Ink jet sensor method and apparatus

In an ink jet printer, improved ink droplet sensing method and apparatus. The disclosed full width ink jet printer includes a number of ink jet nozzles which direct ink droplets to specific regions of a print plane. The present sensing technique insures ink drops from the multiple nozzles "stitch" together properly across the printing plane. Multiple sensing sites (two for each nozzle) are comprised of a light input optical fiber and two output fibers coupled to circuitry to monitor light intensity at the sensing site. The sensitivity of the site is enhanced by electroformed input and output masks which reduce the effective send and receive area of the optical fibers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to ink jet printing and more particularly 
relates to ink droplet sensing method and apparatus. 
2. Prior Art 
As is known in the art, ink jet printing is a form of non-impact printing 
wherein ink droplets are caused to impinge upon a recording medium such as 
paper or the like. A typical ink jet system includes a droplet generator 
which starts a stream of ink along an initial trajectory, a transport for 
moving paper past the generator to allow droplets to impinge upon the 
paper, and a means for directing the ink droplets to impinge upon specific 
paper locations, thereby encoding the paper with information. According to 
one ink jet system, ink droplets are continuously generated at a fixed 
frequency along a trajectory toward the recording medium. The continuous 
drop system also includes a gutter to catch those droplets which are not 
to impact the paper. The trajectory of the droplets is determined at the 
point of droplet break-off by placing a selected net charge on the 
droplet. In this way, some droplets contact the paper at appropriate 
positions and other droplets strike the gutter and are recirculated for 
subsequent use. 
Within the so called continuous drop subset of ink jet printers, there 
exists a variety of architectures. One architecture, for example, 
comprises a single drop generator nozzle which traverses back and forth 
across the ink jet page as droplets are generated. Other architectures 
include multiple nozzles each of which direct ink droplets to selected 
portions of a paper width. 
One multiple nozzle architecture known in the art makes particular use of 
the present droplet sensing method and apparatus. According to that 
architecture, the droplets are charged both positively and negatively to 
varying degrees depending upon the desired droplet trajectory. A 
deflection field located downstream from the droplet break-off point 
interacts with the positively and negatively charged droplets to deflect 
them away from their initial trajectory in a direction transverse to a 
direction of paper movement. Each of the multiple ink jet nozzles of this 
system throws droplets to a specific portion of the paper width and when 
the printer is functioning properly, the ink droplets from the multiple 
nozzles "stitch" together at stitch points across this width. In this 
system, the gutter droplets are more highly charged so that they deflect a 
large amount away from their initial trajectory to gutters comprising part 
of the drop deflection apparatus. Further details regarding this type of 
ink jet printer quite similar to this architecture can be obtained by 
reference to U.S. Pat. No. 4,238,804 to Warren entitled "Stitching Method 
and Apparatus For Multiple Nozzle Ink Jet Printer". 
As droplets are generated and deflected along varying trajectories to the 
paper or the gutter, there is a need to check the performance of the ink 
jet generator, charging electrode and deflection field. This checking 
process must be performed periodically so that the calibration of the 
printing system never deteriorates. One particularly important feature in 
the ink jet process is the stitching together of drops from the nozzles 
along the ink jet array. Neither droplet overlap nor gaps between nozzles 
can be permitted if the ink jet printing image is to be uninterrupted 
across the paper width. For this reason, it is important that some sensing 
of droplet trajectory be conducted to insure that droplets are responding 
to the charging and deflection in a desired manner. 
Prior art drop sensors are known. One such sensor is disclosed in U.S. Pat. 
No. 4,255,754 to Crean et al assigned to the Xerox Corporation, assignee 
of the present invention and incorporated herein by reference. Apparatus 
disclosed in that patent includes a light source and light sensor mounted 
on opposite sides of a droplet trajectory. The sensor interprets changes 
in light intensity transmitted by the light source to sense both the 
presence of ink droplets and to determine their speed of travel to the 
recording medium. U.S. Pat. No. 4,344,078 to Houston filed Nov. 6, 1980 
and issued on Aug. 10, 1982 discloses one method for practicing the 
sensing technique disclosed in the Crean et al patent. According to the 
method disclosed in that application, the light source and light receiver 
include optical paths which are photo-fabricated to a support substrate. 
