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Description  |
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BACKGROUND OF THE INVENTION
1. Object of the Invention
This invention relates to liquid ink recording devices. In particular, this
invention relates to controlling the print mode of thermal ink jet
printing device based on temperature of the printhead and density of the
printed image.
2. Description of Related Art
In liquid ink recording apparatuses, an image is formed on a substrate by
depositing wet ink on the substrate in a predetermined pattern. One type
of liquid ink printing apparatus is a thermal ink jet printer, which
utilizes a printhead having a plurality of aligned nozzles that eject ink
droplets onto the recording medium. Thermal ink jet devices are designed
to give the optimum ink dot size at room temperature. However, as the
ambient temperature increases, the ink dot size begins to grow causing
adjacent ink drops to overlap. Overlapping of still wet ink dots causes
image degradation problems such as bleeding and misting and creates an
image that is excessively bold. Further, at higher temperatures, the ink
jets tend to ingest air that causes intermittent firing of the jets, which
also affects the quality of the image In particular, misfiring leads to a
grainy appearance of the image within the solid fill regions. Therefore,
it is desirable to maintain a constant drop size by reducing the ink drop
size at elevated temperatures to obtain a clear and accurate image.
One method for reducing the drop size is to operate the ink jet printhead
in a checkerboard printing mode that utilizes two passes of the printhead
while ejecting the required dots in an alternating pattern for each swath
of printing. Under this mode for example, when printing left to right, the
jets fire in an alternating odd, even, odd etc. pattern and, when printing
right to left, the jets fire in an alternating even, odd, even etc.
pattern, thus firing every other jet for each pass of the printhead across
the printing medium. The benefits to using the checkerboard printing
include allowing an ink jet twice as long to refill since each jet is only
required to fire at every other dot column. Also, firing every other ink
jet in this manner cuts the ink supply demand through the cartridge in
half. The additional refill time and reduced ink supply demand reduces
misfirings. Further, since diagonally adjacent pixel areas are deposited
in the same pass, there is no overlap of ink dots from adjacent pixel
areas when the ink is still flowable. This prevents the dots from
blurring. An example of checkerboard dot deposition for liquid ink
printing is disclosed in U.S. Pat. No. 4,748,453 to Lin et al., which
employs a checkerboard printing mode based on the printing medium to
prevent blurring of the image when printed on the substrate having poor
ink absorptive properties.
Another reason for choosing a checkerboard printing mode is when the
density of the printed image is high thus requiring the deposition of
numerous closely spaced dots, which can result in blurring. An example of
using the Checkerboard printing mode based on image density is discussed
in U.S. Pat. No. 5,237,344 to Tasaki et al. To more accurately predict
when the use of checkerboard printing mode is appropriate, both the
density of the image and the estimated temperature of the printhead is
used in U.S. Pat. No. 4,653,940 to Katsukawa.
Another means for controlling drop size in a liquid ink recording apparatus
is to vary the frequency at which the ink droplets are deposited on the
substrate. In an ink jet printhead, the frequency can be varied by
reducing the ejection frequency of each ink droplet from the printhead or
by lowering the scanning speed of the recording head. Several devices that
vary the frequency of the ejection of droplets when temperatures are
elevated are disclosed in U.S. Pat. No. 5,300,968 to Hawkins, U.S. Pat.
No. 5,172,142 to Watanabe et al., and U.S. Pat. No. 5,166,699 to Yano et
al.
However, the above solutions to controlling the dot size require
complicated and expensive methods to select the appropriate printing mode.
None account for both the actual temperature of the printhead and the
density simply and inexpensively. For example, several of the above
methods controlling dot size involve selecting the printing mode based on
the substrate composition or based on certain environmental conditions,
such as estimated temperature or humidity. Other methods that control the
frequency of the droplet ejection rate are based solely on the density of
the printed image and do not account for the problems caused by elevated
temperatures. Therefore, there is a need to simply and inexpensively
control the dot size to maintain a high quality printed image.
SUMMARY OF THE INVENTION
An object of this invention is to simply and inexpensively control the ink
dot size during the formation of an image.
Another object of this invention is to ensure a high quality and accurate
reproduction of an image.
An additional object of this invention is to control dot size at elevated
temperatures of a printer and at different image densities.
