U.S. patent number 3,683,396 [Application Number 05/061,111] was granted by the patent office on 1972-08-08 for method and apparatus for control of ink drop formation.
This patent grant is currently assigned to A. B. Dick Company, Chicago, IL. Invention is credited to Henry A. Dahl, Robert I. Keur, Sandra Miller.
United States Patent |
3,683,396 |
|
August 8, 1972 |
METHOD AND APPARATUS FOR CONTROL OF INK DROP FORMATION
Abstract
In an ink drop writing system it was found that the drops formed
by vibrating a nozzle would be accompanied by smaller satellite
drops which could be detrimental to printing. This condition was
considerably improved by designing the nozzle so that it would have
a mechanical resonance at the frequency at which it formed drops.
In order to insure a most efficient transfer of power from the
driving source into the drop forming mechanism, the nozzle had to
be designed to provide a fluid resonance condition. If the nozzle
be considered as a closed pipe, then the frequency of vibration of
the fluid in the pipe at which its length is an odd multiple of a
quarter wavelength of sound through the fluid in the pipe, results
in the highest power transfer.
Inventors: |
Robert I. Keur (Niles, IL),
Sandra Miller (Lafayette, IN), Henry A. Dahl (Mount
Prospect, IL) |
Assignee: |
A. B. Dick Company, Chicago, IL
(N/A)
|
Family
ID: |
22033652 |
Appl.
No.: |
05/061,111 |
Filed: |
August 5, 1970 |
Current U.S.
Class: |
347/75; 347/47;
131/280 |
Current CPC
Class: |
H04N
1/034 (20130101); B41J 2/025 (20130101) |
Current International
Class: |
B41J
2/015 (20060101); B41J 2/025 (20060101); H04N
1/032 (20060101); H04N 1/034 (20060101); G01d
015/18 () |
Field of
Search: |
;346/75,140 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Sweet, Richard G.; High-Frequency Oscillography With
Electrostatically .
Deflected Ink Jets; SEL-64-004; SEL-TR-1722-1; Standford
Electronics .
Labs, Stanford Univ, Calif.; March 1964; pp. 52-62 and
83-90..
|
Primary Examiner: Joseph W. Hartary
Attorney, Agent or Firm: Lindenberg, Freilich &
Wasserman
Claims
1. In an ink drop printing system of the type wherein ink under
pressure is applied to flow through a nozzle and an
electromechanical transducer is employed for applying perturbations
to the ink, as it flows through said nozzle, at a predetermined
frequency, a method of improving the operation of said nozzle
comprising: making the length of the internal cavity in said nozzle
that length which produces fluid resonance at the desired frequency
of operation of the
2. In an ink drop printing system as recited in claim 1 wherein the
internal cavity of said nozzle has a length which produces fluid
resonance at a frequency which is displaced from said desired
frequency of operation by the change in fluid resonance frequency
caused by a change in the ink
3. In an ink jet printing system of the type wherein ink is applied
under pressure to a nozzle to flow therethrough and mechanical
perturbations are applied to the ink flowing through said nozzle by
means of an electromechanical transducer operated at a
predetermined frequency, the improvement comprising a nozzle having
an internal length for establishing fluid resonance during
operation at said predetermined frequency to provide the most
efficient power transfer between said electromagnetic transducer
and said fluid at said predetermined frequency of operation.
4. Apparatus as recited in claim 3 wherein there is included means
for establishing the pressure of the ink applied to said nozzle at
a value
5. Apparatus as recited in claim 4 wherein the internal length
L.sub.f of said nozzle equals N/4 .sup.. V.sub.f /E where F equals
the operating frequency, N equals a desired odd harmonic of
operation at said frequency, and V.sub.f equals the speed of sound
through the ink to be used with said
6. In an ink jet printing system of the type wherein ink is applied
under pressure to a nozzle to flow therethrough and mechanical
perturbations are applied to the ink flowing through said nozzle by
an electromechanical transducer operated at a predetermined
frequency, the improvement comprising: a nozzle having an internal
length to establish fluid resonance at a frequency displaced from
said predetermined frequency by the change in said fluid resonance
frequency caused by a change in the ink temperature
7. In an ink jet apparatus wherein ink is applied at pressure to a
selected nozzle to flow therethrough, and perturbations are applied
to the ink flowing through said nozzle by means of an
electromechanical transducer mounted on said nozzle in order to
cause the ink stream to break up into drops after leaving said
nozzle, the improved method of determining the frequency of fluid
resonance of said nozzle comprising: varying the frequency applied
to said electromechanical transducer until the observed breakoff
time of said ink jet from the tip of said nozzle is a minimum, and
detecting the frequency at which said breakoff time is a minimum,
which is the fluid resonance frequency.
