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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to thermal ink jet printers, and, more particularly,
to control of the temperature of the print head ejectors of such printers
during printing operations.
Printers are devices that print characters onto a printing medium such as a
sheet of paper or a polyester film. Printers of many types are available,
and are commonly controlled by a computer that supplies the images, in the
form of text or figures, that are to be printed.
Some printers used a colored liquid, such as an ink or a dye, but generally
termed herein a colorant, to form the images on the printing medium. (By
contrast, other printers use a dry toner to form the image.) Such printers
deliver the colorant to the medium using a print head that creates the
proper patterning of colorant to record the image.
One important type of printer is the thermal ink jet printer, which forms
small droplets of colorant that are ejected toward the printing medium in
the pattern of dots. The droplets are formed when an electrical current is
passed through an electrical resistor in the ejector, vaporizing a small
volume of colorant. The vaporized colorant expands, driving a droplet of
colorant out of a nozzle to deposit as a dot on the printing medium. When
viewed at a distance, the collection of dots form the image, in much the
same manner that images are formed in newspapers. Ink jet printers are
fast, producing a high output of printed image, and quiet, because there
is no mechanical impact during formation of the image except for the
droplets of ink striking the printing medium.
Typically, a thermal ink jet printer has an ejector with a large number of
individual colorant ejection nozzles in a print head, with one resistor
for each nozzle, supported in a carriage and oriented in a facing, but
spaced-apart, relationship to the printing medium. The carriage and
supported print head traverse relative to locations on the surface of the
medium, with the nozzles ejecting droplets of colorant, at appropriate
times under command of the controller, to produce a swath of droplets. The
droplets strike the medium and then dry to form "dots" of color that, when
viewed together, form one swath of the permanently printed image. The
carriage is then moved an increment in the direction normal to the
traverse (or, alternatively, the printing medium is advanced), and the
carriage again traverses the page with the print head droplet ejector
operating to deposit another swath of dots. In this manner, the entire
pattern of dots that form the image is progressively deposited by the
print head during a number of traverses of the page. To achieve the
maximum output rate, the printing is preferably bidirectional, with the
print head ejecting colorant during traverses from left-to-right and
right-to-left.
One of the key operating parameters of the print head and ejector is its
temperature of operation. Thermal energy is generated with each operation
of an ejection resistor. Some of the energy leaves the printer in the
ejected droplet, but some remains in the print head to heat it. The print
head is constantly cooled by conduction to the surrounding air. The actual
temperature of the print head is the result of a balancing of heating and
cooling of the print head.
A typical thermal ink jet printer has specified minimum and maximum
operating temperatures of the ejector, that define its operating range. If
the operating temperature is less than the minimum, the ejection resistors
cannot impart enough energy to each droplet to achieve proper ejection. If
the operating temperature is greater than the maximum, there may be
spurious ejection, irregularities in the ejected droplets, and choking of
the nozzles as gas dissolved in the ink leaves solution to form bubbles in
the ink flow channels.
These minimum and maximum values are temperatures measured at the ejector
of the print head, and do not correspond directly to the air temperature
where the printer is operated. However, the air temperature plays a part
in determining whether the printer can stay within the specified
temperature range. That is, a cold air temperature tends to cause the
ejector to be nearer the low end of its range, and a warm air temperature
tends to cause the ejector to be nearer the high end of its range. To be a
viable commercial product, the thermal ink jet printer must be able to
operate over a range of air temperatures, and still maintain the ejector
temperature within the acceptable range.