The present invention comprises an alternate drop sensing method and 
apparatus which offers ease in sensor fabrication and improvements in drop 
sensing sensitivity. 
SUMMARY OF THE INVENTION 
The present invention comprises structure for sending and receiving light 
signals to monitor droplet travel from a drop generator to a print medium. 
The structure can be easily fabricated and maintained while offering a 
high degree of droplet sensing sensitivity that is required where the 
sensor is used in stitching together droplets from a multi nozzle ink 
generator. 
The invention has particular utility when used in conjunction with an ink 
jet printer having multiple nozzles for directing a number of ink streams 
toward a recording medium. The apparatus includes optical light fibers for 
directing light to a number of sensing positions along the width of the 
printer so that droplets following certain trajectories intercept this 
light in a region of high light intensity. Additional output fibers are 
positioned in spaced relationship to the input fibers for detecting 
changes in light intensity caused by passage of the droplets past the 
sensing position. Additionally, masks are interposed between the input and 
output light fibers in the regions of ink droplet travel to mask light and 
define optical sending and receiving sites having an area less than the 
fibers which they mask. 
Use of the optical masking technique allows utilization of bulk optic 
fibers having a cross sectional dimension greater than the dimension 
required by the ink droplet sensing apparatus. These bulk fibers can be 
easily routed away from the sensing sites and can be flexed and bent as 
needed to route them to light intensity detecting circuitry. According to 
a preferred embodiment, the masking of these fibers is accomplished by 
positioning electroformed metal masks over the end surface of these output 
fibers. The light transmitting regions as defined by these masks are 
separated along the dimension of the path of travel while being closely 
spaced in the direction of droplet deflection. This closely adjacent 
positioning of output fibers enhances sensing sensitivity to aid in the 
stitching together of ink droplets in a printing array. The fiber 
separation in the direction of drop travel eases sensor assembly. 
According to one assembly technique, the input and output fibers are 
mounted to a mounting plate through which mounting holes are drilled. The 
optical fibers are inserted through the plate and then secured in place by 
a potting compound which firmly secures the fibers to the plate without 
limiting bending or flexing of the optical fibers. 
From the above, it should be appreciated that one object of the present 
invention is the provision of a structurally sound yet optically sensitive 
ink jet droplet sensor for sensing ink drops during their trajectory 
toward a recording medium. Other objects, features and advantages of the 
present invention will become better understood when a preferred 
embodiment of the present invention is described in conjunction with the 
accompanying figures.

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawings, FIG. 1 illustrates a liquid drop printer 10 
using a sensor array constructed according to the present invention. The 
sensing array 12 is a device for detecting the location of drops in flight 
relative to the coordinates of an x, y and z orthogonal coordinate system. 
As used herein, the drops are in flight generally parallel to the z axis. 
A plurality of drop streams are described which are all directed to the 
same x-y print plane. Printing is done in a raster pattern comprising 
multiple scan lines or print lines of pixels. A single drop is placed onto 
a single pixel. The role of the sensor is to insure that the drop 
placement relative to the pixels within a scan line are accurate. That is, 
any errors in drop placement detected by the sensor are correctable. 
The scan or print lines are deposited onto a target along the x axis while 
the target and drop streams move relative to each other along the y axis. 
The relative movement gives rise to the two dimensional raster image 
composed of multiple, parallel print lines. The presence or absence of a 
liquid drop at each pixel is the means by which an image is constructed. 
The system of FIG. 1 is especially benefited by the present invention. It 
is of a type similar to the printing system described by W. Thomas Warren 
in U.S. Pat. No. 4,238,804 filed Feb. 28, 1979 and issued Dec. 9, 1980. In 
the Warren patent, a plurality of drop streams can collectively deposit 
drops at all the pixels within a scan line. The segments must be aligned 
to each other to create a continuous scan line across the target. The 
Warren patent discloses the use of an array of drop position sensors to 
insure the segments "stitch" together. 
