Ink jet multiple field electrostatic lens

An ink jet printer is disclosed of the type wherein a plurality of nozzles emit parallel streams of droplets toward a target. Each nozzle has a charging electrode to charge droplets selectively depending upon whether a particular droplet is intended for the target or a gutter. A pair of deflection plates span the paths of the droplet streams and deflects the charged droplets according to information to be printed. A multiple field electrostatic lens is used to align charged droplets from different nozzles to a common line on the target despite misalignments between nozzles relative to the common line.

CROSS REFERENCE 
This application includes a common specification and drawing to a copending 
application of one of the present inventors, Peter A. Crean, titled 
"Electrostatic Lens for Ink Jet" filed currently with this application. 
BACKGROUND OF THE INVENTION 
This invention relates to ink jet printers. More specifically, this 
invention relates to a novel component for ink jet printers herein called 
an electrostatic lens for aligning or changing the trajectories of charged 
droplets emitted at high velocities from a nozzle. 
The trajectory of charged ink droplets are difficult to align because the 
droplets are small, typically from 1 to 25 mils in diameter, and 
consequently the nozzle orifices are small and difficult to maufacture and 
assemble. A coarse alignment must be achieved to align the trajectory of 
droplets emitted by a nozzle with a charging tunnel and a pair of closely 
spaced deflection plates. The charging tunnel diameter is normally only 
about from 3 to 10 times the droplet diameter whereas considerable larger 
spacing separates the deflection plates. Once a coarse alignment is 
obtained, a vernier or fine alignment is often desired yet difficult to 
achieve. 
The alignment difficulty is compounded in multiple jet printers. For 
example, in a multi-jet printer as disclosed in U.S. Pat. No. 3,373,437 to 
Sweet and Cumming, the trajectories of the multiple jets must be aligned 
relative to each other so as to print a straight row of droplets to match 
a line of print or pixel positions on a target. Heretofore, electrical 
techniques have been used to correct for the misalignment of one 
trajectory relative to another. The target is moved at a constant velocity 
past the print line. The electrical command to place a droplet at a given 
pixel position is delayed (or accelerated) a small amount to allow the 
target to move a distance corresponding to the misalignment of the jet 
trajectory for that pixel position. Alternately, the charge on the errant 
droplet is increased (or decreased) to vary its deflection and hence 
placement on the target. Clearly, the alignment is achieved at the expense 
of increased complexity to the electrical control circuits for the 
printer. 
SUMMARY 
Accordingly, it is a primary object of our invention to overcome the 
alignment problem or prior art, charged ink droplet systems. 
Another ojbect of this invention is to devise an electrostatic lens for 
focusing charged droplets following different trajectories to a common 
point or line. 
A specific object of this invention is to build a cylindrical electrostatic 
lens, analogous to an optical half-cylinder glass lens, that focuses 
generally parallel, charged ink droplet streams to a line (rather than a 
point). The axis of a cylindrical electrostatic lens is a plane that 
intersects the focal line and along which moving charged droplets are not 
diverted by the lens. 
The foregoing and other objects of our invention are achieved by 
establishing a focusing electric field along the intended trajectory of a 
droplet. The focusing field extends in a direction generally parallel to 
the trajectory of a droplet in contrast to the generally normal direction 
of the electric field created by conventional deflection plates. The 
focusing field is preferably given symmetry at least on two sides of a 
droplet's trajectory thereby allowing properly aligned droplets to 
traverse the focusing field without having a course correction imparted to 
it. 
The cylindrical lens effect is achieved with four linear electrodes. At an 
upstream position, an electrode is placed equidistant above and below the 
intended droplet trajectory. At a downstream position, the remaining two 
electrodes are placed equidistant above and below the intended droplet 
trajectory. The four electrodes are substantially parallel and orthogonal 
to the trajectory. A potential difference coupled between the upstream and 
downstream fields creates two electric fields whose boundaries resemble 
two half cylinders abutting at a tangent plane parallel to their bases. 
The tangent plane defines a path over which a charged droplet is not 
deflected. Droplets that enter the field above or below the tangent plane 
are focused to a line on that plane a determinable distance downstream. 
The focal line is constant for droplets having substantially the same 
velocities and mass to charge ratios. 
THE PRIOR ART 
The Sweet Pat. No. 3,596,275 having an effective filing date of July 31, 
1963, describes the prior art high velocity ink jet device for which the 
instant invention is especially suited. Therein, a fluid ink is forced 
from a volume through a small nozzle under high pressure. The natural 
tendency of the resultant stream emitted from the nozzle to break up into 
droplets is promoted by accoustically stimulating the ink at a frequency 
of about 120 kilohertz. The droplets tend to form at regular intervals and 
at a constant size. The ink is conductive. As the droplets separate from 
the fluid column emitted from the nozzle, the droplets pass a charging 
electrode, often a closed tunnel, where charge is induced on it by a 
voltage coupled to the charging electrode. 