The embodiments of this invention accomplish these objectives by providing
a method of controlling printing of an image with an ink jet printer based
on stored data of the image. The method comprises the steps of sensing an
internal temperature of the ink jet printer, determining density of the
stored image to be printed, and selecting a printing mode from one of a
single pass 100% coverage printing mode and a double pass checkerboard
printing mode based on the sensed temperature and the determined density.
The objectives of this invention are also accomplished by the embodiments
herein that provide a method of printing an image based on image data
using an ink jet printhead that comprises the steps of sensing an internal
temperature of the printhead, determining density of the image,
automatically setting the printhead droplet ejection rate based on the
sensed temperature and the determined density, and printing the image
using the set ejection rate.
This invention also accomplishes the above objectives with an ink jet
printer having a printhead and comprising a memory that stores print data
corresponding to an image to be printed, a temperature sensor that senses
an internal temperature of the printer adjacent the printhead, and a
density determiner that determines density of the image to be printed from
the stored print data. A controller, coupled to the memory, the
temperature sensor, and the density determiner, automatically selects one
of a single pass print mode and a double pass print mode and automatically
sets a printhead droplet ejection rate based on the sensed temperature and
the determined density. A printing mechanism is coupled to the controller
that prints the image based on the stored print data in the selected print
mode and the set printhead droplet ejection rate.
Using the methods and device of this invention, ink dot size can be
controlled by switching print modes based on ambient temperature. The
print mode can be varied by changing the printing frequency or by using
checkerboard printing. When the temperature rises above a predetermined
temperature, checkerboard printing mode is selected. Also, when a high
density image is to be printed at or below the predetermined temperature,
the droplet ejection rate is reduced. Thus, ink throughput is reduced for
elevated temperatures and for printing high density images merely by
changing printing modes, which requires no additional complexity and cost
to the device.
Other objects, advantages and salient features of the invention will become
apparent from the following detailed description, which taken in
conjunction with the annexed drawings discloses preferred embodiments of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings that form a part of this original disclosure:
FIG. 1 is a schematic view of the primary elements of a printer employing
this invention;
FIG. 2 is a flowchart depicting the method of selecting the printing mode
according to this invention;
FIG. 3 is a table showing examples of selected printing frequency and
printing modes at different densities and temperatures;
FIGS. 4A and 4B graphically depict an array of print data according to a
first embodiment for determining image density; and
FIG. 5 graphically depicts an array of print data according to the second
embodiment for determining image density.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
This invention is described as applied in the thermal ink jet printer
having a printhead. However, this invention may be employed in other
printing applications, such as plotters or facsimile machines.
FIG. 1 shows the primary components of a printing apparatus 10 that
includes a central processing unit (CPU) 12, a printing mechanism 14, and
a temperature sensor 16. CPU 12 includes a memory 18, a density determiner
20, and a print controller 22. CPU 12 is a microprocessor or similar
processing apparatus. CPU 12 also includes standard known printer control
systems and includes an interface for the operation panel. CPU 12 controls
various motors such as the sheet feeding motor and the carriage driving
motor. Memory 18 stores print data for an image to be printed and includes
a ROM memory for storing control programs and various data and a RAM
memory for temporarily storing various data such as the print data of the
image to be printed. Preferably, the print data is stored in an array of
ON and OFF pixels. Density determiner 20 is designed to determine the
density of the image to be printed from the stored print data in memory 18
as discussed in detail below. Print controller 22 controls printing
mechanism 14 based on the determined density and the temperature sensed by
temperature sensor 16.
Printing mechanism 14 is preferably a thermal ink jet printhead having a
plurality of aligned nozzles each activated by a resistor in a
conventional manner that causes an ink droplet to be ejected from the
nozzle. The printhead is supported by a carriage and oriented to face the
printing medium. The carriage and supported printhead traverse the
printing medium with the nozzles ejecting ink droplets or dots as directed
by the print controller. Each pass of the printhead prints a pattern of
dots known as a swath. Each swath, which represents one pass of the ink
jet printhead, includes a plurality of rasters, which represent one ink
jet moving across the swath. In the preferred embodiment of this
invention, the printhead is configured to have 128 vertically aligned ink
jets, which results in 128 rasters per swath.
Temperature sensor 16 is provided to measure the temperature inside the
printer, specifically the temperature in the vicinity of the printhead.
