U.S. patent number 3,836,912 [Application Number 05/313,913] was granted by the patent office on 1974-09-17 for drop charge sensing apparatus for an ink jet printing system.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to John Ghougasian, Jon Hart, Hans Yohanan Juliusburger, Paul Lowy.
United States Patent |
3,836,912 |
Ghougasian , et al. |
September 17, 1974 |
DROP CHARGE SENSING APPARATUS FOR AN INK JET PRINTING SYSTEM
Abstract
An inductive charge sensing device is disclosed in accordance
with the teachings of the present invention for use with an ink jet
printing system wherein ink under pressure is applied to a nozzle
and ink emitted by the nozzle thereafter breaks up into a series of
drops which are electrostatically charged and subsequently
deflected in order to achieve controlled printing upon a recording
surface moved in front of said apparatus. The charging and
deflection of individual droplets is effected under control of an
applied video signal. In order for the proper information to be
recorded, the charging and deflection operation must be performed
in precise synchronization with the ink droplet formation. As
droplets are emitted from the nozzle, the charge sensor detects
charges impressed on said droplets passing adjacent to but in
non-impinging relationship with said sensor and a signal is
developed which may be used to control an electrical or
electromechanical drop forming means associated with said nozzle
and ink supply.
Inventors: |
Ghougasian; John (Mahopac,
NY), Hart; Jon (Cambridge, MA), Juliusburger; Hans
Yohanan (Putnam Valley, NY), Lowy; Paul (Peekskill,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
23217717 |
Appl.
No.: |
05/313,913 |
Filed: |
December 11, 1972 |
Current U.S.
Class: |
347/81; 324/71.4;
347/80 |
Current CPC
Class: |
B41J
2/115 (20130101) |
Current International
Class: |
B41J
2/115 (20060101); B41J 2/07 (20060101); G01d
015/18 () |
Field of
Search: |
;346/75 ;324/71CP,32
;73/194E,432PS ;128/214E ;317/3 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
schneider et al., An Apparatus to Study the Collision and
Coalescence of Liquid Aerosols; Journal of Colloid Science 20,
1965, pp. 610-616. .
Lindblad et al., Method of Producing and Measuring Charged Single
Droplets; The Review of Scientific Inst., Vol. 38, No. 3, March
1967, pp. 325-327..
|
Primary Examiner: Hartary; Joseph W.
Attorney, Agent or Firm: Schlemmer; Roy R.
Claims
What is claimed is:
1. In an ink jet recording system including an ink supply, a
nozzle, means for projecting a high pressure ink stream from said
nozzle which breaks up into droplets downstream therefrom, means
for applying an electrical charge to individual droplets as they
break off from said stream, means for deflecting said droplets and
a recording medium on which said droplets impinge to produce a
visible record, the improvement which comprises an inductive charge
sensing means for detecting a charge on said droplets,
said means comprising an elongated rod shaped conductive member
mounted downstream from said charging means, having a substantially
flat end in close proximity to, but in non-impinging relation to
said droplet stream to receive, by induction, a charge on said
member corresponding to a charge on said droplets, whereby the
magnitude of said charge may be determined, and wherein the spacing
of the end of said rod shaped member from said stream is between
5-10 drop diameters and wherein the width of said rod shaped member
is between 5-20 drop diameters, and
means connecting said rod shaped element to a signal utilization
means for comparing the signal with a predetermined norm.
2. An inductive charge sensing element as set forth in claim 1
including a pair of shield plates disposed on either side of said
element and substantially perpendicular to the path of said ink
droplet stream, each of said shield plates containing a hole
therein through which said ink droplet stream can freely pass and
circuit means connected to said shield plates whereby the charge
sensing element is protected from stray fields.
3. In an ink jet printing system including a linear array of
closely spaced nozzles, each producing a high pressure ink jet
which breaks up into a series of droplets shortly after leaving
said nozzle, individual charging means located adjacent each nozzle
for placing appropriate charges on said droplets, separate
deflection means for each said droplet stream and a recording
medium upon which said droplets impinge, the improvement which
comprises an array of inductive charge sensing elements for
individually detecting charges placed upon the droplets of each
individual stream by their respective charging elements, said
inductive charge sensing array comprising:
a plurality of rod-shaped elements constructed of a highly
conductive material mounted in a common insulating support member
such that the end of each rod is located adjacent to but in a
non-impinging disposition with respect to each stream of ink
droplets;
means connecting each said element to individual utilization
circuitry means therefor and,
two shielding plates closely spaced to and on opposite sides of
said linear array of charge sensing elements, individual holes
located in said plates so that each said ink droplet stream can
pass through a pair of holes in said plates in non-impinging
relationship thereto and, passes the end of an associated charge
sensing element located between said plates, said plates being
disposed substantially perpendicular to said stream of ink
droplets, and means for grounding said shield plates whereby said
inductive charge sensing elements are shielded from stray field
effects.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is related to U.S. Patent application Ser.