It is known to use heaters and fan coolers within the printer, operating
under control of a temperature sensor, to assist in maintaining the
temperature of the ejector within the proper operating range. See, for
example, U.S. Pat. No. 4,704,620, which emphasizes that the temperature
control of the print heads must be carefully controlled, and provides a
method for ensuring that the heaters will not overheat the print head and
that the fans will not overcool the print head. The approach described
therein utilizes a calculation of the heat transfer coefficients of the
heaters and the fan in an attempt to keep the heat flowing into or out of
the ejector within preconceived limits that will result in maintenance of
the temperature range. The ejector itself is small and has very low
thermal mass, and therefore careful attention is required to avoid
overheating or overcooling. The use of heaters and a fan encourages
increasing the thermal mass to avoid temperature swings through and out of
the acceptable operating range, but the general principles of print head
design call for reduced mass that must be supported on and moved by the
carriage.
Although the system described in the `620 patent and available in the art
is presumably operable, there is a need for an improved thermal control
system for a thermal ink jet printer. Such a control system would
preferably not use a fan to cool the ejector, since this component adds
cost and weight to the printer, and increases the chances of a breakdown.
The control system would also preferably achieve more precise temperature
control than possible using heaters and a fan, without increasing the
thermal mass of the ejector. The present invention fulfills this need, and
further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a thermal ink jet printer, print head, and
thermal control methodology that achieve excellent control of the
temperature of the ejector without the need for a fan. Thermal control is
responsive to the actual printing demands of the printer and heat loading
imposed thereby, and to temperature measurements at the ejectors, and not
just to a preconceived heat loading pattern. The thermal control system of
the invention does not add significantly to the cost or weight of a
printer having no thermal control system, and is less costly than prior
thermal control systems requiring separate heaters and fans.
In accordance with the invention, a thermal ink jet printer comprises print
head means for ejecting droplets of colorant, the print head means
including an ejection heater that heats the colorant; means for supporting
the print head means; means for sensing the temperature of the print head
means; means for predicting the extent of future operation required of the
ejection heater; and means for establishing the temperature of the print
head responsive to the means for predicting.
More specifically, a thermal ink jet printer comprises a print head having
a plurality of ejection nozzles from which droplets of colorant may be
ejected, and a thin film electrical resistance heater associated with each
nozzle, the resistance heaters being deposited upon a substrate; a
carriage that supports the print head and traverses it across a printing
medium in a series of passes; a thin film temperature sensor deposited
upon the same substrate as the thin film resistance heaters of the print
head; means for predicting the heat loading of the ejector during a pass,
prior to the initiation of the pass; and means for controlling the
temperature of the print head responsive to the means for predicting.
The invention also extends to a process for maintaining the ejector
temperature within an acceptable range. In accordance with this aspect of
the invention, a process for controlling the temperature of the ejector
portion of the print head of a thermal ink jet printer comprises the steps
of sensing the temperature of the ejector; predicting the future
temperature of the ejector from the amount of printing to be accomplished
by the ejector during a future period; and controlling the temperature of
the ejector responsive to the prediction of future temperature so that the
actual temperature of the ejector is maintained within an acceptable
operating range.
An important feature of the present invention is the thin film temperature
measurement resistor that is deposited upon the ejector substrate, and
provides an accurate, current measurement of the temperature at the
substrate and the ejector. In accordance with this aspect of the
invention, a thermal ink jet printer comprises a print head having a
plurality of ejection nozzles from which droplets of colorant may be
ejected, and a thin film electrical resistance heater associated with each
nozzle, the resistance heaters being deposited upon a substrate; a
carriage that supports the print head and traverses it across a printing
medium in a series of passes; and a thin film temperature sensor deposited
upon the same substrate as the thin film resistance heaters of the print
head. Since the print head is often provided as a separable unit that is
replaced as necessary, the print head itself utilizing the thin film
temperature sensor is novel. In accordance with this aspect of the
invention, a thermal ink jet print head comprises colorant ejector means
including a plurality of thin film resistors deposited upon a substrate;
and thin film sensing means for sensing the temperature of the colorant
ejector means, the thin film sensing means being deposited upon the same
substrate as the thin film resistors.
