Technical Field
[0001] The present invention relates to the field of continuous ink jet printing and, more
particularly, to a method for improving charge voltage print windows for ink jet printers
which utilize arrays of charging electrodes and orifice arrays.
Background Art
[0002] Ink jet drops are controlled via high voltage electrodes built into a planar array.
The planar array is aligned to an array of orifices and the drops are deflected onto
a catcher surface which is parallel to the charging array. The catch drops flow down
the catcher surface and are returned to a fluid tank to be recycled. Print drops are
selected by switching the voltage off, on specific electrodes. The print drops must
clear the fluid film generated by previous catch drops, while the characteristics
of the fluid film changes depending on the selected drop patterns used to generate
the images.
[0003] A series of print patterns are used to determine a charge voltage, or print, window,
which is the range of voltages which can be applied to the charging electrodes and
still provide perfect print. It is desired to maximize the print window to allow operation
of the printer in a variety of operating conditions including, for example, ambient
temperature, print speed, and related print features.
[0004] The precise location of the drop generation point in an electric field requires parts
that are flat and parallel, (catchers, charge plates, and orifice arrays) at tolerances
utilizing state of the art manufacturing techniques. The voltage required for charging
and deflecting the catch drops is applied to all electrodes. The fluid impact on the
catcher face is uniform when the ideal flat and parallel parts are assembled. Variations
in part tolerances result in a range of impact heights on the catcher face, and this
non-uniformity reduces the print window. If the range is excessive, there is no voltage
that can be applied to provide acceptable operation. Higher density and longer arrays
make it more difficult to fabricate the components to the desired tolerances.
[0005] It is seen, therefore, that an alternative method to increase the print window is
needed.
Summary of the Invention
[0006] This need is met by the segmented charging technique of the present invention.
[0007] In accordance with one aspect of the present invention, means are provided to adjust
the charging voltage to a discrete section of the array, increasing the operating
range of the printhead. The array of charging electrodes are broken into segments
with means to provide relative voltage adjustment to each segment. The voltage to
the charge plate electrodes is then switchably controlled by discrete high voltage
driver chips to enable drop selection for printing. The integrated circuit within
each chip converts serial input data at low voltages (5V typical) to parallel outputs
at high voltage (
∼170 Vdc). The supplied high voltage to each of these chips is varied to compensate
for impact variations on the catcher face caused by non-uniform parts. The compensation
levels required for a set of aligned parts are stored within the printhead memory.
The printhead will then function with the correct compensation levels on any system
in which it is installed.
[0008] Other objects and advantages of the invention will be apparent from the following
description, the accompanying drawing and the appended claims.
Brief Description of the Drawing
[0009]
Fig. 1 is a side view of an aligned printhead indicating the major components required
for a continuous flow ink jet printer;
Fig. 2 is a front view of the printhead of Fig. 1;
Fig. 3 is a front view of a printhead, similar to Fig. 2, but with the drop deflection
modified by changing the voltage on segments of the charging electrodes;
Fig. 4 is a block diagram of a preferred embodiment of the present invention using
a main supply and individual segment regulators; and
Fig. 5 illustrates one embodiment of an individual segment controller, in accordance
with the present invention.
Detailed Description of the Preferred Embodiments
[0010] Present ink jet devices typically use a single voltage level applied to all electrodes
to provide the drop charge required for deflecting the catch drops. When a single
voltage is applied and the parts are uniform, the fluid impact on the catcher face
is uniform. However, the variation in part tolerances causes the impact on the catcher
face to vary, and this variation or non-uniformity reduces the print window. In accordance
with the present invention, a method is provided to compensate for this variation
in part tolerances, by providing different voltages to different regions of the electrode
array. The average impact will be more uniform and the size of the print window will
increase.
[0011] Referring to Fig. 1, there is illustrated a side view of an aligned printhead 14
showing the major components required for a continuous flow ink jet printer. A drop
generator 10 is aligned to a charge plate assembly 20 to cause drops 12 to form within
the electric fields generated by high voltage electrodes 22. The charged drops are
then attracted to a nearby catcher surface 30. The drops which strike the catcher
form a fluid layer 32 which flows down catcher face 28 into the catcher opening.
