided by pressing a short bundle of glass
fibers 73 into the end extension 74 of conduit section 5 as shown
in FIGS. 4 and 5. The resistance element terminates at dashed line
7' which marks the effective inlet end of conduit section 5.
Liquid 2 is supplied from a reservoir, now shown, through supply
conduit section 76 which may be made of any convenient material.
Preferably it is formed of soft plastic and has inside diameter
equal to or larger than the inside diameter of conduit section 5.
If desired, conduit extension 74 may be attached directly to the
reservoir, eliminating section 76.
Another form of acoustic resistance unit that has given good
results has been provided by filling conduit extension 74 with
minute glass beads 77 and then fusing them together and to the
inner wall of extension 74 as shown in FIGS. 6,7.
Still another successful resistance unit has been provided by
pressing a cylinder 79 of porous plastic into conduit extension 74
as shown in FIGS. 8,9. The particular material that was employed
was cut from a "POROSYN" tip for a "TIP-WIK" pen sold by the
Eversharp Pen Company of Janesville, Wisconsin.
The flow resistance R of the acoustic resistance elements of FIGS.
4 to 9 should be approximately equal to the characteristic acoustic
impedance Zo of the liquid filled conduit section 5. The
characteristic impedance Zo is given approximately by:
Zo = (1/S) .sqroot.B .rho.
where
S = .pi. a.sup.2 = cross sectional area of the liquid column
enclosed by conduit section 5
a = inside radius of conduit section 5
B = bulk modulus of liquid 2
.rho. = density of liquid 2
Suitable dimensions and degree of packing of the glass fibers 73 of
FIGS. 4,5; suitable dimensions and degree of fusing of the beads 77
of FIGS. 6,7; or suitable material and length for the porous
cylinder 79 of FIGS. 8,9 to obtain the desired value of R may be
determined by experiment. The experimental resistance element to be
tested should be vacuum impregnated with the liquid to be used, and
then such liquid should be forced through the unit under measured
pressure P taking care to avoid introducing any air. The quantity Q
of liquid which flows in a measured time t should be measured. The
resistance then is calculated from:
R = Pt/Q
As an alternative, experimental resistance units may be installed
in a double transducer assembly similar to that shown in FIG. 1,
but supplied with liquid as in FIG. 4, and then tested in the
set-up of FIG. 2. Resistance which is too low will result in curves
resembling curves 67 or 68 in FIG. 3. When the resistance is too
high the curve should resemble curve 71. The correct resistance
should produce a curve resembling curve 70 of FIG. 3.
FIGS. 10 and 11 show how an acoustic resistance may be provided by
an annular slit at the inlet end 7' of conduit section 5. In this
case, conduit section 5 and the nozzle, not shown, preferably are
made of metal or plastic. Small indentations 82 are formed in
conduit extension 74. A solid cylinder 80 of metal or plastic is
pressed into extension 74 and held in place by indentations 82. The
acoustic resistance R is given approximately by
R = 6.mu.l/t.sup.3 .pi.a
where
.mu. = viscosity of liquid 2
L = axial length of cylinder 80
t = clearance between cylinder 80 and conduit extension 74
a = inside radius of conduit extension 74
The dimensions t and L should be selected to make R approximately
equal to the characteristic impedance Zo of conduit section 5.
FIGS. 4 to 11 show various ways in which energy absorbing means in
the form of acoustic resistance elements may be coupled to the
liquid in the conduit to absorb substantially all of the energy of
a pressure wave traveling in the liquid from transducer 17 toward
the inlet end 7' of conduit section 5. In each case the inlet end
7' may be located inside of transducer 17 as illustrated in FIGS. 4
and 8 or may be external of the transducer as illustrated in FIGS.
6 and 10.
In any of the constructions of FIG. 1 and FIGS. 4 to 11 it is
advantageous to enclose the transducer 17 in a jacket of yieldable
material such as rubber or plasticized vinyl. The assembly may thus
be secured in the apparatus in which it is used, by clamping to the
jacket. With such an arrangement there is little danger of applying
breaking stresses to the assembly, and the clamp does not interfere
significantly with the pulsing changes in diameter of the assembly
as droplets are ejected. The jacket may be in the form of a section
of tubing dimensioned to fit tightly over the transducer as shown
at 78 in FIG. 4, or it may be cast around the assembly. In the
latter case it may extend over the end of the transducer embedding
also the wrapped-around portions of terminal wires 26, 29 and the
exposed portion of conduit section 5 or 74. Another advantage of
such a jacket is that it provides damping of mechanical resonances
of the assembly which might otherwise cause deleterious
effects.