Description
This invention relates to ink drop printing systems and more
particularly to improvements therein.
Briefly and simply described, an ink jet printing system of the
type with which this invention is concerned, comprises a nozzle
which is coupled by suitable means to a reservoir of ink, so that
ink is discharged in a stream from the nozzle under a pressure
determined by the reservoir. The nozzle is either vibrated or
periodically constricted so that a short distance from the nozzle
opening the ink stream breaks off into drops. At or about the break
off point, a conductive ring is placed so that the stream passes
through it. A succession of voltages are applied to the ring at a
frequency synchronous with the formation of the drops.
Each of the drops receives a charge which is determined by the
voltage applied to the ring at the time the drop is formed. The
drop thereafter passes between two plate electrodes to which a
fixed potential is applied. In its passage through these
electrodes, the drop is deflected from the straight line path an
amount determined by the amplitude of the charge upon it. The drop
thereafter falls upon paper, which is moved to provide a new
surface for receiving the drops.
From the foregoing brief description it will be appreciated that
the voltages which are applied to the drops can cause the drops to
be deposited on the paper in a pattern which can form alphanumeric
characters, symbols or waveforms. It should be appreciated that in
order for the system to perform satisfactorily, the charge on each
drop should not be altered once it is established, otherwise, the
drop will not be deflected to the proper location to form the
desired alphanumeric character or symbol.
As the ink stream emitted by the nozzle breaks up into drops, it is
noted that each drop is accompanied by a small drop known as a
satellite. The satellite has a velocity which is often different
from that of the drop. It was found initially that this velocity
could be varied by either varying the frequency driving the
piezoelectric device which constricted or vibrated the nozzle, or
by varying the voltage applied to the piezoelectric device, or by
varying the pressures applied to the fluid whereby the velocity of
the fluid stream could be varied.
Customarily, an electric field is applied to the stream to
establish a charge on a drop or satellite when separation occurs.
Describing the separation in more detail, when a drop is formed,
there is a fine filament of the fluid that connects the drop to the
stream just before separation. This filament forms a satellite. If
the drop separates from the filament before the filament separates
from the stream, the filament will form into a satellite whose
speed will be less than that of the drop resulting in a slow
satellite condition. If the filament separates from the stream
before it separates from the drop, the filament will form into a
satellite whose speed is greater than that of the drop resulting in
a fast satellite condition. There is an intermediate satellite
condition which occurs when the drop and filament separate
simultaneously resulting in the satellite speed being the same as
that of the drop. As a consequence the satellite does not collide
with any droplets, but rather travels at substantially the same
speed as the droplets through the flight. In the fast satellite
condition, only one charge establishing separation occurs for each
satellite drop pair, whereas in the slow satellite condition, two
charges establishing separations occur for each satellite drop
pair.
It will be appreciated that the synchronism of the charge inducing
voltages is much less difficult with the fast satellite condition
than with the flow satellite condition because the voltage need
only be synchronous with one separation per drop period instead of
two separations; i.e., that of the drop and also that of the
satellite. Thus, a system can operate with slow satellites,
however, it is greatly preferred to operate with fast satellites.
The third satellite condition, that of intermediate satellites is
completely unsatisfactory because once the satellites obtain a
charge, they easily deflect into the high voltage deflection plates
due to their low mass. The result is an undesirable accumulation of
ink on the plates which tends to short out the deflection
voltage.
It was found that excessive values of the parameters described were
needed for obtaining a fast satellite condition with early nozzle
designs. It wasn't until the nozzle was designed to be mechanically
resonant near the operation frequency, that fast satellites could
be formed at more reasonable values of the parameters.