Any known temperature sensor can be used. The purpose of temperature
sensor 16 is to inexpensively determine an estimate of the printhead
temperature. Measuring the printhead temperature directly adds additional
costs such as additional printed circuit boards (PCB) on the carriage
assembly, additional wire in the carriage ribbon cable, and additional
connector lead at the carriage and at the main logic board PCB. The
inventor has found that simply measuring the ambient air temperature from
a thermistor mounted directly to the main PCB will yield a reasonable
estimate of the printhead temperature once a correction factor is
subtracted from the thermistor. For example, if the correction factor was
7.degree. C. and the thermistor measured 37.degree. C., the estimate for
the printhead temperature would be 30.degree. C.
In operation according to this invention, temperature sensor 16 senses the
temperature adjacent the printhead and selects either a single pass 100%
coverage print mode or double pass checkerboard print mode for printing as
discussed in detail below. The print mode is determined at the start of
each swath. The single pass 100% coverage print mode is a typical normal
print mode for an ink jet printer. In the single pass print mode, each
swath of printing is printed in one pass. Therefore, all of the intended
dots are deposited in a single pass based on the print data from the
controller. The double pass checkerboard print mode uses two passes for
each swath of printing. For example, when printing left to right, the jets
fire in an alternating odd, even, odd etc. pattern based on the print data
from the controller across the swath. Then, the printhead direction is
reversed from right to left, and the jets fire in an alternating even,
odd, even etc. pattern. Thus, adjacent dots are deposited in different
passes for each swath thereby preventing adjacent wet dots from smearing
and blending together. Checkerboard printing provides each ink jet twice
as long to refill since each jet is only required to fire on a single
pass. Further, firing every other jet in the checkerboard manner reduces
the ink supply demand through the cartridge to one half. Experimental
observation of ink jets firing in a checkerboard pattern indicates that
such a print mode can "fix" nonfiring jets by allowing them sufficient
time to refill and preventing the ingestion of air into the nozzle.
In addition to selecting the print mode based on temperature according to
this invention, the density of the image to be printed is determined, and
printing is controlled in response to that density. Density may be
determined using a variety of methods, such as the basic method of
counting pixels in a swath. However, it is preferable that the method of
determining the density accounts for clustering of pixels within a swath,
which results in areas of high ink concentration. Thus, the image density
according to the preferred embodiments of this invention is determined
using a method of scanning the image density in blocks and determining the
area of concentrated pixels.
FIG. 2 shows a flowchart of the steps used to select the printing mode and
ejection rate. As seen in FIG. 2, print data is first stored in step S1.
Then using temperature sensor 16, the actual temperature adjacent the
printhead is sensed in step S2. If the sensed temperature is higher than a
predetermined temperature (in this case, a normal ambient temperature of
about 30.degree. C.) a double pass checkerboard mode is selected in step
S3. For these higher temperatures, a standard droplet ejection rate is set
in step S4. Typically, this rate is 6.0 kHz. Then, printing mechanism 14
is instructed to print from print controller 22 based on the selected
printing mode and set droplet ejection rate in step S8. When the sensed
temperature is a normal ambient temperature or lower in step S2, the
single pass mode is selected in step S5. Then, the density is determined
in step S6. If the density is high, the standard droplet ejection rate is
set in step S4, and in step S8, the image is printed accordingly. However,
if the density is high in step S6, the droplet ejection rate is reduced
from the standard rate to a lower rate in step S7. For example, it would
be reduced from 6.0 kHz to 4.5 kHz. Then, the image is printed accordingly
in step S8. Thus, for high temperature and high density printing, the
output of the printhead is reduced to prevent the problems discussed above
that degrade image quality.
FIG. 3 shows a chart of typical selections of print mode and ejection rate
based on sensed temperature and density. When the temperature sensed is
higher than normal ambient temperature of about 30.degree. C., which would
normally cause the dot size to grow, a double pass checkerboard print mode
is automatically selected to reduce the throughput of ink in the
individual ink jets. This change of mode provides a simple and inexpensive
solution for printing at elevated temperatures requiring no additional
complex hardware and circuitry. When the temperature is normal, about
30.degree. C., or lower, the single pass 100% coverage print mode is
selected. Then, based on density, the ejection rate is set. When the
density is determined to be low, a standard droplet ejection rate, of 6.0
kHz for example, is selected. This applies to temperatures both above and
below normal ambient. When the density is determined to be high and the
sensed temperature is greater than a normal ambient temperature, the
standard droplet ejection rate is set. However, when the density is
determined to be high and the temperature is a normal ambient temperature
or lower, the droplet ejection rate is changed from the standard rate to a
reduced rate, for example 4.5 kHz.
