EP0600648B1

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Publication number: EP0600648B1
Application number: EP19930309242
Authority: EP
Inventors: Brian Canfield; Clayton Holstun; King-Wah W. Yeung
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1992-11-30
Filing date: 1993-11-19
Publication date: 2001-10-24
Anticipated expiration: 2013-11-19
Also published as: EP0600648A2; JP3408302B2; DE69330991T2; DE69330991D1; SG47457A1; JPH06278291A; EP0600648A3

Description

Field of the Invention

This invention relates generally to the field of thermal ink jet printers and moreparticularly to controlling the temperature of thermal ink jet printheads.

Background of the Invention

Thermal ink jet printers have gained wide acceptance. These printers aredescribed by W.J. Lloyd and H.T. Taub in "Ink Jet Devices," Chapter 13 ofOutput 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 printquickly but quietly because only ink strikes the paper. The typical thermal inkjet printhead (i.e., the silicon substrate, structures built on the substrate, andconnections to the substrate) uses liquid ink (i.e., colorants dissolved ordispersed in a solvent). It has an array of precisely formed nozzles attachedto a printhead substrate that incorporates an array of firing chambers whichreceive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a thermal ink jet firing chamber resistor, located opposite the nozzleso ink can collect between it and the nozzle. When electric printing pulses heatthe thermal ink jet firing chamber resistor, a small portion of the ink next to itvaporizes and ejects a drop of ink from the printhead. Properly arrangednozzles form a dot matrix pattern. Properly sequencing the operation of eachnozzle causes characters or images to be printed upon the paper as theprinthead moves past the paper.
Drop volume variations result in degraded print quality and have prevented therealization of the full potential of thermal ink jet printers. Drop volumes varywith the printhead substrate temperature because the two properties thatcontrol it vary with printhead substrate temperature: the viscosity of the ink andthe amount of ink vaporized by a firing chamber resistor when driven with aprinting pulse. Drop volume variations commonly occur during printer startup,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 printercovers the page with dots).
Variations in drop volume degrades print quality by causing variations in thedarkness 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 primarycolor drops that create the printed color. If the printhead substrate temperatureincreases or decreases as the page is printed, the colors at the top of the pagecan differ from the colors at the bottom of the page. Reducing the range ofdrop volume variations will improve the quality of printed text, graphics, andimages.
Additional degradation in the print quality is cause by excessive amounts of inkin the larger drops. When at room temperature, a thermal ink jet printheadmust eject drops of sufficient size to form satisfactory printed dots. However, previously known printheads that meet this performance requirement, ejectdrops containing excessive amounts of ink when the printhead substrate iswarm. The excessive ink degraded the print by causing feathering of the inkdrops, bleeding of ink drops having different colors, and cockling and curlingof the paper. Reducing the range of drop volume variation would help eliminatethis problem.
EP-A-0 511 602 describes a system for reducing variations in the size of ink drops ejected from thermal printheads by heating the printhead using nonprinting pulses.

Summary of the Invention

For the reasons previously discussed, it would be advantageous to have anapparatus and a method for reducing the range of drop volume variation.
The foregoing and other advantages are provided by the present invention as defined in the claims.
The scope of the present invention includes heating the printhead substrateduring a print cycle (i.e., the interval beginning when a printer receives a printcommand and ending when it executes the last command of that data stream),as well as, heating it at anytime or heating it continuously. The scope of thepresent invention includes heating the printhead substrate by heating the entirecartridge (i.e., the printhead substrate, the housing, connections between theprinthead substrate and the ink supply, and the ink supply if it is attached to theprinthead substrate) by using a cartridge heater or heating the printheadsubstrate more directly by driving the firing chamber resistors with nonprintingpulses (i.e., pulses that do not have sufficient energy to cause the printhead to fire). The scope of the present invention includes using a thermal model toestimate the amount of heat to deliver to the printhead substrate to raise itstemperature to the reference temperature and delivering this energy betweenswaths to avoid slowing the printer output.
Another preferred aspect of the present invention varies the reference temperatureaccording to the print resolution. When a cartridge prints at lower resolution(i.e., skipping every other dot), the space between the printed dots increases.This preferred aspect of the present invention reduces this empty space by increasing the referencetemperature of the printhead substrate so that it produces larger dots. Afurther preferred aspect of the present invention is a darkness knob that allows the userto vary the reference temperature and thereby control the darkness of the printand the time required for it to dry. The present invention preferably includes atemperature sense resistor deposited around the firing chamber resistors of theprinthead substrate.
The present invention has the advantage of reducing the range of drop volumevariation and increasing the quality of the print. Other advantages of theinvention include a reduction in the average drop volume since a smaller dropvolume range allows the designer to set the average drop volume to a lowervalue, a reduction in the amount of ink that the paper must absorb, and morepages per unit ink volume whether the ink supply is onboard (i.e., physicallyattached to printhead substrate so that it moves with it) or offboard (i.e.,stationary ink supply).