The temperature of the ejector of a thermal ink jet printer print head is
determined by a balancing of heat flows in and out of the ejector. Heat
flows into the ejector when the ejection resistors are operated during the
printing operation to eject droplets of colorant that are then ejected
toward the printing medium, and from the general thermal transfer from the
environment if it is warmer than the ejector. Heat flows out of the print
head with the droplets of ejected colorant, and by radiation and
conduction if the ejector is warmer than its environment. Intentional
heating and cooling, if any, also influence the temperature of the
ejector.
Accurate temperature control is not readily achieved unless these various
components of heat transfer are considered. However, the various heat
flows are not easily modeled by any preestablished set of criteria,
because a major influencing factor is the rate of production of heat and
loss of heat by the ejector itself, which is in turn determined by the
amount of printing that is performed. Where there is much printing (that
is, many droplets ejected over a short period of time), for example, the
heat loading into the ejector is high, and the rate of heat transfer out
of the ejector through the droplets is also high.
The present invention therefore incorporates a temperature control system
that utilizes the current temperature of the ejector, measured very
accurately at a location on the ejector substrate, together with
predictions of heat flow during the immediate future period of time. Such
prediction is possible because of the mode of operation of ink jet
printers, wherein the print to be deposited subsequently is decomposed
into a droplet pattern that may be analyzed to determine the future
droplet demand and thence heat loading.
The thermal control of the print head ejector involves several
considerations. The temperature of the print head may be measured at
different locations, and in different manners. Prior approaches measure
the print head temperature at locations remote from the ejector, an
approach that is unacceptable where precise information and control are
required. The temperature at the ejector may differ from that in other
portions of the print head, because the temperature of the ejector can
vary over a range in a short time due to printing demands and because of
the relatively low thermal conductivity of the remainder of the print
head. The present approach preferably utilizes the thin film temperature
measurement resistor deposited directly upon the same substrate upon which
the ejection resistors are deposited. The substrate, which is normally
silicon, has a high thermal diffusivity, so that the temperature measured
by the thin film resistor is very nearly that at the ejector nozzles.
Moreover, the thin film resistor and its associated circuitry can be
deposited upon the substrate at the same time that the ejector resistors
and related circuitry are deposited, so that there is virtually no
additional cost in providing the temperature measurement resistor.
Another key consideration in controlling the temperature of the ejector is
the future demand for ejection of droplets. In the normal mode of
operation of a thermal ink jet printer, the print head is moved
back-and-forth across the face of a printing medium in a series of passes
by the carriage, while the printing medium is moved in a perpendicular
direction relative to the print head between passes. On each pass, the
print head ejector prints a pattern of dots, termed a swath. In this
manner, the entire image is built up as a series of swaths deposited
side-by-side on the printing medium.
The pattern of dots to be printed during any swath is determined by the
driving computer or by a microcomputer built into the printer itself. That
is, prior to the commencement of a swath, the pattern of dots, and thence
the number of droplets required to form the swath, is calculated and
known. The number of droplets to be deposited during the swath is the
printing demand for the ejector during that swath. If the number of
droplets or demand is high, then there will be a predictably large amount
of heating of the ejector by ejection pulses to the ejection resistors
during the swath. Conversely, if the number of droplets or demand is low,
then there will be a predictably small amount of heating during the swath.
From this information, and an accurate measurement of the temperature of
the ejector at the commencement of the swath, the final temperature of the
ejector under various printing patterns can be predicted. If a normal
printing mode causes the predicted temperature at the end of the swath to
exceed an acceptable maximum limit, then the start of the printing of that
swath can be delayed or the printing mode can be modified. If the
temperature at the start of the swath is below the minimum permissible
temperature, then the ejector can be heated before commencement of the
swath by passing a small current through the ejection resistors. This
heating current is calculated to be too small to cause ejection of
colorant, but large enough to heat the ejector to at least the minimum
acceptable operating temperature. Alternatively, a small current can be
passed through the sensing resistor itself to heat the ejectors, between
temperature measurements.
