Method and apparatus for prediction of inkjet printhead lifetime

It has been discovered that inkjet printhead lifetime is related to an amount of accumulated air within the inkjet printhead. The invention, therefore, comprises a method of: determining an amount of ink that is output by an inkjet printhead during a determined period; using the amount of ink so determined to derive an update air accumulation value that is indicative of an amount of air which has accumulated during the determined period within the inkjet printhead; and updating a stored air accumulation parameter in accord with the air accumulation update value. The stored air accumulation parameter is thus related to a projected remaining lifetime of the inkjet printhead. A preferred embodiment stores the air accumulation parameter directly on a memory that is integral with the inkjet printhead. The parameter can further be stored on a memory that is resident on an ink container employed in the inkjet printer.

FIELD OF THE INVENTION 
This invention relates to inkjet printers and, more particularly, to a 
method and apparatus for enabling assessment of remaining lifetime of an 
inkjet printhead. 
BACKGROUND OF THE INVENTION 
Presently, inkjet printers employ two different kinds of inkjet printheads: 
those which include an integral ink supply and are typically thrown away 
when the ink supply is exhausted; and those wherein the printhead is 
connectable to a replaceable container, enabling longer usage of the 
printhead. In the former type of disposable printhead, typically the 
printhead is thrown away prior to an occurrence of any printhead failure 
mechanism. With respect to the latter or "semi-permanent" category of 
printheads, a number of known failure modes have been experienced. 
In printheads which employ heater resistors to cause ejection of droplets 
of ink, resistor burnout has been a problem. However, redesign of resistor 
structures and modification of resistor materials has largely eliminated 
the problem. A further failure mechanism is a buildup of scum within the 
ink chamber, juxtaposed to the heater resistor. Changes in ink composition 
are able to largely overcome this problem. 
The prior art has suggested that inkjet printheads incorporate a parameter 
memory for storage of operating parameters to be used by the inkjet 
printer. Such parameters include: drop generator driver frequencies, ink 
pressure and drop charging values. Such a printhead is described in 
"Storage of Operating Parameters in Memory Integral with Printhead", 
Lonis, Xerox Disclosure Journal, Volume 8, No. 6, November/December 1983, 
page 503. Other patents have suggested that an ink-containing replaceable 
cartridge can be provided with an integral memory for storage of 
information relating to control parameters for a connected inkjet printer. 
For instance, U.S. Pat. No. 5,138,344 to Ujita stores information on a 
replaceable ink cartridge which relates to control parameters for the 
printer. U.S. Pat. No. 5,365,312 to Hillmann et al. describes the use of a 
memory device integral with an ink reservoir for storage of ink 
consumption data. European patent EP 0 720 916 describes an ink reservoir 
which includes a memory for storage of data regarding the identity of the 
ink supply and its fill level. 
It is an object of this invention to provide a replaceable cartridge for 
use in an ink jet apparatus (i.e. a printer, copier, plotter and the 
like), which cartridge includes memory with data that enables a projection 
to be made of further remaining printhead lifetime. 
It is another object of this invention to provide an improved method for 
determining printhead lifetime. 
SUMMARY OF THE INVENTION 
It has been discovered that inkjet printhead lifetime is related to an 
amount of accumulated air within the inkjet printhead. The invention, 
therefore, comprises a method of: determining an amount of ink that is 
output by an inkjet printhead during a determined period; using the amount 
of ink so determined to derive an update air accumulation value that is 
indicative of an amount of air which has accumulated during the determined 
period within the inkjet printhead; and updating a stored air accumulation 
parameter in accord with the air accumulation update value. The stored air 
accumulation parameter is thus related to a projected remaining lifetime 
of the inkjet printhead. A preferred embodiment stores the air 
accumulation parameter directly on a memory that is integral with the 
inkjet printhead. The parameter can further be stored on a memory that is 
resident on an ink container employed in the inkjet printer.

DETAILED DESCRIPTION OF THE INVENTION 
It has recently been discovered that inkjet printhead failure can occur as 
a result of temperature-induced outgassing of air from ink passing through 
the printhead. This problem especially appears when inks are used that are 
adapted for "plain paper" and that further provide a high edge acuity in 
the printed characters. These inks tend to be mostly water-based. Water is 
known to have a relatively steep solubility curve, such as shown in FIG. 
