Cross-Reference to Related Applications
[0001] The present invention is related to the following pending and commonly owned Eurpoean
Patent Applications: 93309242.1 and 92107065.2 which are herein incorporated by reference.
Field of the Invention
[0002] This invention relates generally to the field of thermal inkjet printers and more
particularly to controlling the ejected ink drop volume of thermal inkjet printheads
by controlling the temperature of the printhead substrate.
Background of the Invention
[0003] Thermal inkjet printers have gained wide acceptance. These printers are described
by W.J. Lloyd and H.T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices
(Ed. R.C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Patents
4,490,728 and 4,313,684. Thermal inkjet printers produce high quality print, are compact
and portable, and print quickly and quietly because only ink strikes the paper.
[0004] An inkjet printer forms a printed image by printing a pattern of individual dots
at particular locations of an array defined for the printing medium. The locations
are conveniently visualized as being small dots in a rectilinear array. The locations
are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation
can be viewed as the filling of a pattern of dot locations with dots of ink.
[0005] Inkjet printers print dots by ejecting very small drops of ink onto the print medium,
and typically include a movable carriage that supports one or more printheads each
having ink ejecting nozzles. The carriage traverses over the surface of the print
medium, and the nozzles are controlled to eject drops of ink at appropriate times
pursuant to command of a microcomputer or other controller, wherein the timing of
the application of the ink drops is intended to correspond to the pattern of pixels
of the image being printed.
[0006] Color thermal inkjet printers commonly employ a plurality of printheads, for example
four, mounted in the print carriage to produce different colors. Each printhead contains
ink of a different color, with the commonly used colors being cyan, magenta, yellow,
and black. These base colors are produced by depositing a drop of the required color
onto a dot location, while secondary or shaded colors are formed by depositing multiple
drops of different base color inks onto the same dot location, with the overprinting
of two or more base colors producing secondary colors according to well established
optical principles.
[0007] The typical thermal inkjet printhead (i.e., the silicon substrate, structures built
on the substrate, and connections to the substrate) uses liquid ink (i.e., colorants
dissolved or dispersed in a solvent). It has an array of precisely formed nozzles
attached to a printhead substrate that incorporates an array of firing chambers which
receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor,
known as a thermal inkjet firing chamber resistor, located opposite the nozzle so
ink can collect between it and the nozzle. When electric printing pulses heat the
thermal inkjet firing chamber resistor, a small portion of the ink next to it vaporizes
and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot
matrix pattern. Properly sequencing the operation of each nozzle causes characters
or images to be printed upon the paper as the printhead moves past the paper.
[0008] Print quality is one of the most important considerations of competition in the color
inkjet printer field. Since the image output of a color inkjet printer is formed of
thousands of individual ink drops, the quality of the image is ultimately dependent
upon the quality of each ink drop and the arrangement of the ink drops on the print
medium. One source of print quality degradation is improper ink drop volume.
[0009] Drop volume variations result in degraded print quality and have prevented the realization
of the full potential of thermal ink jet printers. Drop volumes vary with the printhead
substrate temperature because the two properties that control it vary with printhead
substrate temperature: the viscosity of the ink and the amount of ink vaporized by
a firing chamber resistor when driven with a printing pulse. Drop volume variations
commonly occur during printer start- up, during changes in ambient temperature, and
when the printer output varies, such as a change from normal print to "black-out"
print (i.e. where the printer covers the page with dots.)
[0010] Variations in drop volume degrades print quality by causing variations in the darkness
of black-and- white text, variations in the contrast of gray-scale images, and variations
in the chroma, hue and lightness of color images. The chroma, hue and lightness of
a printed color depends on the volume of all the primary color drops that create the
printed color. If the printhead substrate temperature increases or decreases as the
page is printed, the colors at the top of the page can differ from the colors at the
bottom of the page.
[0011] Reducing the range of drop volume variations will improve the quality of printed
text, graphics, and images.
[0012] Additional degradation in the print quality is caused by excessive amounts of ink
in the larger drops. When at room temperature, a thermal ink jet printhead must eject
drops of sufficient size to form satisfactory printed dots. However, previously known
printheads that meet this performance requirement, eject drops containing excessive
amounts of inkwhen the printhead substrate is warm. The excessive ink degraded the
print by causing feathering of the ink drops, bleeding of ink drops having different
colors, and cockling and curling of the paper. Reducing the range of drop volume variation
would help eliminate this problem.