The precise method of predicting the temperature during any swath depends
upon the specific printer, and a preferred approach is presented
subsequently. However, in each case the future printing demand is utilized
to predict the temperature at the end of the swath and possibly at
intermediate points along the swath, to be certain that the temperature
does not stray from the permitted range. By utilizing a swath by swath
approach, the heating or cooling demand for a reasonable period of time
into the future is predicted, obviating the need for highly precise
thermal models. That is, on a swath by swath basis, relatively simple
linear thermal models may be used and are normally acceptable. However,
more complex models reflecting an advanced understanding of the effects of
thermal inputs and losses may also be utilized.
The present invention provides an important advance in the thermal control
of ink jet printers, that improves the performance of the printers. A
predictive process provides the predicted heat loading for a future period
of time, which is used in conjunction with the current temperature,
measured very accurately at the ejector, to determine the temperature of
the ejector in the future. The printing behavior can then be modified, if
necessary, to ensure that the temperature limits of operation are not
exceeded. Other features and advantages of the present invention will be
apparent from the following more detailed description of the preferred
embodiment, taken in conjunction with the accompanying drawings, which
illustrate, by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a thermal ink jet print head assembly;
FIG. 2 is a schematic side view of an ejector;
FIG. 3 is a plan view of a portion of an ink jet printer;
FIG. 4 is a side sectional view of the printer of FIG. 3, taken along lines
4--4;
FIG. 5 is a plan view of the electrical leads for the ejector resistors and
the thermal measurement resistor deposited upon the substrate; and
FIG. 6 is a flow chart illustrating the process for thermal control of the
ejector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The approach of the present invention is used in conjunction with a thermal
ink jet printer. A thermal ink jet printer utilizes as the basic print
head an assembly that creates and ejects microdroplets of ink by
vaporization of a small bubble of colorant. A thermal ink jet print head
assembly 10, used to eject droplets of colorant toward a print medium in a
precisely controlled manner, is illustrated in FIG. 1. Such a print head
assembly is discussed in more detail in U.S. Pat. No. 4,635,073whose
disclosure is incorporated by reference.
The print head assembly 10 includes an ejector 12 having a silicon
substrate 14 and a nozzle plate 16, depicted in FIG. 2. The nozzle plate
16 has a plurality of nozzles 18 therein. Droplets of colorant are ejected
from the individual nozzles 18. (As used herein, the term "colorant" means
generally a fluid that is deposited upon a printing medium to produce
images, including but not limited to inks and dyes, and is not restricted
to any narrow sense of that term as may be found in other arts.)
Droplets of colorant are ejected through the nozzles 18 by localized
heating of the silicon substrate 14 with a heater 20. To effect such
heating, the silicon substrate 14 has deposited thereon a plurality of
tantalum-aluminum alloy planar resistors 22 with gold leads 24, one of the
resistors being located adjacent each nozzle 18. An electrical current is
passed through the portion of the resistor 22 between the ends of the
leads 24 rapidly heating the resistor. A small volume of colorant adjacent
the resistor 22 is thereby rapidly heated and vaporized, causing some of
the colorant 26 in a reservoir 28 to be ejected through the nozzle 18 and
thereafter to be deposited as a dot 30 on a printing medium 32 (such as
paper or polyester). An optional passivation layer 34 overlies the
resistor 22, to p protect it from corrosion and cavitation damage by the
colorant.
Returning to FIG. 1, the ejector 12 is mounted in a recess 36 in the top of
a central raised portion 38 of a plastic or metal manifold 40. The raised
portion has slanted side walls 44, and end tabs 46 which facilitate its
handling and attachment to a carriage mechanism in the printer, to be
described subsequently.
External electrical connection to the leads 24 and thence to the resistors
22 is supplied through a set of traces 48 on the silicon substrate 14,
connected to a flexible interconnect circuit 50, which may be of the type
sometimes known as a TAB circuit. The circuit 50 fits against the side
walls 44, with one end extending to the traces 48 and the other end to
external connections to the controllable current source that supplies
current to the resistors 22. The general features, structure, and use of
such flexible interconnect circuits 50, and their fabrication, are
described in U.S. Pat. No. 3,689,991, whose disclosure is incorporated by
reference.