1. There, changes of air solubility in water is plotted against 
temperature (degrees Centigrade), showing an exponential decrease in 
solubility with increases in temperature. It is clear from the curve of 
FIG. 1, that air solubility in water decreases rapidly as temperature is 
increased. 
Many ink jet printheads employ heater resistors to enable the ejection of 
ink droplets and, further, are often supplied with additional heating to 
assure constant performance characteristics over a wide range of 
temperatures. The additional heating is known as pulse-warming. The 
resulting increased temperatures tend to exacerbate the outgassing of air 
from ink passing through the inkjet printhead. 
If an inkjet printhead is used in a high use-rate environment, such as 
large format printing or high speed copiers, it has been determined that 
the problems arising from outgassing become more severe. In such 
applications, a printhead will tend to be semi-permanent. More 
specifically, multiple ink containers are used over the lifetime of the 
printhead to supply ink to the printhead. Thus, over a printhead's 
lifetime, multiple liters of ink will pass through the printhead, thereby 
enabling substantial air accumulation to occur within the printhead 
structure. 
Referring to FIG. 2, a sectional view of a printhead, with some internal 
parts missing, is illustrated. Inkjet printhead 10 employs a hollow needle 
12 that mates with an inlet conduit from an ink supply cartridge (not 
shown). The ink travels up hollow needle 12, through channel 14 and down 
to a valve 16. Valve 16 is normally closed, but will open in response to a 
vacuum condition within upper ink chamber 18, thereby enabling an inflow 
of ink thereinto. Ink flows from upper ink chamber 18, through a filter 
element 20, into lower ink chamber 22, and thence into ink pen element 24 
(shown in phantom). Further description of the structure of printhead 10 
and ink pen element 24 can be found in U.S. Pat. No. 5,278,584, the 
disclosure of which is incorporated herein by reference. 
It has been found that air accumulates within lower ink chamber 22 both 
above and below filter element 20. If air accumulates to a sufficient 
degree below filter element 20 (and in lower ink chamber 22), the print 
pen 24 becomes starved for ink, as the accumulated air blocks the path of 
ink flow. If air accumulates to an even greater extent, both above and 
below filter element 20, temperature excursions may cause an expansion of 
the air and create a pressure situation within printhead 10 which will 
cause a "drooling" of ink from ink pen element 24. Such drooling can 
result in printer damage. 
It has been assumed that keeping track of the number of ink droplets 
ejected from printhead 10 would be sufficient to enable a calculation of 
the amount of outgassed air from ink passing through printhead 10. Such a 
value would enable a signalling of when the accumulated air had reached a 
critical level. It has been found, however, that a calculation of air 
outgassed derived from a count of ink drops fired (and a conversion of the 
count to an ink volume value) provides a less than satisfactory indication 
of air accumulation. In this regard, it has been found that a residence 
time of ink within printhead 10 has a significant effect on the outgassing 
value. This is as a result of the fact that the longer ink is resident 
within printhead 10, the longer the ink is subjected to an elevated 
temperature, as a result of heat applied to pen element 24, and the more 
outgassing occurs as a result of that exposure. 
The effect of residence time can be explained further as follows. Ink that 
flows into the lower ink chamber 22 and is finally ejected through 
ejection elements 24 has a certain amount of dissolved air. With a 
convection mechanism, the ejection elements 24 warm the ink as it enters 
lower chamber 22. Because the solubility of air in the ink decreases as 
the ink is warmed, the ink can become supersaturated as it approaches 
ejection elements 24. This supersaturation causes air to diffuse into 
bubbles in lower ink chamber 22 and to a lesser extent into bubbles in 
upper chamber 18. As is well known, the total mass diffused across an 
interface (in this case from ink to a bubble) increases with the initial 
concentration gradient (affected by the temperature) and time. In the 
limit as the residence time gets sufficiently large, air diffusion will 
take place until the ink in lower ink chamber 22 is no longer 
supersaturated--i,e, . all of the "excess" air will have diffused into the 
bubbles in ink chamber 22. On the other hand, as the residence time gets 
short, there is very little time for diffusion and hence less total air 
diffuses out of the ink (per unit volume of ejected ink). 