No. 313,914 filed concurrently with the present application of H.
Juliusburger et al. entitled "Digital Ink Jet Pulse Edge Phase
Control System" (now U.S. Patent No. 3,769,632). This referenced
patent discloses a particular phase control system which utilizes
the inductive proximity drop charge sensor of the present invention
in a specific synchronization control system. As such it represents
a preferred utilization of the device of the present invention.
BACKGROUND OF THE INVENTION
The need for improved low volume extremely high speed printers has
increased drastically in recent years. A particular application for
such printers is in the computer printout area wherein the actual
printing devices utilized to produce human readable records has
long been a major bottleneck in the overall computer system wherein
data which is produced by the system must often be held in
temporary storage such as magnetic tapes, discs, drums, etc. for
many hours before the particular printing devices can produce the
required outputs. Most currently available printers in this general
area today are of the impact type where a printing element must
actually be moved forceably against a record member to produce a
visible letter or symbol. In recent years ink jet printing has been
developed wherein ink is applied under pressure to a suitable
nozzle. The ink is caused to break up into individual droplets. The
droplet formation is controlled by a number of different methods
available in the art including physical vibration of the nozzle,
pressure perturbations introduced into the ink supply feed to the
nozzle, etc. The result of applying such external perturbations to
the ink jet apparatus is to cause the jet stream emerging from the
nozzle to break up into uniform drops at a predetermined frequency
and at a somewhat variable distance from the tip of the nozzle. It
is necessary, however, that this droplet formation and the
application of video charging signals to the ink droplet stream be
synchronized. The rate of drop formation in such systems is
determined by a signal applied to the physical perturbation means,
e.g., vibrating the nozzle. A means for applying an electrostatic
charge to each drop produced by the nozzle is provided in such
systems adjacent to the location where the ink stream begins to
form such droplets. Conventionally, this means is a hollow tube,
channel, plates, etc. surrounding the emerging stream and connected
to a suitable charging source. Video signals are applied between
the nozzle and the charging electrode in response to which a drop
will assume a charge determined by the amplitude of the particular
video signal on the charging electrode at the time that the drop
breaks away from the jet stream.
The drop thereafter passes through a fixed electric field and the
amount of deflection is determined by the amplitude of the charge
on the drop at the time it passes through said deflecting field. A
suitable recording surface is positioned downstream from the
deflecting means with the result that the droplet strikes such
recording surface and forms a small spot. As will be appreciated,
the position of the drop on the writing surface is determined by
the deflection the drop experiences which in turn is determined by
the charge on the droplet. Thus, by suitably varying the charge,
the location at which the droplet strikes the recording surface may
be controlled with the result that by applying suitable video
signals to such a system, a visible human readable printed record
may be formed upon the recording surface. U.S. Pat. No. 3,596,275
of Richard G. Sweet entitled "Fluid Droplet Recorder" discloses
such a recording or printing system.
As will be further appreciated with such a system, the time that
the drop separates from the fluid stream emerging from the nozzle
is quite critical since the charge carried by the droplet is
normally produced by electrostatic induction. The field established
by the video signal is maintained while the drop separates. The
drop will carry a charge determined by this video signal and
proportional to the magnitude thereof. However, if at the time of
separation the video signal is in the process of either rising or
falling or is not present at all at the time of drop separation,
the exact charge on the drop will be some time function of the
maximum video signal rather than being proportional thereto in
accordance with some predetermined and fixed relationship. It is
thus necessary in order to place exact predetermined charges on
individual droplets in accordance with successive video signals, to
know exactly the time of droplet separation in relationship to the
timing of the video signal. Stated differently, the droplet
separation time and the application of the video signal must be
very precisely synchronized. Failure to properly synchronize
droplet formation and the video signal results in very imprecise
control of the printing process with attendant severe degradation
of the uniformity, clarity, and generally the quality of the final
printer result.