The charged droplet is propelled along a trajectory toward a target that is 
moving at right angles to its flight. Before the droplet reaches the 
target it passes between parallel plates. A steady state electric field 
normal to the path of the droplet is created by a 2000-14 4000 volt 
potential difference coupled between the deflection plates. The amount of 
charge on the droplet determines the amount of deflection imparted to it 
by the deflection field. 
There is no teaching in this basic Sweet patent of the use of electric 
fields that extend generally in the direction of the droplet trajectory. 
As such, the Sweet patent is understandably silent on the present concept 
of focusing. 
The Sweet and Cumming U.S. Pat. No. 3,373,437 mentioned above describes a 
binary ink jet system in which two deflection plates are shared by a 
plurality of linearly aligned nozzles. The binary feature is that the 
droplet from a given nozzle either is charged and deflected toward a pixel 
position on the target or remains uncharged and is collected in a gutter. 
The charge on the droplets sent to the paper is intended to be equal. Here 
as above, there is no suggestion of a focusing field of any kind. 
The Loeffler et al. U.S. Pat. No. 3,877,036 discloses an ink jet alignment 
electrode. The electrode, however, is positioned to act on the fluid 
column at a location prior to droplet formation. Also, the deflecting 
field is generally normal to the fluid column and does not include a path 
through the field that will not bend a properly aligned column as with the 
present focusing fields.

DETAILED DESCRIPTION 
Herein, the ink jet system described is of the Sweet type disclosed in the 
above named U.S. Pat. No. 3,596,275 and that disclosure is hereby 
expressly incorporated by reference. Briefly, a transducer modulates or 
stimulates ink in a chamber or tube coupled to a nozzle. The ink is 
subjected to pressures of from about 20 to 150 psi. The modulation of the 
ink causes a stream of discrete droplets of like velocity, mass, shape and 
trajectory to be emitted from the nozzle. The modulating apparatus and 
circuitry is not shown to simplify and thereby clarify the present 
discussion. For details on that apparatus, the reader is referred to the 
above Sweet patent. 
FIGS. 1 and 2 are a side view and plan view of a multiple nozzle ink jet 
printer. Like elements in the various figures have the same reference 
numbers. The printer includes the nozzle 1 that emits a stream of droplets 
along a trajectory indicated by dashed line 2. The droplets are charged at 
charging electrode 3 as indicated by the circle 4 having the minus sign 
indicating a net negative charge. For the polarities given, the negatively 
charged droplets are deflected upwardly along the path indicated by dashed 
line 5 by the deflection plates 6 and 7. The deflected droplets head 
toward the target 8 and the uncharged, low charged, or oppositely charged 
droplets are collected by the gutter 9. The cylindrical, electrostatic 
lens 10 focuses the charged droplets to a common focal line 12 on the 
target. The droplet 4 is diverted over the path indicated by the dashed 
line 13 by the lens. The dashed line 14 (actually a plane) is the 
centerline or axis of lines 10. Charged droplets that travel through the 
lens along the centerline do not have their trajectories altered. 
The lens 10 can also be located upstream of the deflection plates. 
Specifically, lens 10 can be positioned between the charging electrode 3 
and the deflection plates 6 and 7. 
The printer of FIGS. 1 and 2 is a binary printer similar to that disclosed 
in the Sweet and Cumming Pat. No. 3,373,437 mentioned at the outset. The 
disclosure of that patent is incorporated herein by reference. Printing is 
achieved by moving the target 8 at generally right angles to the ink jet 
path or trajectory 2. The target is moved at a constant velocity in the 
upward direction in FIG. 1 as indicated by arrow 15. Four drive rollers 
16a, b, c and d are coupled to an appropriate drive source (not shown) to 
advance the target. 
Referring to FIG. 2, a plurality of nozzles 1 through 1c are representative 
of the multiple nozzles of a printer. For good quality image reproduction, 
a printer should have about 100 nozzles per inch. This means that to cover 
an 8.5 inch standard paper width, 850 nozzles are deployed as illustrated 
in FIG. 2. The packing density is reduced if the nozzles are aligned in 
two or more rows with one row offset one nozzle or pixel position from the 
other. The lens 10 is appropriate for the multiple row arrangement of 
nozzles provided allowance is made for one row to be focused to a 
different line than the other. In addition, the offset between rows can be 
made large enough to accommodate a lens for each row. 
In FIG. 2, each nozzle 1-1c has a separate charging electrode 3-3c that 
charges droplets traveling the generally parallel paths 2-2c. The object 
is to place a droplet--when called for by a video signal--at adjacent 
pixel positions 18-18c on the target. The scan line of 18-18c pixels 
should be straight. However, any misalignment of the nozzles or any error 
in the amount of charge placed on a droplet by the charging electrodes 
causes the droplet to miss the pixel location. The result is a distortion 
of an image constructed from a raster pattern of multiple pixel lines. 