The residence time of ink within printhead 10 is directly related to the 
print density produced by printhead 10 during the course of a print 
action. For instance, a graphics print job and a text print job may result 
in considerably different residence times of ink within printhead 10. 
Thus, a particular user's use pattern will have a major influence on how 
much ink can be delivered through a printhead before that printhead 
experiences a level of air accumulation which can cause a failure of the 
printhead. 
Referring to FIG. 3, the phenomena of air accumulation, with changes in 
print density will become more apparent. Shown is the outgas rate plotted 
against print density for an exemplary printhead structure. (It is to be 
understood that the indicated outgas relationship will change according to 
printhead design, ink type, pulsewarming algorithm, etc.) The vertical 
axis indicates the outgas rate in cubic centimeters of air outgassed into 
lower ink chamber 22 per liter of ink that is ejected by ejection elements 
24. The horizontal axis indicates the area coverage, where 100% indicates 
a "blackout" area fill (a drop ejected at every dot matrix location) and 
lower percentages indicating the fraction of area coverage. 
Note that as the print density decreases, the amount of air accumulated 
within printhead 10, per liter of ink expelled onto media, substantially 
increases. This can be understood by realizing that when a printhead 
prints at a low print density, less ink is utilized by the printhead, 
thereby leading to a longer residence time of the ink within the printhead 
and a greater opportunity for outgassing of air therefrom. Thus, as print 
density increases, residence time of the ink within the printhead lessens 
and the opportunity for air outgassing likewise decreases. 
Prior to describing the method of the invention, reference should be made 
to FIG. 4 which is a perspective view of an inkjet printer 31 which 
incorporates the invention. A tray 32 holds a supply of input paper or 
other print media. When a printing operation is initiated, a sheet of 
paper is fed into printer 31 and is then brought around in a U-direction 
towards an output tray 33. The sheet is stopped in a print zone 34, and a 
scanning cartridge 35, containing plural removal color printheads 36 is 
scanned across the sheet for printing of a swath of ink thereon. The 
process repeats until the entire sheet has been printed, at which point it 
is ejected into output tray 33. 
Printheads 36 are respectively, fluidically coupled to four removable ink 
cartridges 37 holding, for example, cyan, magenta, yellow and black inks, 
respectively. Since black ink tends to be depleted most rapidly, the black 
ink cartridge has a larger capacity than the other ink cartridges. As will 
be understood from the description which follows, each printhead and ink 
cartridge is provided with an integral memory device which stores data 
that is used by printer 31 to control its printing operations and to 
enable a printhead lifetime value to be calculated and stored. 
In FIG. 5, a schematic view of elements of inkjet printer 31 shows host 
processor 40 connected thereto. Host processor 40 connected thereto. Host 
processor 40 provides both control and data signals for inkjet printer 31 
and is adapted, in the known manner, to receive a memory media cassette 42 
which includes operating program data for control of inkjet printer 31. A 
replaceable ink cartridge 44 includes a reservoir 45 which holds a supply 
of ink, a fluidic coupler 46 and an electrical connector 48, both of which 
couple to mating connectors within ink jet printer 31 upon installation of 
ink cartridge 44. A memory chip 49, installed on ink cartridge 44 is 
coupled to connector 48 and upon insertion of ink cartridge 44, is 
electrically coupled to a microprocessor within inkjet printer 31. 
A printhead 50 also includes a fluid coupler region 52, a resident memory 
54 and an electrical connector 56 which connects to memory 54. Other sense 
and control devices are present within printhead 50, such as heater 
resistors for causing ejection of ink droplets from pen segment 58. 
FIG. 6 illustrates inner connections within inkjet printer 31 between a 
microprocessor 60, which controls the operation of inkjet printer 31, ink 
cartridge 44 and printhead 50. An ink flow path 62 provides a flow path 
between ink cartridge 44 and printhead 50. 
Memory chip 54 on printhead 50 includes a variety of parameters recorded 
therein, one of which, preferably, is an air accumulation parameter that 
is indicative of an amount of air accumulated within printhead 50. Memory 
54 can also include a variety of other parameters, one of which is a value 
which enables droplet volume to be determined by microprocessor 60. 
Turning to FIG. 7, a logic flow diagram is shown which illustrates the 
procedure employed to determine air accumulation update values for the air 
accumulation parameter stored in printhead memory 54. Initially (box 100), 
ink cartridge memory 49 is accessed and a parameter indicative of the 
slope of the air solubility curve for the ink in ink cartridge 44 is read. 