At the same time that it is desirable to design a nozzle so that it
is mechanically resonant near its frequency of operation, it is
also desirable to transfer energy as efficiently as possible to the
fluid stream. This is advantageous first, for the obvious reason
that it provides a saving in power consumption and component costs,
but additionally, lower power usage reduces heat dissipation at the
nozzle and thereby prevents extreme temperature excursions of the
fluid stream, whereby a much more stable operating system is
provided. The condition at which the system operates most
efficiently, as far as operating parameters are concerned is one in
which the nozzle is also designed to produce "fluid resonance."
This is advantageous also because it provides a method of
compensation for ambient temperature changes. The design of the
nozzle to achieve fluid resonance could affect the design of the
nozzle to achieve mechanical resonance, and thus it is important to
reconcile the two.
An object of this invention is to provide an ink jet nozzle design
which is "fluid resonant" near the desired operation frequency.
Another object of this invention is to provide a nozzle design
having fluid resonance which enhances the formation of fast
satellites.
Still another object of the present invention is the provision of a
novel and useful ink jet nozzle construction.
The foregoing and other objects of the invention are achieved by
designing the nozzle so that its internal length is determined by
the formula F = (n/4) .sup.. (V.sub.f /L.sub.f) where N = 1, 3, 5,
etc., V.sub.f = speed of sound through the fluid in the nozzle.
L.sub.f is the length of the nozzle, and F is the desired frequency
of resonance.
It has been found that the nozzle cavity holding the fluid acts
like a closed pipe. Closed pipes are resonant when their lengths
match odd multiples of quarter wavelengths of sound in the cavity.
The formula indicated in the foregoing paragraph is the one which
expresses the relationship in a closed pipe between frequency of
resonance and pipe length for a given odd multiple of the sound
wavelength.
After on has designed for fluid resonance, then the nozzle design
proceeds for mechanical resonance which essentially has the same
formula, F = (N/2) .sup.. (V.sub.M /L.sub.m) where F equals the
desired operating frequency of resonance, N = 1, 2, 3, etc.,
V.sub.M is the speed of sound through the structure of the nozzle,
and L.sub.M is a distance over which resonance occurs. The
significant length is one which includes the major masses combined
that are associated with the nozzle. In designing a nozzle it is
convenient to have the mechanical resonant length shorter than the
fluid resonant length.
FIG. 1 is a block schematic diagram of an ink jet printing system
shown to assist in an understanding of the invention.
FIG. 2 illustrates an ink jet nozzle construction without a
piezoelectric driver unit mounted on it.
FIG. 3 illustrates an ink jet nozzle construction with the
piezoelectric driver thereon.
FIG. 4 is a Rayleigh curve.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic drawing of the presently known arrangement
which is shown to afford a better understanding of the invention. A
pump 10 applies pressure to an ink reservoir 11 so that it can
provide ink under pressure to tubing 12 which is flexible. An
electromechanical transducer 14 is usually placed adjacent to or
around the tubing. The transducer is driven in response to signals
from a source 16. The transducer serves to vibrate and/or compress
the tubing 12 in the region of the nozzle 18. This results in an
ink jet 20 being emitted which at a short distance downstream
breaks up into drops 22 which are formed at a rate determined by
the frequency of the vibration. In the region where the stream 20
breaks down into drops, a charging tunnel 24 is provided. This
comprises a conductive cylinder to which video signals from a video
signal source 26 are applied. The video signals establish a field
within the charging tunnel so that the ink drops which are formed
therein assume a charge determined by the amplitude of the video
signal present at the time the drop separates from the ink jet
20.
Downstream of the charging tunnel there are usually placed a pair
of electrodes 28 which are connected to a field bias source 20. As
a result, there is established between the electrodes a constant
electric field. The ink drops, which bear charges in accordance
with the video signal, enter this field and are deflected by an
amount which is proportional to the amplitude of the charge. This
enables intelligent writing to occur on a writing medium 32, which
is moved at some synchronous rate past the electrodes. Drops which
do not bear a video charge are captured by a tube or trough 34
which is judiciously placed at one side so as to capture these
drops. It leads to a waste reservoir 36. The paper 32 moves into
the plane of the drawing whereby its motion, together with the
deflection of the drops, may be used for forming intelligible
characters.
Referring now to FIG. 2, there may be seen a nozzle without its
piezoelectric driver unit. It consists of a tube 40 having a
threaded portion 42 near its rear, and an enlarged region 44 near
its front end. As indicated by the dotted lines, the center of the
tube is hollow and the opening therethrough at the front or
emitting end 46, is reduced to a diameter which is very much
smaller than the diameter of the opening through the remainder of
the tube.