Heretofore, the alignment of the nozzles to the pixel locations has 
included electrical techniques. For example, should nozzle 1a tend to 
place its droplets slightly above pixel position 18a on the target, the 
video signal applied to electrode 3a is delayed relative to nozzles 1, 1b 
and 1c, a short duration to allow the target to move the amount of the 
offset. Alternately, the amount of change induced on the droplet is 
increased or decreased to vary the deflection an amount to correctly place 
a droplet at a given pixel position. The delay or magnitude change are 
applied to subsequent droplets. 
The present invention uses lens 10 for the alignment of droplets. In FIG. 
2, the lens 10 is seen in plan view as shared by all the nozzles. 
Referring to FIGS. 3 and 4, lens 10 is made up of an insulating member 20 
having a rectangular tunnel or hole 21 for passage of droplets. The 
upstream face of the insulator 20 has rectangular electrodes 22 and 23 at 
the long sides of the rectangular entrance to the tunnel 21. The upstream 
electrodes 22 and 23 are coupled to ground potential, by way of example. 
The downstream face of insulator 20 has rectangular electrodes 24 and 25 
at the long sides of the rectangular exit to the tunnel 21. The downstream 
electrodes are coupled to a high positive voltage indicated by the +A 
symbol. As an example, the insulator 20 is a phenolic insulator board of 
the type used for printed circuit boards and the electrodes 22-25 are 
copper strips formed by conventional evaporation and chemical etching 
techniques. The +A voltage is preferably about 1500 volts for a 60 mil 
thick board 20. The length of the tunnel 21 is about 61 mils, i.e. the 
conductors are about 0.5 mils in thickness. 
Briefly referring to FIG. 1, the lens 10 establishes a field that focuses 
droplets to a line 12 that corresponds to the scan line of pixels 18-18c. 
The focusing field is better described in connection with FIG. 4. The 
focusing electric field is represented by the dashed lines 27 and 28 
emanating from the edges of the upstream and downstream electrodes 22-25 
and confined substantially within the region defined by the semi-circles 
27a and 28a along the length of the electrodes. The envelope of the field 
lines is analogous to two half-cylinders abutting at a tangent plane 
parallel to their bases. The abutting tangent plane is normal to the 
drawing and is conveniently defined by centerline 14. 
The plane defined by centerline 14 is a path through the focusing field 
comprising fields 27 and 28 over which a charged droplet remains 
unaffected. However, a droplet such as the negatively charged droplet 29 
that is on a trajectory 31 offset from the centerline is focused to the 
focal line 12 by the focusing field. Likewise, the droplet 30 below the 
centerline 14 is focused to the focal line 12. All other droplets 
traveling trajectories lying above, below or between the paths 31 and 32 
are also focused to line 12. 
The focusing fields 27 and 28 extend in the direction of droplet travel 
from the upstream electrodes 22 and 23 to the downstream electrodes 24 and 
25. At the entrance to the tunnel 21, the focusing fields include a high 
density flux region that has vertical force components of significant 
magnitude. These forces are represented by the vectors 34 and 35. In the 
center region of the fields 27 and 28, the field and force vectors are 
parallel to the centerline 14 and have the same direction as the droplet 
for the polarities shown. These parallel forces accelerate the charged 
droplets shown. As a result, the charged droplets are under the influence 
of the focusing forces 34 and 35 longer than they are corresponding 
defocusing forces at the tunnel exit represented by vectors 37 and 38. 
When the +A potential is coupled to the upstream electrodes 22 and 23 and 
the ground potential is coupled to the downstream electrodes 24 and 25, 
the charged droplets are decelerated as they enter the tunnel 21. In this 
case, the charged droplets once again are under the influence of the 
focusing forces for a longer time than the defocusing forces. With this 
reversed polarity, the defocusing forces are at the entrance to the lens 
10 and the focusing forces are at the exit to the lens. Similarly, a 
positively charged droplet will be focused by the field shown in FIG. 4 by 
first being decelerated and then accelerated. The focusing forces always 
predominate over the defocusing forces regardless of the relative 
polarities. 
Experimentation shows that the focusing forces represented by the vectors 
34 and 35 are not offset by the effects of the defocusing forces 
represented by the vectors 37 and 38. In otherwords, despite what appears 
to be equal and opposite forces, the focusing forces represented by vector 
34 prevail over forces represented by vector 37 and bend the trajectory 31 
of a droplet 29 so as to intersect the centerline 14 at the focal line 12. 
This is because the time spent in the region of the focusing fields is 
greater than the time spent in the regions of the defocusing fields. 