[0013] Thermal inkjet cartridge performance can vary widely due to the temperature of the
ink firing chamber and therefore the ejected ink. Due to changes of the physical constants
of the ink, the nucleation dynamics and the refill characteristics of a thermal inkjet
printhead due to substrate temperature, the control of the temperature is necessary
to guarantee consistently good image print quality. The cartridge substrate temperature
can vary due to ambient temperature, servicing (spitting) and the amount of printing
done with the cartridge.
[0014] Heating of the printhead before the start of the printing swath has been used to
control substrate temperature. This method has the disadvantage of having to predict
the required temperature and adjust the delivered energy at the start of the printing
zone to compensate for all possible changes of temperature during the printing swath.
Temperature excursions can be great and very difficult to predict. Heating during
the printing swath has been tried by adding additional heating elements or additional
electronics to energize the print element heaters in parallel with the printing pulses.
This method adds to the cost and complexity of the control and power electronics.
[0015] For the reasons previously discussed, it would be advantageous to have an apparatus
and a method for reducing the range of temperature and drop volume variation by heating
the printhead during print.
Summary of the Invention
[0016] The foregoing and other advantages are provided by the present invention which reduces
the range of the drop volume variation by maintaining the temperature of the printhead
substrate above a minimum value known as the reference temperature. The present invention
includes a temperature sense resistor deposited around the firing chamber resistors
of the printhead substrate to measure temperature. The present invention includes
the steps of selecting a reference temperature that can reduce the range of drop volume
variation, measuring the printhead substrate temperature, comparing the printhead
substrate temperature with the reference temperature, and keeping the printhead substrate
temperature above the reference temperature to reduce the range of drop volume variation.
[0017] The present invention includes using a thermal model to estimate the amount of heat
to deliver to the printhead substrate to raise its temperature to the reference temperature
and delivering this energy during printing swaths. The present invention includes
heating the printhead substrate during the printing of a swath by driving the firing
chamber resistors with non-firing pulses synchronized with the firing pulses. The
use of non-nucleating pulses synchronized with the printing pulses to control the
temperature of the printhead substrate has been shown to dramatically improve the
print quality of images printed at all operating conditions including the extremes
of printhead parameters. By using synchronized pulses, significant cost and complexity
can be reduced as compared to other controlled temperature systems.
Brief Description of the Drawings
[0018]
FIG. 1 is a block diagram of the present invention.
FIG. 2 is a plot of the thermal model of the printhead substrate used by the preferred
embodiment of the invention.
FIG. 3 shows the temperature sense resistor for the preferred embodiment of the present
invention.
FIG. 4 shows the composite pulse waveform generated by OR-ing the heating pulses and
printing pulses.
Detailed Description of the Invention
[0019] As discussed above, ink drop volume in an inkjet printer varies with printhead substrate
temperature. The present invention reduces the range of drop volume variation by heating
the printhead substrate to a reference temperature before printing begins and controlling
that temperature during printing by using non-firing pulses synchronized with the
firing pulses used to eject printing drops.
[0020] FIG. 1 is a block diagram of the preferred embodiment of the present invention. The
invention uses a thermal model of the printhead substrate to estimate how long to
drive the printhead substrate at a particular power level to raise its temperature
to the reference temperature of the printhead substrate. It consists of a printhead
substrate temperature sensor 22, a cartridge temperature sensor 24 measures the ambient
temperature of the cartridge, and a reference temperature generator 26. The outputs
of these three devices are fed into a Thermal Model Processor/Comparator 28 which
calculates the non-printing pulse width to apply to the heater resistors. Non-printing
pulses are pulses that heat the printhead substrate, but are insufficient to cause
nucleation by the firing chamber resistors and eject drops of ink. As used herein,
the terms "non-printing," "non-firing," "heating," and "non-nucleating" pulses are
synonymous. Also as used herein, firing chamber resistors 38 and heater resistors
are synonymous. The output of the Synchronized OR-ing Controller 30 signals a Printhead
Driver 32 when to drive the firing chamber resistors 38 with one or more packets of
nonprinting pulses having the pulse width specified by the Thermal Model Processor/Comparator
28, based on input from the Print Data Memory 34 and the Printhead Position Sensor
36.