In the above described embodiment, a threshold temperature of 30.degree. C.
is used and a standard droplet ejection rate of 6 kHz is used with a
reduced rate of 4.5 kHz. However, other threshold temperatures and other
appropriate droplet ejection rates may be employed.
The preferred method for determining the density of the image includes
filtering an array of data using successive blocks in the array to
determine a maximum number of ON pixels in a block. Basically, image
density is dependent on the maximum number of pixels that fill a given two
dimensional area within a swath. A swath represents one pass of printhead.
Each ink jet within a printhead across a swath produces a raster, which is
a line of printed data within a swath.
In the first embodiment for determining the image density, a filter
analyzes the print data on a raster by raster basis as shown in FIG. 4A.
Using the raster by raster filtering method to determine density, first, a
window is formed at the upper left edge of an array of print data, which
represents the top raster in a swath, as shown in FIG. 4A. According to
this embodiment, the window has a size of n.times.1. n may be any integer,
but, for illustrative purposes in this embodiment, n is preferably 48. For
purposes of simplicity however, n is shown in FIG. 4A as 5. First, the
n.times.1 window begins at the left edge of the top raster. The number of
ON pixels is counted. The window then moves to the right, as shown by the
dashed box in FIG. 4A. The window can be moved one pixel as shown or at
greater pixel intervals, such as eight pixel intervals. The number of 0N
pixels in this window is then counted. The process continues across the
array as shown in FIG. 4A until the window reaches the end of the raster.
The maximum number of ON pixels found in a window is recorded. The same
procedure is used for each of the remaining rasters. For example, in a
printhead having 128 vertically aligned ink jets that produces 128 rasters
per swath, 128 values representing the maximum fill of any n.times.1
window within each raster is recorded. These values are stored as a data
array as shown in FIG. 4B. For example, in an ink jet having an 128
vertically aligned jets, the data array of maximum numbers would be
1.times.128.
Next, a second window is formed at the top of the array of maximum numbers.
This window has a size of 1.times.m. Preferably, in this embodiment, m is
48. However, for illustrative purposes, in FIG. 4B, m is shown as 5. The
average for all the data within the second window is computed. Then, the
1.times.m window is moved down the array calculating averages within each
window as shown in FIG. 4B. The maximum average value is determined from
the set of calculated average values. The maximum average value is a
representation of the maximum image density for that swath.
According to a second embodiment of this invention to determine density,
the print data is analyzed in a column format, as shown in FIG. 5. In this
embodiment, a window is also formed at the top left edge of an array of
print data representing a swath. As shown in FIG. 5, this window has the
size of p.times.128, with 128 representing the number of vertically
aligned ink jets. The preferred value of p in this embodiment is 48.
However, for purposes of illustration, p is shown in FIG. 5 as 4. In
operation, if p is too small, it is difficult to discern between double
rows of small text versus one row of large text. It is undesirable to make
p substantially larger than 48. If p is much larger than 48, it becomes
much more difficult to discern between dispersed dot patterns and
clustering of dots in a confined region.
Using the second embodiment to determine density, the total number of ON
pixels within the window p.times.128 is counted. The window is then
incremented to the right and the total number of ON pixels is counted.
Preferably, the window is incremented at eight pixel intervals to decrease
the time required to determine density and to correspond to the recorded
bits of information. However, to increase resolution, the window can be
incremented one pixel at a time. The process continues across the swath
until the p.times.128 window reaches the right edge of the array. The
maximum number of ON pixels found in any of the windows is determined.
This value is a representation of the maximum density for that swath.
Although the above examples of determining density were described with
respect to a conventional data array read from left to right, the method
of determining the density can be employed in a data array that is read
right to left or from top to bottom and bottom to top.
While advantageous embodiments have been chosen to illustrate the
invention, it will be understood by those skilled in the art that various
changes and modifications can be made therein without departing from the
scope of the invention as defined in the appended claims.
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