As seen in FIG. 1, the sensing array 12 is positioned close to a print line 
14 on a target. Liquid drops 16 in a drop stream represented by the dashed 
lines 18 fly toward the target to strike a given pixel within a print line 
or to be deflected into a print gutter 20. A test gutter 22 is located 
downstream of the target whereas the print gutters 20 are located upstream 
of the target. In the embodiment of FIG. 1, the sensor array 12 is 
normally employed only when the target is removed from the vicinity of the 
test gutter. The sensors are used to calibrate or adjust various system 
parameters affecting the drops 16. The drops are continuously generated 
from a liquid column 24 in a manner predicted by Lord Rayleigh. The column 
of liquid is issued from nozzles 26 in the drop generator 28. The 
generator includes a piezoelectric transducer coupled to the liquid in the 
generator. The piezoelectric transducer creates pressure variations within 
the liquid at a given frequency. These pressure variations in turn cause 
the drops 16 to form at the same rate, at a fixed distance from a nozzle 
26 and with a uniform size and spacing. 
A number of charging electrodes 29 (one for each drop stream) are located 
at the region of drop formation near the end of column 24. A voltage 
applied to an electrode 29 induces charge in the liquid column 24 which is 
trapped in a drop 16 when the drop breaks away from the column. Typically, 
the liquid is electrically grounded through the body of the generator 28. 
Charged drops are deflected approximately in the x-z plane by an 
electrostatic field established between adjacent deflection electrodes 30. 
The drops in the embodiment of FIG. 1 are deflected either to the print 
gutter 20 or to a pixel within the scan line 14 at the target. The 
deflection field is created between the deflection electrodes 30 by the 
ground and +B potentials coupled to the electrodes. 
During a printing operation, voltages are applied to the charging 
electrodes 29 to affect a sequential positioning of printed drops within 
the length 32 of the segments within the scan line 14. The points 35 and 
36 represent adjacent pixels in the scan line located in adjacent 
segments. Pixel 35 is addressed by a drop trajectory 38 from the rightmost 
drop stream and pixel 36 is addressed by a drop following a trajectory 40 
from the neighboring stream to the left. In a raster image printing system 
like that of FIG. 1, it is essential that the drops from adjacent streams 
are "stitched", that is, aligned to the ideal pixel locations within the 
scan line 14. Drops not intended for the target during a printing 
operation are charged to a level causing them to follow a trajectory 
intersecting a print gutter 20. The trajectories 42 and 44 are those 
followed by gutter drops in the two leftmost streams and are typical for 
other streams. Zero charge level drops fly a path to the trajectory 
unaffected by the deflection field between plates 30. However, the zero 
charge level is not used unless it results in placement of a drop at one 
of the evenly spaced pixels within scan line 14. 
The sensor array 12 is used for drop stitching. The array is an assembly 
including a plurality of drop sensor sites 46a-e spaced apart a fixed 
distance apart along the sensor array. The sensor spacing is equal to that 
of the nozzle to nozzle spacing 34. The sensor positioning is chosen so 
that each drop stream can have drops deflected to trajectories over two 
sensor sites. This is important for the stitching operation. A drop can be 
charged, for example, to a voltage causing it to fly through a trajectory 
47 over sensor 46c. Similarly a drop can be charged to cause it to follow 
a path 49 over sensor 46b. The electrostatic deflection process is 
approximately linear. Consequently, knowing the voltages that cause drops 
to follow the two sensor trajectories 47,49 means that all the pixels 
within a segment can be accurately addressed by charging the droplets to 
voltages calculated by interpolation. Each drop stream is calibrated in 
this way to print or place drops at the ideal pixel locations within the 
line segment within its reach. The drop placement process is then 
"stitched" since the end pixel positions in adjacent segments each are 
able to have drops placed on them. The number of sensor sites equals the 
number of drop streams plus one. Adjacent drop streams share a sensor site 
but one more sensor than the number of drop streams is needed to provide 
two sensors for an end drop stream. 
The stitching or calibration process need not be performed constantly. The 
charging voltages affecting aligned flight over the sensors hold steady 
for periods from seconds to tens of minutes. Consequently, it is adequate 
to reset or check the charging voltages at intervals. A convenient 
interval is that provided between completion of printing on one target and 
the start of printing on another target. 