[0021] FIG. 2 is a plot of the thermal model of the printhead substrate as described in
copend'ing commonly assigned application serial number 07/983,009, filed November
30, 1992, entitled METHOD AND APPARATUS FOR REDUCING THE RANGE OF DROP VOLUME VARIATION
IN THERMAL INK JET PRINTERS which is incorporated herein by reference. As set forth
above, the inputs to the thermal model include the reference temperature, the cartridge
temperature (i.e., the temperature of the air inside the cartridge that surrounds
the printhead substrate,) and the printhead substrate temperature. The output parameter,
Δt, shown in FIG. 2 is the length of time the firing chamber resistors 38 should be
driven at power P to heat the printhead substrate to the reference temperature.
[0022] FIG. 3 shows the temperature sense resistor 22 used by the invention. Temperature
sense resistor 22 measures the average temperature of a printhead substrate 40 since
it wraps around all nozzles 42 of printhead substrate 40. The temperature of the ink
in the drop generators is the temperature of greatest interest, but this temperature
is difficult to measure directly, so temperature sense resistor 22 measures it indirectly.
The silicon is thermally conductive and the ink is in contact with the substrate long
enough that the temperature averaged around the head is very close to the temperature
of the ink by the time the printhead ejects the ink.
[0023] The output of the printhead substrate temperature sensor 22 is compared to the reference
temperature output of reference temperature generator 26 by the Thermal Model Processor/Comparator
28. If the printhead substrate temperature is less than the reference temperature,
the Thermal Model Processor/Comparator 28 will enable heating pulses and send the
heating pulse width to the Synchronizing OR-ing Controller 30. This process is repeated
as required during the print cycle.
[0024] The advantage of the thermal model is that the printhead substrate reaches the reference
temperature with reduced iterations of measuring the printhead substrate temperature
and heating the printhead substrate. However, the thermal model is part of a closed-loop
system and the system may use several iterations of measuring and heating if needed.
[0025] The present invention, sets the reference temperature equal to T
APCT because it has the advantage of eliminating half the temperature range and half the
range of drop volume variation due to temperature variation. Alternate embodiments
could set the reference temperature equal to any temperature, such as above the maximum
temperature, equal to the maximum temperature, somewhere between T
APCT and the maximum temperature, or below T
APCT without departing from the scope of the invention.
[0026] Raising the reference temperature has the advantage of reducing the range of printhead
substrate temperature variation and if the reference temperature equals the maximum
temperature, the printhead substrate temperature will not vary at all. But raising
the reference temperature places increased stress on the printhead substrate and the
ink and the likelihood of increased chemical interaction of the ink and the printhead
substrate. This results in decreased reliability of the printhead. Also, a printhead
substrate with a higher reference temperature will require more time for heating.
Another disadvantage of raising the reference temperature is that all inkjet printer
designs built to date have shown a higher chance of misfiring at higher printhead
substrate temperatures.
[0027] The printhead substrate is heated to the reference temperature only during the print
cycle. This has the advantage of keeping the printhead substrate at lower and less
destructive temperatures for longer. The temperature of the printhead substrate is
measured as it moves across the paper. If the substrate temperature is below the reference
temperature the printer will send either a printing pulse if the plot requires it
or a nonprinting pulse as described below.
[0028] Another aspect of the invention, is a darkness control knob 25, shown in FIG. 1,
that allows the user to change the reference temperature and thereby adjust the darkness
of the print or the time required for the ink to dry according to personal preference
or changes in the cartridge performance. Adjustments of the darkness control knob
25 can cause the reference temperature to exceed the maximum temperature.
[0029] The preferred embodiment of the invention heats the printhead substrate by using
packets of nonprinting pulses. The power delivered by these packets equals the number
of nozzles times the frequency of the nonprinting pulses (which can be much higher
than that of the printing pulses since no drops are ejected from the printhead) times
the energy in each nonprinting pulse. This power parameter is used to create the thermal
model shown in FIG. 2. The number of nozzles and the frequency of the nonprinting
pulses are constant and set by other aspects of the printhead design. Alternate embodiments
of the invention can vary the frequency of the nonprinting pulses and pulse some but
not all of the nozzles without departing from the scope of the invention.