FIGS. 3 and 4 illustrate a portion of one type of ink jet printer 60, which
can utilize print heads of the type just discussed. The printer 60
includes platens 62 between which a sheet of the printing medium 32 is
captured. One or both of the platens 62 are rotatably driven by a stepping
motor 64 that causes them to controllably rotate in either direction.
Rotation of the platens 62 advances the printing medium in the selected
direction.
A carriage 66 is supported above the printing medium 32 on bearings 68 from
rails 70. The carriage 66 slides along the rails 70 under the control of a
traversing motor 71 acting through a wire or belt 72 that extends from the
motor 71 to the carriage 66. The direction of movement of the carriage 66
along the rails 70 is termed the "traversing direction", indicated by
numeral 73. The traversing direction 73 is perpendicular to the direction
of the advance of the printing medium through rotation of the platens 62,
termed the "paper advance direction" and indicated by numeral 74.
One or more of the print heads 10 is supported in the carriage 66, in a
generally facing but spaced apart relationship to the printing medium 32,
in the manner illustrated in FIGS. 2 and 4, so that ink droplets ejected
from the ejector 12 strike the printing medium. If the printer is only for
printing of single colors, then only one print head is required. Multiple
print heads are needed where a variety of colors are to be printed. The
present invention is applicable whether one or multiple print heads are
used, but is discussed herein in relation to a single print head for
simplicity. Where multiple print heads are used, then the most limiting
conditions must be considered in determining a printing strategy.
The print head 10 is mounted in a support 76 on the carriage 66. The
support 76 preferably includes a body 78 and an aperture 80 therethrough.
The print head 10 slides into the aperture 80 to rest against a shoulder
82. A retainer clip 84 holds the print head 10 in position within the
aperture 80 and against the shoulder 82. Plug-in electrical connectors 86
extend to the print head 10 from the control circuitry of the printer.
FIG. 5 presents an enlarged plan view of a detail of the substrate 14 with
traces 48 to the ink ejection resistors 22 shown thereon (and the nozzle
plate 16 removed). A thermal sensing resistor 94 is deposited upon the
same substrate 14, with measurement leads 96 extending thereto. The
resistor 94 is made of a material whose temperature coefficient of
resistance is sufficiently high that measurements of resistance can be
converted directly to a temperature value for the resistor 94. An
acceptable resistor material is aluminum or an aluminum-copper alloy with
less than about 5 percent by weight copper. Because the resistor 94 is
positioned directly adjacent the ejector 12 on a substrate of relatively
high thermal conductivity, its temperature provides a close approximation
to that of the ejector 12. For the same reason, the temperature of the
resistor 94 follows changes in the temperature of the ejector 12 quite
closely. The illustration of FIG. 5 depicts the presently preferred
approach wherein the resistor 94 is deposited as a single length or
resistance material at one end of the ejector. Alternatively, the resistor
94 may be deposited with portions in different locations around the
ejector, as on the sides and at both ends, to provide an even more
accurate measurement of the actual temperature in the neighborhood of the
nozzles 18. At the present time, the configuration of FIG. 5 has been
found satisfactory for temperature measurement and control. In any event,
the leads 96 to the resistor 94 are attached to the flexible interconnect
circuit 50 in the same manner as the traces 48, so that the temperature
can be measured externally.
To accomplish the measurement of temperature externally to the print head,
the four-wire measurement technique is preferably used, requiring that
there be four leads 96, two to each end of the resistor 94. A current is
passed through the resistor 94 using one pair of the leads 96, and the
voltage drop across the resistor 94 is measured with the second pair of
leads at the opposite ends of the resistor 94. The voltage drop and
current are converted to electrical resistance, which is a known function
of temperature and is stored in the computer as a formula or table.