Printhead memory 54 is then read and the following parameters are read: a 
drop volume parameter; and certain constants (a, b and c) that will be 
used in calculating an outgas rate for the ink as it passes through 
printhead 10 (box 102). 
During operation of printhead 10 in printing a swath, the following data is 
accumulated: a count of fired ink droplets and a measure of the average 
temperature of a die within printhead 10 (box 104). A print density (Pd) 
value is then calculated by microprocessor 60. The Pd value is a value 
which varies between zero and one. For a full black swath, the Pd value is 
set at one, and for a full white swath, the print density is set to zero. 
The Pd value can be calculated by knowing that approximately 1 cubic 
centimeter of ink provides a 100% print density on a normal 8-1/2.times.11 
paper sheet. Thus, by knowing the number of ink droplets fired after the 
printing of a swath, the volume of ink emitted can be calculated, 
utilizing a drop volume parameter from printhead memory 54. Based upon the 
ratio of the calculated volume of ink placed on a swath page vs. the 
amount of ink required to produce a 100% print density swath, a value 
between zero and one is determined that is indicative of the respective 
swath's print density. 
Concurrent with the calculation of print density, the die temperature (T) 
is accessed and, utilizing the air solubility slope parameter value and 
constants a, b and c from printhead memory 54, an outgas rate is 
calculated (box 106) using the following relation: 
##EQU1## 
The above relationship is used to calculate the outgassing rate to enable 
an amount of air outgassed to be calculated. Constant a is an overall 
constant of proportionality that takes into account unit conversions. 
"Slope" is an approximate slope of the solubility curve in the temperature 
range of interest. Although the solubility curve shown in FIG. 1 is not 
linear, an approximate slope value can be used, between T.sub.amb (ambient 
temperature of roughly 25.degree. C.) and the operating temperature 
(typically roughly 50.degree. C.). Note that a particular ink will have 
its own curve that is similar to FIG. 1; however, many inks tend to have 
curves that are not as steep over the temperature range of interest. 
Constant b is approximately 1, but may be adjusted to help take into 
account the solubility curve non-linearity. 
Constant c is used to match the flow rate of ink to the shape of an 
empirical curve as shown in FIG. 3. To take into account the effect 
illustrated in FIG. 3, the outgas rate has a denominator that is 
proportional to the flow rate of ink through the printhead, raised to a 
power c (an empirical constant). 
Thereafter, the air outgassed is calculated (box 108) in accordance with 
the expression: 
EQU Air amount=outgas rate.times.no. of droplets.times.droplet volume 
The resultant number is the amount of air in cc's that is has outgassed 
from the ink (assuming the outgas rate is in cc's per liter and the 
droplet volume is in liters). This calculation may be done on a per swath, 
per portion of a page or full page basis, or for some total number of 
dots, depending on what is best for a particular printing system 
controller. 
Thereafter, using the calculated air outgassed amount, a stored air 
accumulation value is updated (box 110) and the updated air accumulation 
value is compared to a pre-set threshold (decision box 112). If the air 
accumulation value is less than the threshold value, the procedure 
recycles. If the air accumulation value equals or exceeds the threshold 
value, microprocessor 60 provides a printhead lifetime warning to the user 
(box 114) indicating an imminent requirement to change the printhead. 
As an alternative, the updated air accumulation value may be compared to 
plural threshold values, with a lower threshold value being utilized to 
provide a warning to the user and a higher or last threshold value being 
causing a disabling of further printing until the printhead is changed. 
Accordingly, the invention enables a printhead lifetime parameter to be 
accumulated, based upon usage and ink residence time within the printhead. 
Further, by recording the air accumulation value directly on the 
printhead, if the user transfers the printhead from one printer to 
another, the lifetime procedure does not change, as the air accumulation 
value is continually updated as a result of the procedure shown in FIG. 7. 
Further, the air accumulation parameter can be stored on the memory that 
is resident on the ink cartridge. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications can be devised by 
those skilled in the art without departing from the invention. 
Accordingly, the present invention is intended to embrace all such 
alternatives, modifications and variances which fall within the scope of 
the appended claims.