Similarly, the trajectory 32 of a droplet 30 below the centerline 14, is 
bent by the focusing forces represented by vector 35 to intersect the 
focus point despite the defocusing forces represented by vector 38. 
The symbol f in FIG. 4 is representative of the focal length of the lens. 
For convenience it is measured from the entrance to tunnel 21 to the 
empirically determinable focus line 12. As mentioned earlier, the focus f 
varies for a change in the focusing field potential. When +A is decreased, 
f is increased and when +A is increased, f is decreased. Also, when the 
amount of charge on droplets 29 and 30 are increased, f is decreased and 
when the amount of charge on the droplets is decreased, f is increased. 
FIG. 3 shows the lens 10 looking from the target upstream toward the 
nozzles 1-1c. The insulator board 20 is shown with the conductive copper 
everywhere but along the narrow rectangular sides of the exit to tunnel 
21. Since electrodes 24 and 25 (as well as electrodes 22 and 23) are 
coupled to the same potential, the two electrodes could be electrically 
coupled by copper deposited on the vertical, exposed areas of the board 
20. The vertical, conductive edges should be spaced a significant distance 
from the end nozzles 1 and 1c so the distortion to the cylindrically 
shaped fields 27 and 18 are minimized. 
FIG. 5 illustrates another embodiment of the instant invention employing 
multiple, cylindrical focusing fields. The lens 40 is similar in 
construction to lens 10 but includes an intermediate electrode 41 between 
upstream electrodes 42 and 43 and downstream electrodes 44 and 45. 
Electrode 41 is a metal plate having a rectangular hole or tunnel 46 in it 
that matches the rectangular tunnels 47 and 48 in insulators 49 and 50 
abutted against member 41. The intermediate electrode 41 is fabricated 
from 63 mil thick aluminum sheet and the insulators 49 and 50 from 60 mil 
phenolic board. The upstream and downstream electrodes 42-45 are on the 
parallel, long edges of the tunnel orifices as in the case of the 
electrodes 22-25 on lens 10. The height of the tunnels 46-48 is about 50 
mils for droplets of about 1 to 10 mils in diameter. 
Upstream and downstream electrodes 42-45 are all coupled to a high voltage 
(represented by the symbol +A) of about 1500 volts, for example, and the 
intermediate electrode is grounded. Alternately, the intermediate 
electrode 41 can be coupled to +1500 volts, for example, and the upstream 
and downstream electrodes 42-45 to ground. 
There are two focusing fields associated with lens 40 including the 
upstream field made up of the upper and lower cylindrical fields 55 and 56 
and the downstream field made up of the upper and lower cylindrical fields 
57 and 58. The centerline 60 defines the path over which the trajectory of 
a charged droplet is not bent. For the polarities shown, the upstream 
field extends in a direction opposite to the flight of the droplet, and 
the downstream field extends in the same direction of the flight of the 
droplet. The defocusing forces represented by vectors 61 and 62 at the 
entrance to lens 40 and vectors 67 and 68 at the exit to the lens are 
found not to prevent the focusing of offset charged droplets 52 and 53 at 
the focal line 70. The focusing forces represented by the vectors 63-66 
are predominant because of the greater time spent in the focusing region. 
That is, the acceleration and deceleration of the droplets always act to 
favor focusing rather than defocusing. The opposing polarity of the fields 
of lens 40 are selected so that no net accelerating or decelerating energy 
is given to the droplets passing through it. In contrast, the single field 
lens, e.g. lens 10, imparts a very small amount of accelerating or 
decelerating energy to a charged droplet. The amount of net energy change 
is negligible yet, surprisingly, the focusing effect is realized. 
The focal distance f is measured, for convenience, from the edge of the 
upstream edge of the intermediate electrode 41 to the focal line 70. 
The function of lens 40 was tested by directing a stream of droplets 
through the lens and charging every third droplet. The uncharged droplets, 
by definition, are not effected by an electric field but they establish a 
base line for measurements. A lens was constructed like lens 40 above. 
About +1500 volts was coupled to the intermediate electrode 41. A ground 
potential was coupled to the upstream and downstream electrodes 42-45. 
Every third droplet emitted by a nozzle 1 was charged negatively by 
synchronously coupling about +650 volts to a charging tunnel 3. The 
uncharged droplet trajectory was about 10 mils offset from the centerline 
of the lens. The charged droplets were focused at about 1.2 inches 
downstream from the lens. 
The foregoing described lenses are novel components for ink jet 
applications. The focusing fields associated with lenses 10 and 40 operate 
on charged droplets analogously to a half-cylinder, glass lens that 
focuses light rays entering its flat base to a line in space parallel to 
the base. Other focusing field shapes including portions parallel to the 
droplet trajectories can be devised that are analogous to sperical and 
other optical lenses. Modifications of that type are within the scope of 
this invention.