FIG. 3 is a drawing of the appearance of the nozzle with the
piezoelectric driving unit mounted thereon. This consists of two
piezoelectric crystals respectively 50 and 52, which are separated
by a conductive electrode 54. The tubes are pressed against the
enlarged region 44 by means of a retaining mass 56, which is
mounted upon the threaded portion 42. The piezoelectric crystals
are polarized so that they expand and contract axially in response
to the application of potential thereto. This serves to push
against the retaining mass and the enlarged section 44 whereby the
nozzle is caused to elongate and contract which in turn results in
constrictions being applied to the fluid flowing through the
nozzle.
Referring again to FIG. 2, the length to be considered in designing
the nozzle for fluid resonance is represented by L.sub.fluid on the
drawing and extends from one end of the nozzle to the other. In the
FIG. 3, the length to be considered for mechanical resonance is the
length L.sub.mechanical which extends from one end of the retaining
mass to the other end of the enlarged section. Effectively, it
includes the section of the nozzle covered by the two masses
retaining the piezoelectric crystals together with the length of
the two crystals.
It should be noted that the back end of the nozzle, which is
adjacent the threaded portion 42, is usually connected to a fluid
reservoir by means of other tubing not shown.
An efficient transfer of energy to the fluid stream is indicated by
the time required for an element of fluid to pass from the end of
the nozzle to the point where the continuity of the stream ends and
thereafter becomes drops. This is known as a break-off time. A
shorter break-off time indicates a more efficient transfer of
energy. Lord Rayleigh determined that there was a relationship
between the break-off time and a ratio expressed as .lambda./d .
.lambda. comprises v/f where V is the velocity of the stream of the
fluid being used, f is the frequency of the disturbance applied to
the fluid, and d is the diameter of the issuing fluid.
FIG. 4 represents a curve 60 which is derived when the relationship
is plotted, at a particular driving voltage applied to the
piezo-electric crystal. It is known as a Rayleigh curve. The curve
60 has the breakoff in microseconds plotted as the ordinate, and
the abscissa has the value of .lambda./d . The voltage applied to
drive the crystals for the purposes of this curve was 25 volts.
Decreasing the driving voltage would cause a substantial
duplication of this curve to occur but is placed higher on the
graph. Increasing the voltage would cause a substantial duplication
of this curve to occur but is placed lower on the graph.
It was found, for each one of these curves, that when the value
.lambda./d increased beyond a certain value, that conditions
favorable for fast satellites would occur. For the curve shown,
this took place at the point indicated at a value of .lambda./d of
approximately 4.8. For each curve the same point beyond which fast
satellites would occur was found to occur. As the driving voltage
increased the required value of .lambda./d decreased. Thus, values
of .lambda./d greater than a minimum value for a particular
operating voltage assists in obtaining the desirable fast satellite
operation.
To insure an efficient transfer of energy while at the same time
obtaining fast satellites, it was necessary to design the nozzle so
that fluid resonance was obtained. In observing the behavior of the
nozzle over a range of frequencies it was found that the nozzle
cavity holding the fluid acted like a closed pipe. Closed pipes are
resonant when their lengths match odd multiples of quarter
wavelengths of sound in the cavity. As previously indicated this
phenomenon is represented by the equation F = N/4 .sup..
V.sub.fluid /L.sub.fluid .
It was further found that operation of a nozzle in a non-resonant
condition may involve either great waste of energy or even a poor
drop formation. Accordingly it was realized that because of fluid
resonances, the first step in design is to build a nozzle wherein
the fluid resonance should be near or close to the frequency at
which it is desired to form droplets.
It was found that the velocity of sound varies quite differently
for various fluids. Also, when the ink jet printing apparatus is
employed, a temperature rise occurs within the fluid as a result of
ambient conditions. Within the temperature range involved, it was
found that some fluids, for example, water, have a positive
coefficient of the velocity of sound which increase in temperature,
that is the velocity of sound goes up with temperature, and for a
number of other fluids including oil base fluids, the speed of
sound decreases with a rise in temperature. The temperature
coefficient is negative.