A number of devices have been used in the past to detect the timing
of droplet formation in such ink jet printing systems. Two of these
are U.S. Pat. Nos. 3,465,350 and 3,465,351 of Keur et al., both of
which are entitled "Ink Drop Writing Apparatus." In both of these
systems, however, actual impingement of the ink droplet on some
receiving member is required, such as an ink gutter or the like. In
virtually all such ink jet printing operations where the droplet
formation occurs rapidly, extremely precise control of the
synchronization of the system is mandatory. Such detection elements
as disclosed in the two above-referenced Keur et al patents cause
problems due to ink buildup on the receiving element which
decreases the sensitivity of the system. This causes attendant lack
of precise control over the synchronization of such systems in
extremely high speed applications. The buildup also causes
serviceability problems as the ink must be periodically
removed.
SUMMARY AND OBJECTS OF THE INVENTION
It has now been found that improved sensing is possible with such
ink jet printing systems for the purpose of detecting the precise
times at which droplets are formed by utilizing an inductive charge
sensing element comprising a rod-like conductor placed proximate to
the ink jet stream, but in such a position that the ink droplets do
not impinge upon said member but which allow charging of said
member by inductive electrostatic effects in much the same way that
the ink droplet is initially charged.
It is accordingly a primary object of the present invention to
provide an improved element for detecting ink drop separation times
for use in an ink jet recording system.
It is a further object to provide such an element utilizing a
charge carried by individual droplets to detect such separation
times.
It is yet another object of the invention to provide a switching
element wherein the charge on the droplets is sensed by
electrostatic induction.
It is yet another object to provide such an element which avoids
problems of ink fouling of the element itself.
It is a still further object to provide such an element which
allows an improved signal-to-noise ratio in the sensing circuitry
associated with such element.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 comprises a combination functional block diagram and a
simplified perspective organizational view illustrating the
principle mechanical and electrical components of an ink jet
recording system utilizing the inductive drop charge sensing
element of the present invention.
FIG. 1B comprises a series of curves illustrating certain of the
critical time relationships in the system of FIG. 1A.
FIG. 2 comprises a set of curves illustrating a preferred method of
charging ink drops during a test phase portion of the system
operation cycle.
FIG. 3 comprises a set of curves illustrating a different
embodiment for applying a test signal to various ink droplets
during the test phase portion of an operating cycle.
FIGS. 4A and 4B illustrate the location of a given ink droplet with
respect to the sensing element tip at times t.sub.1 and
t.sub.2.
FIG. 5 is a graphical illustration of the charge produced in the
sensing element as a result of a charged drop passing said
element.
FIGS. 6A-6D are curves representing the series of individual
induced charges in the sensing element and the cumulative charge
effect induced in said element.
FIG. 7 is a graph illustrating the currents which would be induced
in the sensing circuitry as a result of the induced charge
illustrated in FIG. 6D.
FIG. 8 is a perspective view of a preferred embodiment of the
present invention comprising a matrix of sensing elements and
illustrating the application of the invention to monitor the
synchronizing of a plurality of individual ink jet streams.
FIG. 9 comprises a fragmentary cross-sectional view of FIG. 8
illustrating a single detecting element of the type shown and also
the nozzle, charging apparatus, and deflection means as they would
be located in such a system.
DESCRIPTION OF THE DISCLOSED EMBODIMENT
The objects of the present invention are accomplished in general by
a sensing element for use in an ink jet printing system wherein
individual ink droplets are electrostatically charged and
subsequently deflected electrostatically. The element comprises a
conductive member placed downstream from the charging station
proximate to, but in non-impinging relationship with the droplet
stream. A pair of shielding plates are located on each side of said
sensing element substantially perpendicular to said ink stream and
each plate has a small hole therein adapted to allow the ink
droplet stream to pass therethrough.
The sensor is connected to a suitable current amplifier which
develops a pulse output in accordance with the inductive charges
detected by the element. According to a preferred embodiment of the
invention, a test cycle is utilized wherein a special test charge
is placed on the ink droplets and a plurality of successive
droplets are utilized to cumulatively build-up a charge in the
sensing element which greatly improves the signal-to-noise ratio
and thus the accuracy of the element.
Referring now to FIG. 1A a more or less typical ink jet recording
system utilizing electrostatic deflection of the ink droplets is
shown with the inductive sensing element of the present invention
in place between the charging station and the deflecting station.