[0012] As the fluid flows down the catcher face 28, the fluid slows down and the fluid layer
32 gets thicker. If the fluid layer gets too thick, the fluid layer can interfere
with the non-deflected print drops. This can cause areas of missing print. On the
other hand, too thin of a fluid layer can allow some catch drops to print, producing
a dark defect. As the fluid layer thickness depends on the height at which the drops
strike the catcher, there is a range of impact heights on the catcher that produces
defect-free print.
[0013] The impact height of the drops on the catcher depends on the amount of charging and
deflection produced by the charge plate. The charging and deflection depend on the
charging voltage and on the spacing between the charge plate and the jets. The impact
height further depends on the recess distance of the catcher behind the charge plate.
Deviations in jet directionality, charge plate flatness, catcher flatness or the parallelism
of the components all result in variations in impact height.
[0014] Fig. 2 illustrates a front view of the printhead 14. Ink is seen to flow down the
catcher face below the impact line 35 of drops on the catcher. As discussed, the impact
line 35 varies up and down the catcher face. Fig. 2 also shows the boundaries for
the acceptable drop impacts, defined by pick out of the print drops in areas with
the impact line high up in the catcher face such as at 34 and printing of catch drops
where the impact line is low on the catcher face such as at 36.
[0015] To ensure maximum reliability, it is desirable to minimize the variation in impact
heights within the acceptable range of impacts. This makes the printer less sensitive
to variations in pressure, ink properties or other environmental variables.
[0016] In accordance with the present invention, it is possible to minimize the impact variation
by using different charge voltage levels for different sections of the ink jet array.
Fig. 3 shows the same impact on the catcher face from Fig. 2, but the drop deflection
has been modified by changing the voltage on the charging electrodes. The voltage
in segment D has been raised relative to segments C and E to minimize the depth of
the low impacting center section of the array. Similarly segments B and F have also
been raised relative to segments C and E. The impact line 35 shows discrete jumps
or breakpoints at the voltage steps. With the total impact variation reduced, the
print latitude improves accordingly.
[0017] The number and width of each fixed voltage segment may vary as desired. While the
best print window would be achieved by adjusting the charging voltage electrode by
electrode, one electrode segments, such an implementation would add considerable cost
to the electronics. A more cost effective option, which still provides most of the
virtue provided by the single electrode segmenting option, is to chose a segment size
which corresponds to the number of electrodes controlled by a charge voltage driver
IC. In one such implementation of the invention, the segment size is 64 jets or charge
leads, since the charge voltage switching electronics, Supertex HV34, which produces
the necessary print voltage pulses, are designed to control 64 charging electrodes.
In a still more cost effective embodiment the typical impact profile was considered,
so that the segment width, corresponding to the number of charge driver IC's per segment,
is varied across the array. In areas that tend to have flat impact profiles, the segment
width was wider, the segment includes a larger number of charge driver IC's. In areas
where the impact height tends to change more rapidly, the segment width is narrower,
the number of charge driver IC's is less. For this particular embodiment, forty two
charge driver IC's are separated non-uniformly into 18 segments.
[0018] The high voltage supplied to each segment, or can be provided by either individual
high voltage supplies or by a main supply with individual segment regulators. When
using the individual segment regulators, voltage may be added to or subtracted from
the main supply voltage. In the particular embodiment described here the individual
segment regulators subtract voltage from the main supply voltage. To prevent damage
to the charge driver IC's from excessive reverse current, it is important to block
reverse current from one high voltage segment to the next. This can be accomplished
using any suitable means such as, for example, by using a diode in the high voltage
supply line to each charge driver IC.
[0019] Fig. 4 shows a block diagram 50 of a preferred embodiment of the present invention
for supplying high voltage to each segment. The embodiment illustrated in Fig. 4 uses
a main supply 56 and individual segment regulators 54. High voltage is supplied to
the individual segment regulators 54 by the main high voltage supply 56. Current sensing
electronics at block 58 measure the current drawn by the individual segments. This
allows shorts to be detected. Furthermore the shorts can be located to the individual
charging segment. The segment regulators 54 then supply the voltage to the charge
drivers, as indicated by block 60.