Therefore, in accordance with this invention it was discovered that
by displacing the frequency to be employed in the formula for fluid
resonance, to one side or the other of the actual desired frequency
of drop formation which side depending upon whether the fluid to be
used has a positive coefficient or a negative coefficient, the
change in efficiency with change in temperature could be
compensated for. With increasing temperature we see an increase in
the resonant frequency for water based solutions. Simultaneously,
the inherent increase in .lambda./d with increasing temperature
makes the system easier to drive. As a consequence, when the
resonance is higher than the operating frequency, the frequency
shift makes it harder to drive and the .lambda./d change makes it
easier to drive.
Lowering temperature causes the role of each to be the opposite.
When the resonance is lower than the operating frequency, the two
changes act in the same way causing overdriving to occur sooner.
Obviously for practical purposes, the former case above is
preferable as a wider temperature range is tolerated. In one
practical example an aqueous solution of polyethyleneglyco was used
in a system set to separate at a frequency of 66 KHz. A fluid
resonant frequency of 68 KHz, 2 KHz above the operating point, was
used in the design formula for determining the required cavity
length. The foregoing allows for ambient temperature drift of the
ink jet printing system.
Techniques for measuring the speed of sound at a given temperature
or over a temperature range for a particular fluid are well known
and thus, from the information given one can design a nozzle to
have a desired fluid resonance whereby most efficient transfer of
power and temperature compensation within the nozzle is made to
occur.
While the determination of mechanical resonance is not a part of
this invention, at the time of designing the nozzle for fluid
resonance, consideration must be given to the mechanical resonance
design to insure that the length of the mechanical masses which are
mounted on the nozzle are not incompatible with the required length
for fluid resonance. It is convenient to have mechanical resonance
length slightly shorter than the fluid resonance length and the
length should be realistic in defining a nozzle.
To summarize the foregoing, in accordance with this invention, one
can operate the ink jet producing structure of an ink jet printing
system in a .lambda./d range which gives fast satellites and also
which operates at or near a resonance point where energy is most
efficiently transmitted to the fluid stream and temperature
compensation is obtained. Initially, a desired droplet formation
rate is selected, and that indicates the frequency to be applied to
the nozzle to cause perturbations in the fluid stream. For example,
assume that it is desired to obtain 66,000 drops from the first
nozzle and 16,500 from a second nozzle. This indicates frequencies
(F) of 66 KHz and 16.5 KHz respectively.
A second step in the design is to determine the velocity of sound
for the particular fluid in the particular cavity at a temperature
determined as the operating temperature. The velocity of sound
(V.sub.f) in the inks to be used with the two nozzles is 1,575
meters per second. Assume now that N is 3 for the first nozzle and
N is 1 for the second nozzle.
From the foregoing, one can determined, by using the resonant pipe
formula, F=N/4 .sup.. V.sub.f /L.sub.f the required length of the
fluid cavity. Thus, for the first nozzle, L.sub.f = .0240
meters.
Finally, one determines the length of the nozzle required for
achieving mechanical resonance. Using the formula F = N/2 .sup..
V.sub.M/L for the first nozzle, L.sub.M = 0.0150 and for the second
nozzle, L.sub.M = 0.0200 meters. If it is desired to compensate for
temperature effects in the first nozzle, for example, then, on the
assumption that the ink has a positive temperature coefficient,
assume the frequency F at 68 KHz, instead of 66 KHz (i.e., 2,000
KHz more). Then the fluid length L.sub.f calculates as 0.0174
meters.
After building the nozzle, it is operated at a desirable .lambda./d
ratio and the applied voltage is adjusted until operation is
definitely in the fast satellite region. This can be determined by
observing the behavior of the satellites after they pass through
the charging ring. The foregoing design technique assures a most
efficient usage and a desirable fluid stream configuration for good
printing.
Should a nozzle which is being employed not have a smooth fluid
cavity but have a step configuration along its length with
different diameters, the speed of sound through the fluid in that
cavity may not be precisely predictable. By varying the frequency
applied to that nozzle while observing the break-off time, one can
determine where the resonant point is. From this and a knowledge of
the length of the nozzle, the closed pipe formula may be employed
to give the effective velocity of sound. Once the effective
velocity of sound is obtained, the formula may be again used with
the desired resonant frequency, and the length of the nozzle may be
calculated. The nozzle may then be shortened or lengthened until
this calculated length is reached.
In the foregoing, there has been described a novel and unique
method and means for designing a nozzle.
* * * * *