The mechanical portion of the system comprises a reservoir 10 in
which the ink is stored under pressure from whence it flows through
the conduit 12 to a nozzle 14. The ink jet stream 16 emanates under
pressure from the nozzle 14 and subsequently forms droplets 18. The
details of the formation of the droplets from the stream are shown
more clearly in FIG. 9, it being noted that the droplet formation
is not immediate but occurs at some point downstream from the
nozzle and within the charging station 20. This as will be readily
appreciated, is necessary since in order for the charge to be
placed on the drops inductively, a current path must exist within
the ink while under the influence of the charging electric field
just prior to the formation of the droplets 18. This feature, as
stated above, is clearly illustrated in FIG. 9.
Subsequent to leaving the charging station, the droplet passes
through shielding plates 24 having appropriate holes therein and
pass in close proximity to the drop charge sensor 22 which in the
disclosed embodiment comprises an elongated metal rod. The details
of the placing of the rod with respect to the droplet stream are
shown more clearly in figures which will be described subsequently.
The droplets then reach the deflection plates 24 wherein the
deflection of individual droplets is a result of the charge on each
droplet together with the magnitude of the charge on the deflection
plates. In the more widely used system such as exemplified in the
previously referenced Sweet U.S. Pat. No. 3,596,275, the deflecting
field is maintained constant and the charge upon individual
droplets is varied. However, it should be understood that the
present sensor would work equally well with a system wherein the
charge was fixed and a variable field was placed on the plates. The
ink droplets impinge upon the recording surface 28 subsequent to
passing through the deflecting field, their particular location
being determined by the video signal applied to the charging
element 20.
It will be noted that a drop producing transducer 30 is shown
attached to the nozzle 14. In conventional apparatus as referenced
above this transducer is a piezo-electric crystal which
mechanically vibrates the nozzle to form the subsequent droplets
18. The frequency of the vibration of the crystal determines the
rate of the droplet formation. Obviously the phase of the vibration
of the crystal will determine the exact time, especially with
respect to the video signal source, at which droplets are produced.
As stated previously, if the time of droplet formation is not in
correct synchronization with the applied signals from the video
signal source 32 given droplets will either not be charged at all
or have an incorrect charge thereon which will result in very poor
print quality. In the disclosed system a source of synch signals at
34 is utilized to drive both the transducer 30 and act as the basic
timing source for the video signal source 32. This source would
normally be a conventional high stability oscillator producing the
desired frequency for drop formation. The synch signals going to
the video signal source 32 pass through a phase changing network 36
wherein a phase change may be introduced in accordance with the
signal received from the amplifier 38 connected to the detecting
element 22. Thus, if the droplets are breaking off too soon or too
late with respect to the application of the charging signal, a
phase change may be introduced into the network 36 to correct this
situation as will be understood.
Referring briefly to FIG. 1B, the three curves illustrate just what
happens when a drop separates from the ink stream in phase or
completely out of phase with the charging signal. In this FIG.,
curve A represents the signal applied to the drop producing
transducer 30. The two arrows on this curve indicate the exact
instant of drop separation from the stream. Curve B illustrates the
charging signal placed on the charging element 20 by the video
signal source 32 substantially in phase with the drop separation
and curve C illustrates the charging signal source being applied
completely subsequent to drop separation. As will be understood
when the situation of curve C exists, virtually no charge will be
placed on the droplet with resultant erroneous operation of the
system.
It is thus necessary to have accurate and reliable synchronization
means in such an ink jet recording system to insure optimum
operation of the system. As stated previously, prior systems
required direct interception of ink droplets to make such timing or
synchronization measurements. As will be well understood, with ink
buildup on these elements, various types of problems are possible.
One problem is lack of sensitivity as ink builds up and dries on
the element. It can also interfere with adjacent apparatus. In
order to avoid such buildup or to minimize its effects, periodic
shutting down of the machine and cleaning of the sensing element
has been found necessary. In accordance with the teachings of the
present invention, the inductive charge sensor never intercepts the
individual ink droplets but merely picks up the signal inductively
which is then amplifed by an appropriate amplifier.
In a preferred embodiment it has been found that best results can
be obtained if a special test cycle is utilized periodically with
the system to test the current synchronization of the droplet
formation and charging.
FIGS. 2 and 3 represent two possible ways of applying test signals
to the droplets. In both FIGS., curves A represent the transducer
signal with the arrow indicating the moment of drop separation and
curves B indicate the signal applied to the charging plates. In
FIG. 2 the test signal comprises a series of short duration pulses
successively displaced across the transducer signal period. Thus,
the FIG. 2 cycle requires a series of test pulses applied to a
predetermined number of ink droplets with each series being
slightly displaced. When a predetermined signal magnitude is
detected by the sensor and amplifier, it will be apparent that the
droplet is being separated at some particular time with respect to
the phase of the transducer signal. The phase changing network 36
may be controlled to, in effect, center the charging video signal
appropriately around the detected separation point.