[0030] In the preferred embodiment of the invention, the nonprinting pulses have the same
voltage as the printing pulses so that the various time constants in the circuit are
the same for printing pulses and non- printing pulses. The pulse width and energy
delivered by printing pulses are adjusted according to the characteristics of each
particular printhead. The width of nonprinting pulses is equal to or less than .48
times the width of the printing pulse so that it has little chance of ever ejecting
ink from the printhead.
[0031] By applying non-nucleating pulses to the heater elements during periods of inactivity
the substrate temperature can be controlled. The complexity of the control electronics
can be significantly reduced and printhead operation can be improved if the pulses
normally used to eject printing drops are reduced in width when used as heating pulses.
The print pulses can be extended to the pulse width required to eject a drop when
printing is required. By simple control of the pulse width of the non-nucleating pulses
the temperature of the substrate can be increased or lowered as required. Increasing
the pulse width increases the substrate temperature and decreasing the pulse width
lowers the substrate temperature.
[0032] Heating pulses synchronized with the printing pulses can be generated by combining
(OR-ing) the data for the heating pulses and the printing pulses in the Synchronizing
OR-ing Controller 30 during each firing cycle. At each firing period either the heating
pulse width, or the printing pulse width is applied. By Or-ing the data, the excess
heating of the substrate is only applied during the non-firing periods. This method
allows all elements of the printhead to be used for both printing and warming with
minimal additional electronics. By using all the elements to heat the substrate, a
more even temperature over the whole substrate is achieved. FIG. 4 shows an example
of the printing and heating pulses for a particular firing chamber resistor 38. The
first row shows the heating pulses and the heating pulse width to be sent to the firing
chamber resistor 38. The second row shows the printing pulses and the printing pulse
width to be sent to the firing chamber resistor 38. The third row shows the printing
pulses and heating pulses to be sent to the firing chamber resistor 38 as a result
of the OR-ing process.
[0033] In summary, the preferred embodiment uses a thermal model of the printhead substrate,
having inputs of the reference temperature, the cartridge temperature, and the printhead
substrate temperature, that calculates how long the firing chamber resistors 38 of
the printhead substrate 40 should be driven with packets of nonprinting pulses of
a specified power, to the printhead substrate during printing swaths to raise the
printhead substrate temperature to the reference temperature.
[0034] All publications and patent applications cited in the specification are herein incorporated
by reference as if each publication or patent application were specifically and individually
indicated to be incorporated by reference.
[0035] The foregoing description of the preferred embodiment of the present invention has
been presented for the purposes of illustration and description. It is not intended
to be exhaustive nor to limit the invention to the precise form disclosed. Obviously
many modifications and variations are possible in light of the above teachings. The
embodiments were chosen in order to best explain the best mode of the invention. Thus,
it is intended that the scope of the invention be defined by the claims appended hereto.
1. A method for controlling print quality in an inkjet printer that includes a printhead
having a printhead substrate and ink firing resistors (38) disposed on the printhead
substrate, comprising the steps of:
selecting a reference temperature;
measuring a temperature of the printhead substrate;
comparing the printhead substrate temperature with the reference temperature; and
heating the printhead substrate to the reference temperature periodically by delivering
synchronized heating pulses and printing pulses to the ink firing resistors during
selected print firing periods wherein either a heating pulse or a print pulse, but
not both, occurs during a selected print firing period.
2. The method of Claim 1 wherein the step of heating includes the steps of:
generating a heating signal that includes heating pulses;
generating a print signal that contains printing pulses; and
combining the heating signal and the print signal to produce the synchronized heating
pulses and printing pulses.
3. An inkjet printer comprising:
a ink jet printhead including a substrate and ink firing resistors (38) formed on
said substrate;
a printhead substrate temperature sensor (22);
means (26) for generating a reference temperature;
pulse generating means (28, 30, 32, 34) responsive to said printhead temperature sensor
and said reference temperature generating means for driving each of said ink firing
resistors with a print signal during a plurality of print periods, said print signal
containing during each of selected print periods a single pulse that is either an
ink firing print pulse or a non-firing heating pulse.
4. The ink jet printer of Claim 3 wherein said pulse generating means comprises:
means (32) for generating a heating signal that includes non-firing heating pulses;
means (34) for generating a print signal that includes print pulses; and
means (30, 32) for combining said heating signal and said print signal to produce
said pulse signal.