FIG. 6 illustrates the presently preferred process for determining the
printing strategy that permits printing without exceeding the allowed
temperature range, on either the high end or the low end. From the image
to be printed 100, which is supplied by the computer, the dot pattern to
be deposited is calculated from well known algorithms. See, for example,
"Principles of Interactive Computer Graphics", by William M. Newman and
Robert F. Sproull, McGraw Hill, 1979, pages 213-243 and the "Hardware
Support Manual for Hewlett Packard 7600 Series Printers, For Models 240D
and 240E Electrostatic Plotters", Hewlett Packard Corp., 1988, at pages
5-1 to 5-4, both of which publications are incorporated by reference.
Those procedures are well known, and performed by existing ink jet
printers as a matter of course.
The printing demand is calculated, numeral 102, from the number of dots
required for the swath. It has been found convenient to define an area
fill fraction as the number of dots printed during a swath divided by the
total number of possible dots in a swath. The area full fraction provides
a direct indicator of the printing demand during the swath, which in turn
is used to predict heat loadings. The area fill fraction can be determined
as a function of position in a similar manner, so that the printing demand
as a function of position is known. This information would be particularly
useful where images appear on one side of the page, and large portions of
the other side of the page are blank, for example. However, at the present
time it has been found sufficient to determine the overall area fill
fraction during a pass, and work with only the beginning and ending
temperatures.
The current or beginning value of temperature T.sub.b is measured, numeral
104, prior to the initiation of the printing of the swath using the
thermal sensing resistor 94 and the measurement procedure previously
described.
The predicted temperature T.sub.f at the end of the swath is then
calculated, numeral 106, using the following formulation:
T.sub.f =T.sub.b +dT.sub.print +dT.sub.environment.
where dT.sub.print is the change in temperature due to the printing
demands, and dT.sub.environment is the change in temperature that would
normally occur due to heating or cooling of the print head as it is moved
through the ambient air.
dT.sup.print is determined from a table lookup or corresponding formula
expressing the relationship between printing demand and the heat flow
during printing. The ejector normally heats during printing. Heat flows
into the ejector in the form of electrical energy that is converted to
heat by the resistors 22. Some of that heat flows out of the ejector as
heated colorant and heated gas, during ejection of each droplet. The net
heat flow per droplet (the heat input less the heat lost per droplet) and
the increase in temperature of the ejector are calculated or measured, and
expressed as a function of the area fill fraction. For example, an
increase in the area fill fraction means that the total net heat retained
in the ejector will increase, and that the temperature of the ejector will
increase. The preferred approach is to establish a calibration table or
curve of dt.sub.print by direct measurement of print head operation as a
function of area fill fraction for the print head, and store that
calibration in the computer for use in finding dt.sub.print. Such
measurements are performed by the manufacturer prior to sale to the user,
so that the thermal control is not apparent to the user.
dT.sub.environment is similarly determined from a table lookup or
corresponding formula expressing the heat flow into or out of the ejector
as it moves through the ambient air. The temperature of the ambient air is
measured by a temperature resistor positioned well away from the ejector,
preferably on the frame of the printer, such as the resistor 95
illustrated in FIG. 3. The resistor 95 is used to sense ambient air
temperature using the same four-point measurement technique previously
described in relation to the resistor 94. For example, if the air
temperature is cool and the print head moves through it without any ink
ejection, the print head and ejector are expected to cool down. The value
of dT.sub.environment is ascertained from the table of calibration
measurements or a formula wherein the average coefficient of thermal
transfer is multiplied by the difference in temperature of the ejector and
the environment. Again, the preferred method for establishing this
relationship is measurements conducted by the printer manufacturer prior
to sale of the product to the user, so that the calibration procedures
need not be of concern to the user.