By known means, such as the gutter 27 a way is provided to
intercept the charged, or partially charged test drops, so they do
not interfere with the recording operation. If requried, the test
drops would be charged opposite to those used for normal
recording.
In FIG. 3 an analog system is disclosed wherein a ramp signal is
applied to a successive series of droplets during the test phase
and depending upon the magnitude of the charge detected, the timing
of the ink droplet separation is readily resolvable with respect to
the phase of the transducer signal. Thus, in the illustrated case
the drop separation occurs fairly low down on the ramp; however, if
the drop were found to be separating later in phase a larger ramp
or charging signal would be placed on the droplets with an
attendant larger charge detected by the sensor. A corrective signal
would then be fed into the phase changing network as described with
respect to the embodiment of FIG. 2.
It is believed that the electronics of the present system would be
obvious for one skilled in the art to build the necessary phase
changing network for controlling the moment of application of the
video signal to the charging electrode. It will be noted that a
particular preferred embodiment of a digital system is disclosed in
previously referenced copending application Ser. No. 313,914 of H.
Juliusburger et al. It is the intent of the present application to
set forth and claim the details of the sensing element per se, it
being apparent that the element could be utilized with a number of
different types of actual control circuits and mechanism without
essentially modifying the element.
Referring now to FIGS. 4-7, the actual operation or the charging
effect will be described. FIGS. 4A and 4B merely indicate the
location of an individual ink droplet with respect to time at the
time periods t.sub.1 and t.sub.2. Referring concurrently to FIG. 5
as well as FIGS. 4A and 4B, it will be noted that at time t.sub.1
the maximum induced charge developed when the ink droplet reached
the leading edge of the sensor so that its full charging effect is
felt therein. This is shown in FIG. 5 at time t.sub.1 which is the
beginning of the plateau region. This maximum charge effect is felt
until time t.sub.2, after which time the charge starts to fall off
as the drop leaves the area of the sensor. Similarly, with respect
to the leading edge a charge slowly builds up as the droplet
approaches the sensor element.
FIGS. 6A-6D illustrate the manner in which a series of droplets
utilized during a test phase can be utilized to maximize the total
induced charge q.sub.1, shown in FIG. 6D, to increase the
sensitivity or signal-to-noise ratio of the system. The successive
curves 6A, 6B and 6C merely duplicate curve 5 and are shown to
illustrate the charging effect of a successive series of drops
passing the sensing element.
FIG. 6B shows the total charge built-up in the sensing element
during a sequence of detected charged droplets. The signal or
charge built-up in the sensing element and thus in the sensing
element detection circuit which would preferably be an amplifier
having a very high input impedance is essentially the wave form
shown in FIG. 6D. When applied to the amplifier, this charging
signal wave form will produce a signal current in the input circuit
of the amplifier as exemplified by the curve of FIG. 7.
Thus, depending on the exact circuitry utilized, the signal
appearing at times t.sub.1 or t.sub.2 may be utilized for
controlling the phase changing network.
It will be readily appreciated that the particular preferred
embodiment of the invention shown in cross-section in FIGS. 4A and
4B is a single elongated rod-like element constructed of a highly
conductive material such as copper or possibly aluminum. It should
be readily understood that the inductive charge sensor could have
other shapes such as a plate, ring-shaped member, or U-channel
surrounding the path of the ink jet and suitably connected to the
detection circuitry.
The shielding plates could be any thin metal such as aluminum,
brass, copper, etc. The opening should be large enough so that
there is no possibility of interfering with the passage of the ink
droplets therethrough. The thickness and spacing of the plates is
not overly critical. For example, the plates may be spaced 1/16 of
an inch on either side of the sensing element.
Referring now to FIGS. 8 and 9, there is shown an embodiment of the
present invention particularly adapted for use with an array of
individual ink jet printing nozzles as would be used in a matrix
type printing operation as is well known in the art. In effect, in
this embodiment a separate sensing element is provided for each ink
jet stream making up the matrix. In FIG. 8 only the sensing element
is shown as the overall configuration would be similar to that of
FIG. 1A; however, obviously there would be a multiplicity of
nozzles, electronics, etc.