[0020] Continuing with Fig. 4 and referring also to Fig. 5, there is illustrated in Fig.
5 one embodiment of an individual segment regulator. This segment regulator 54 adjusts
the voltage across capacitor 82. The voltage across the capacitor has a polarity such
that it will oppose or subtract from the voltage from the high voltage supply, 56.
The resulting voltage, supplied high voltage from source 56 minus the voltage drop
across capacitor 56 is applied to the charge driver IC 60 through the reverse blocking
diode 96. By subtracting voltage from the main supply level, this design ensures all
segments drop to zero volts when the main supply voltage is dropped to zero volts.
This is an important consideration during charge plate short recovery.
[0021] During operation, the load current drawn by the HV34 charge driver IC's 60 cause
the voltage across capacitor 82 to increase, decreasing the voltage across capacitor
84. To reduce the voltage increase across capacitor 82, the switch 72 is turned on.
This discharges capacitor 82 through resistor 80, which controls the discharge time
constant. The on time of switch 72 is adjusted by the analog control circuit 62 to
maintain the voltage across capacitor 82 at the desired level.
[0022] The voltage across capacitor 82 is sensed by means of the difference amplifier 90
and two resistive voltage dividers. One voltage divider consisting of resistors 86
and 88 supplies the difference amplifier with a voltage proportional to the output
voltage from the regulator, while the other resistive voltage divider consisting of
resistors 92 and 94 supplies the other input of the difference amplifier with a voltage
proportional to the input voltage to the regulator. The resulting voltage from the
difference amplifier, which is proportional to the voltage across the capacitor 82,
is supplied to the analog control section 62. In this way the output voltage from
each segment regulator can be controlled by its analog control circuit. The set point
for each segment is dictated by a common digital control circuit 66.
[0023] It is desirable to store the segment and main supply voltage information needed by
the digital control circuit 66 in a nonvolatile memory 70 that is part of the printhead.
In this way, the printhead appropriate set point information for each segment, established
during manufacture of the printhead to provide the widest printhead latitude, can
be supplied to the digital control circuit.
[0024] In operation, the digital control electronics 66 reads the configuration data stored
in the printhead and sets the segment voltage appropriately. Should changes in ink
properties require a change in the charge voltage, the baseline high voltage value
would be changed. This maintains the relative voltages between segments.
[0025] For diagnostic purposes, the segment voltage levels and current levels are monitored
by the analog measure circuit 64, which passes the data on to the digital control
circuit 66. In this way, the digital control can perform diagnostics tests related
to the printhead and the segment charging electronics.
[0026] The invention has been described in detail with particular reference to certain preferred
embodiments thereof, but it will be understood that modifications and variations can
be effected within the spirit and scope of the invention.
1. A method for improving charge voltage print windows for ink jet printers comprising
the steps of:
utilizing arrays of charging electrodes;
breaking the array of charging electrodes into segments;
providing relative voltage adjustment to each segment, whereby the charging electrodes
charge the ink drops.
2. A method as claimed in claim 1 wherein each segment has a charge voltage window for
proper charging of the drops and the relative voltage adjustment is used to supply
each segment with a charging voltage near a center of its charge voltage window.
3. A method as claimed in claim 1 further comprising the step of storing relative voltage
adjustment levels in the printhead.
4. A method as claimed in claim 1 further comprising the step of detecting charge plate
shorts in each segment.
5. A method as claimed in claim 1 further comprising the step of carrying out diagnostic
tests related to the printhead and segment charging electronics.
6. A method as claimed in claim 1 further comprising the step of adjusting baseline voltage
while maintaining desired relative voltage adjustment between segments.
7. A system for improving charge voltage print windows for ink jet printers comprising:
arrays of charging electrodes;
means for breaking the array of charging electrodes into segments;
relative voltage adjustment provided to each segment, whereby the charging electrodes
charge the ink drops.
8. A system as claimed in claim 7 wherein the relative voltage adjustment further comprises
individual segment regulators.
9. A system as claimed in claim 7 further comprising means to detect charge plate shorts
in each segment.
10. A system as claimed in claim 7 further comprising means to adjust baseline voltage
while maintaining desired relative voltage adjustment between segments.