The three components of temperature are added according to the above
formula to predict the final temperature T.sub.f, numeral 106. The
beginning temperature T.sub.b and final temperature T.sub.f are then
compared to the permissible temperature range of operation, numeral 108,
and a printing strategy is determined, numeral 110. Normally, dt.sub.print
is positive and causes a temperature increase, and dT.sub.environment is
negative and causes a temperature decrease. Thus, a balancing of
temperature to within the acceptable range is achieved by an appropriate
strategy involving the printing rate, the time permitted for cooling
without printing, and heating pulses introduced, as required.
In the preferred approach wherein only the beginning and ending
temperatures are considered, there are five possible conditions of
operation, which are not mutually exclusive. In the first, both the
beginning and predicted final temperatures are within limits, and the
printing proceeds with no modifications to the printing cycle.
In the second, the beginning temperature is below the acceptable minimum
temperature. In that event, the computer commands the printer to send low
level electrical warming currents through the resistors 22 or 94 to warm
the ejector. The currents are too small to cause ejection of colorant, but
cumulatively warm the ejector to a temperature greater than the minimum
acceptable operating temperature.
In the third, the temperature of the ejector is initially too high. In that
event, the starting of the printing swath is delayed until natural cooling
of the ejector reduces its temperature to below the maximum permitted
temperature.
In the fourth, the predicted temperature of the ejector at the end of the
swath is too low. In that event, small electrical warming currents may be
passed through the resistors 22 during the pass and printing of the swath
at intermediate times when particular resistors 22 are not operating, or
through the resistor 94 when temperature measurements are not taken. As
described previously, the warming currents are too small to cause colorant
ejection, but are sufficient to warm the ejector so that it does not fall
below the minimum acceptable temperature.
In the fifth, the predicted temperature of the ejector at the end of the
swath is too high. In that event, printing of the swath is commenced but
an alternative printing strategy is used. Many different approaches are
possible to reduce the temperature rise during the swath resulting from
printing, and three exemplary strategies are listed. In one, where the
beginning temperature is near the high end of the range or perhaps
exceeding the maximum temperature, the initiation of printing is delayed
to permit cooling, so that both the beginning and final temperatures are
within the acceptable temperature range. In a second, printing of the
swath is initiated immediately but at a reduced rate of carriage movement
and droplet output, permitting environmental cooling to balance the heat
input from the printing demand. In a third, printing of the swath is
initiated immediately at the normal rate of carriage movement, but only a
fraction, typically half, of the dots are printed on the pass, and the
remaining dots are printed on the next pass without advancing the printing
medium. Of course, other and more complex printing strategies can be
envisioned.
As noted previously, more complex strategies regarding the temperature
distribution of the ejector at points along the swath can also be adopted,
but these consume processing and memory of the computer. At the present
time, the outlined approach of beginning and ending temperature
determinations has been found sufficient and is preferred.
As a further diagnostic aid in assessing the operation of the printer, the
predicted temperature at the end of the swath T.sub.f is compared with the
measured temperature at the beginning of the next swath. Or alternatively,
the ending temperature at the end of the swath is measured using the
resistor 94, and compared with the predicted ending temperature T.sub.f.
If the actual measured temperature is significantly greater than the
predicted temperature, a plugged nozzle or deprimed nozzle is indicated.
Such a problem causes a degraded printed image. That is, where no colorant
is ejected from a particular nozzle even though heating pulses are sent to
its ejection resistor 22, the temperature of the ejector rises much faster
than predicted by the model, because some of the heat produced by the
ejection resistors 22 is not being carried away from the ejector as in
normal operation. This information of an unexpectedly large temperature
rise can be used to indicate to other automated systems in the printer the
need to correct the problem, or to signal the user if the problem cannot
be corrected automatically by the printer.
The present invention provides a thermal control system and strategy that
permits the ejector of a thermal ink jet printer to be maintained within
acceptable operating limits without the need for a fan or other expensive
cooling device. Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various modifications
may be made without departing from the spirit and scope of the invention.
Accordingly, the invention is not to be limited except as by the appended
claims.
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