Referring now to FIG. 8, it will be noted that a plurality of
individual rod-shaped elements 22 are shown mounted in a
non-conductive plastic block 40. The shielding plates 42 are
cemented or otherwise fastened on either side of block 40 and the
apertures 44 therein provide a path for the ink droplet stream to
pass through the sensing assembly. A plurality of individual ink
droplet streams are clearly illustrated in the drawing and are
designated by the numeral 46. As will also be apparent, each ink
stream subsequently passes to its own set of deflection plates
which for the case of most matrix type printing systems will either
cause the individual ink droplets to strike a gutter or some other
shield or allow them to pass through and form a dot at a
predetermined location on the recording surface.
Similarly, as indicated in the FIG., an amplifier 48 is connected
to each of the sensing elements 22 through an appropriate impedance
element such as the resistance 50. Alternatively the sensing
element may be connected to the input of a current to voltage
amplifier such as described in the book "Operational Amplifiers,"
Design and Application, Toby, Graemp, and Huelsman, McGraw Hill,
New York, 1971. The output of each amplifier, as described
previously, is fed to an associated phase shifting or control
network for controlling the phase of the charging signal for the
associated nozzle and charging electrode.
The relative configuration of the overall system is shown more
clearly in FIG. 9 which illustrates the nozzle, the ink stream
breaking up into droplets, the charging plate, the sensing station
including the sensing element and finally the deflection plates.
This FIG. clearly shows the droplets forming within the area of the
charging electrode 20. As stated previously, this is necessary
since, for the charging voltage to have the desired effect on a
droplet, the droplet just prior to break-off must be under the full
influence of the charging field to allow electrons to flow out of
the affected droplet through the ink stream and to ground, as will
be well understood. The placement of the charging electrode is
fairly standard and the point of droplet fomation is relatively
constant since the primary system physical variables are in
viscosity, temperature, and the like. The variable which causes the
problem which is solved by the present invention is synchronizing
the precise drop break-off time with the application of the
charging signal to the charging electrode. As will be apparent from
the FIG., and also with reference to FIG. 1B and specifically curve
C thereof, if there is no charging signal on the electrode at the
time of drop break-off, the droplet will either receive no charge
or a very slight charge due to the effects of the preceding
charging signal.
The shape and location of the sensing element 22 together with the
shielding plates 42 having apertures 44 therein, is clearly shown
in FIG. 9. As stated, the element must be placed sufficiently close
to the stream of ink droplets to be able to sensitively detect
charges thereon and yet be spaced far enough away that there is no
reasonable likelihood of the ink droplet impinging upon the sensing
element. The magnitude of the signal is not materially affected by
the width of the sensor, since only approaching and departing
charged drops contribute to the sensor output. In practice, sensors
between about 5 and 20 times the ink drop diameter have been used
for ease of alignment. The spacing of a droplet from the stream
should be between about 5 and 10 times the diameter of a droplet.
Typical values of voltage developed across the 10 megohms resistor
50 are 50-150 mV for 16 drops at 100 KHz and for dimensions as
shown above.
While for the purpose of describing the preferred mode of the
invention a plurality of drops during the test period has been
disclosed, this is not absolutely necessary. For example, a single
droplet could be used, however, the signal-to-noise ratio of the
sense amplifier would be more important. A test signal of increased
magnitude with respect to the print signal may also be utilized
with a single or a plural test drop sequence to provide a layer
detected signal.
Having thus described the operation of the sensing element of the
present invention, it is reiterated that the essential elements are
a conductive charge sensing member and a set of shield plates. The
conductive element is located in an inductive charging relationship
with the ink stream but is sufficiently spaced therefrom to insure
that the ink stream cannot impinge upon said element. Thus, in
operation, the element senses the occurrence of a charge on the
droplet inductively and further senses the amount of charge on the
droplet which can be utilized to give a positive indication of the
effectiveness of the charging signal. In the latter case the
amplitude may be utilized to time synchronization of the charging
signal with the precise break-off time of the ink droplet from the
stream. It is possible to readily enhance the overall accuracy of
the system by accumulating energy from consecutively charged
droplets to provide larger output signals and better
signal-to-noise ratios. It is also possible to utilize a high
impedance in the sensing circuitry which also improves
signal-to-noise ratios. Finally, the element can be utilized in
such systems to in effect, monitor other parameters than the
specific charge synchronization; such, for example, as velocity
measurements by utilizing a plurality of such elements spaced at
precisely known distance apart along the ink stream.
While the invention has been disclosed utilizing certain preferred
embodiments, it will be readily understood that various
substitutions may be made by a person skilled in the art without
departing from the essential spirit and scope of the invention.
* * * * *