It is not necessary to employ a cylindrical piezoelectric
transducer surrounding the conduit as shown in FIGS. 1 and 4 to 11.
Other transduction principles may be used, for example,
electrostriction and magnetostriction. Further, other geometric
configurations and methods of coupling the transducer to the liquid
may be employed. For example, in FIGS. 12 and 13 the transducer is
a piezoelectric disc which is coupled to the liquid by direct
contact.
In FIGS. 12, 13, piezoelectric disc 83, preferably of lead
zirconate-lead titanate ceramic, has electrodes 85,86 to which
terminal wires 88, 89 are attached by solder or conductive epoxy
91,92.
Piezoelectric disc 83 is clamped between metal or plastic cover
plates 94,95 by O-rings 97,98 which fit into grooves 100,101 in the
cover plates. Terminal wires 88,89 extend through openings 103,104
in the cover plates.
Also clamped between covers 94,95 is a sheet 106 of viscoelastic
material. The assembly is held together by screws 99 which exert
sufficient compressive force on sheet 106 and O-rings 97,98 to
prevent leakage of liquid from the conduit formed as described in
the next paragraph.
Sheet 106 has an elongated cut-out 109 intersecting circular
cut-out 112 which surrounds piezoelectric disc 83 and O-rings
97,98. Tubular member 107 of metal or plastic is secured to cover
95 and communicates with cut-out 109 at one end thereof. Opening
110 through cover 94 communicates with cut-out 109 at the other
end. Thus there is formed a conduit comprising tubular member 107,
cut-out 109 enclosed by covers 94, 95, opening 110, and an annular
space formed by cut-out 112, the rim of piezoelectric disc 83,
O-rings 97,98 and cover plates 94,95. The conduit is terminated at
one end by sapphire watch jewel 113 which serves as a droplet
ejecting nozzle. The other end of the conduit, i.e., the open end
of tubular member 107, may be immersed in liquid in a reservoir,
not shown, or may be coupled to liquid in a reservoir by an
additional conduit member such as a flexible tube. The entire
conduit and the opening 115 in nozzle 113 are filled with the
liquid.
To facilitate further description, the section of the above
described conduit extending from dashed line 118 to the face of
watch jewel 113 at dashed line 116 will be identified as conduit
section 118-116. Line 118 marks the inlet end and line 116 marks
the outlet end. The location selected for line 118 is not critical
but preferably it is considered to be near or at conduit member
107. The internal cross sectional areas of the various components
of conduit section 118-116 are selected so that pressure waves in
the liquid may travel from end-to-end of the section without the
occurrence of significant reflection within the section.
The polarization of piezoelectric disc 83 is in the thickness
direction. Thus, when a voltage of suitable polarity is connected
between terminals 88 and 89, the diameter of the disc increases.
When the voltage is reduced to zero, the disc returns to its
original diameter.
The rim of piezoelectric disc 83 forms part of conduit section
118-116 and is in direct contact with the liquid. O-rings 97,98
which also form part of the conduit prevent the liquid from
contacting electrodes 83,85. Thus, when a voltage pulse with
polarity that causes increase of diameter is applied to transducer
83 the liquid surrounding the transducer is momentarily compressed.
This causes a pressure wave to travel through the liquid in conduit
section 118-116 to the outlet end 116 thereof and eject a droplet
from nozzle 113. It also causes a pressure wave to travel through
the liquid toward inlet end 118. As the latter wave progresses from
the rim of transducer disc 83 it causes elastic deformation of the
viscoelastic material of sheet 106 progressively along the length
of conduit section 118-116, with consequent absorption of wave
energy as described in connection with FIG. 1. After the wave
passes inlet end 118 it at some point encounters an impedance
discontinuity and therefore it is at least partially reflected. As
the reflected wave progresses toward nozzle 113 it experiences
further attentuation due to energy absorption in the viscoelastic
walls of the conduit. The conduit section 118-116 is made
sufficiently long so that the reflected wave energy reaching nozzle
113 is too weak to interfere with ejection of subsequently
initiated droplets.
While there have been described what are at present considered to
be the preferred embodiments of this invention, it will be obvious
to those skilled in the art that various changes and modifications
may be made therein without departing from the invention, and it is
aimed, therefore, in the appended claims to cover all such changes
and modifications as fall within the true spirit and scope of the
invention.
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