FIG. 2 shows the sensor array 12 in perspective and includes the 
co-ordinate axis defined in FIG. 1. Each sensor site 46 includes a fiber 
optic input light guide 50 for directing light from a light source 52 and 
two fiber optic output light guides 54A,54B for directing light to two 
light detectors 58,60. Other sensor sites across the array 12 also have 
output light guides coupled to the same sensors 58,60. 
Droplets are sensed by measuring the light intensity transmitted by the 
fibers 54A,54B. When a droplet passes between the input 50 and either of 
the output fibers 54A,54B comprising one sensor site the intensity of the 
light transmitted by that fiber is diminished and the output from an 
associated detector 58,60 also diminishes. The two detectors 58,60 have 
outputs coupled to an operational amplifier 62 which functions as a 
difference amplifier (FIG. 5). When no drop is in the sensing zone or when 
a drop is symmetrically aligned between the two input ends of the A and B 
fibers, the outputs from the two detectors are equal and the differential 
amplifier output is zero. When the moving droplets unequally shadow one or 
the other of the two fibers, however, the differential amplifier output 
goes either positive or negative depending upon which detector produces 
the larger output. Further details regarding the method for interpreting 
outputs from the sensor site may be obtained by referencing the '754 
patent to Crean et al. 
The differential amplifier output is coupled to an analog to digital 
converter 64 which in turn presents a digital signal to a printer 
controller 66. The controller monitors and controls the stitching 
operations. The controller tests and calibrates each nozzle sequentially, 
allowing the use of a minimum of two detectors 58,60. The controller 
calculates voltage values needed to place a droplet at all the pixels 
within a segment. This calculation is based on the voltages that align the 
drops over the left and right sensor sites accessable by each stream. 
These numbers are typically unique for each drop stream and are calculated 
by interpolation from the voltages needed to deflect droplets over the 
sensor sites. 
A typical fiber optic output fiber has a cross sectional diameter in the 
range 10 to 15 thousandths of an inch. It is desirable, however, to define 
the optical receive and transmit sites more accurately. It is also 
desirable to mount the fibers in as efficient yet reliable a manner as 
possible. FIGS. 2 and 3 show the preferred method of accomplishing those 
results. 
The output fibers 54A,54B are routed through a mounting block or plate 72 
with holes or a slot extending therethrough which allow a slide fit of the 
output fibers 54A,54B. The outer surface 74 includes recesses which allow 
potting and polishing of the fibers. Once the fibers are in place an epoxy 
or other suitable potting glue 76 (FIG. 3) provides strain relief to the 
fibers 54A,54B. 
Further definition is given to the sensing sites by two electroformed light 
masks 78,80 (FIG. 2) which define light transmitted by the input fibers 50 
and more specifically define the light receiving sites of the output 
fibers 54A,54B. The input mask 80 has one light transmitting portion or 
hole for each input fiber 50 of a cross section about one third to one 
fourth the fiber cross section. 
The arrangement for defining the output portion of the sensing sites is 
seen in FIG. 4. The electroformed mask 78 defines two light transmitting 
regions 82,84 (holes) spaced in a direction generally parallel to a 
typical ink droplet trajectory. The output fibers 54A,54B abut the mask 80 
so that only a portion of a fiber core 88 and none of a fiber cladding 90 
is exposed to light from the input fiber 50 to that sensing site. 
The present sensing site definition method significantly improves the 
printer. The electroformed metal masks for transmit and receive sites 
improve the accuracy of the sensing and thus the accuracy of droplet 
stitching and placement. Use of the precise mask positioning allows 
non-critical fiber positioning so long as the mask holes cover a portion 
of the core area of the fibers. In FIG. 4 it is noted that there is zero 
separation of the receive site boundaries in the drop deflection 
direction. For any droplet traversing this boundary there will be a 
maximum gradient of differential signal output which allows precise 
stitching when compared to prior art sensors which separated the sensing 
sites along the direction of droplet detection due to the cladding 
material thickness of the output fibers. 
The present invention has been described with a degree of particularity. It 
is the intent, however, that all modifications or alterations included 
within the spirit or scope of the appended claims be protected.