Brief Description of the Drawings

Figure 1 is a block diagram of the present invention.
Figure 2 is a plot of the thermal model of the printhead substrate used by thepreferred embodiment of the invention.
Figure 3 is a block diagram of an alternate embodiment of the presentinvention.
Figure 4A is a histogram of the distribution of print-cycle temperatures that apopulation of printheads substrates without the present invention wouldexperience over a typical range of user plots.
Figure 4B is a histogram of the distribution of print-cycle temperatures that apopulation of printheads with the present invention would experience over thesame typical range of user plots where the reference temperature equals 40°C.
Figure 5A is a plot of the distribution of drop volumes for a printhead substratewithout the present invention.
Figure 5B is a plot of the distribution of drop volumes for a printhead substratemade according to the preferred embodiment of the invention.
Figure 6 shows the temperature sense resistor for the preferred embodimentof the present invention.
Figure 7A shows print having a resolution of 300x600 dots per inch and
Figure 7B shows print having a resolution of 300x300 dots per inch.
Figure 8 shows the effect of increasing the drop size when printing at aresolution of 300x300 dots per inch.

Detailed Description of the Invention

A person skilled in the art will readily appreciate the advantages and featuresof the disclosed invention after reading the following detailed description inconjunction with the drawings.
Drop volume varies with printhead substrate temperature. The presentinvention uses this principle to reduce the range of drop volume variation byheating the printhead substrate to a reference temperature before printingbegins and keeping it from falling below that temperature during printing. Thepreferred embodiment uses a thermal model of the printhead substrate toestimate how long to drive the printhead substrate at a particular power levelto raise its temperature to the reference temperature of the printhead substrate.
Figure 1 is a block diagram of the preferred embodiment of the presentinvention. It consists of a printheadsubstrate temperature sensor 22, alsoshown in Figure 6, a cartridge (i.e., the box that holds the ink and the printheadsubstrate) temperature (i.e., the air temperature inside the cartridge which is theambient temperature of the printhead substrate) sensor, and a referencetemperature generator. The outputs of these three devices are fed into athermal model processor/comparator which calculates how long to drive thefiring chamber resistors with nonprinting pulses having a known power. Thepreferred embodiment of the invention heats the printhead substrate onlybetween swaths so it has a printhead position sensor that detects when theprinthead is between swaths. The output of the thermal model and the outputof the printhead position sensor goes to a nonprinting pulse controller thatdetermines when the firing chamber resistors should be driven with nonprintingpulses. The output of the nonprinting pulse controller signals a pulse generatorwhen to drive the firing chamber resistors with one or more packets ofnonprinting pulses having the duration specified by the thermal modelprocessor/comparator.
Figure 2 is a plot of the thermal model of the printhead substrate. Theprinthead substrate has an exponential temperature rise described by:
T_{printheadsubstrate} -T_cartridge =A(1-exp^-^t /^τ). A and τ are constants of the system. The inputs to the thermal model include: the reference temperature, the cartridgetemperature (i.e., the temperature of the air inside the cartridge that surroundsthe printhead substrate), and the printhead substrate temperature. The outputparameter, Δt, shown in Figure 2 is the length of time the firing chamberresistors should be driven with a Power₁ to heat the printhead substrate to thereference temperature. The equation that defines this time is:t = τ(ln(TCartridge +A -TprintheadsubstrateTcartridge +A -Tref))
The advantage of the thermal model is that the printhead substrate reaches thereference temperature with reduced iterations of measuring the printheadsubstrate temperature and heating the printhead substrate. However, thethermal model is part of a closed-loop system and the system may use severaliterations of measuring and heating if needed.
Figure 4A is a histogram that represents the distribution of print-cycletemperatures that a population of printheads without the present inventionwould see over a typical range of user plots. The average print-cycletemperature of these printhead substrates without the invention is T_APCT andequals 40°C. The preferred embodiment of the invention sets the referencetemperature of a printhead substrate equal to T_APCT. This has the advantage ofeliminating half the temperature range and, thus, half the drop volume variationdue to temperature variation.
The preferred embodiment of the invention heats the printhead substrate to thereference temperature only during the print cycle. This has the advantage ofkeeping the printhead substrate at lower and less destructive temperatures forlonger. Additionally, the preferred embodiment of the invention heats theprinthead substrate only between swaths (i.e., passes of a printhead across thepage) to reduce the load on the processor and prevent a reduction in the printspeed. An alternate embodiment of the present invention heats the printheadsubstrate continuously. It measures the temperature of the printhead substrate as it moves across the paper. If it is below the reference temperature themachine will send either a printing pulse if the plot requires it or a nonprintingpulse. Alternate embodiments of the invention may heat the printhead substrateat anytime without departing from the scope of the invention.
The preferred embodiment of the invention heats the printhead substrate to thereference temperature by driving the firing chamber resistors with nonprintingpulses (i.e., pulses that heat the printhead substrate but are insufficient to causethe firing chamber resistors to eject drops). Alternate embodiments of theinvention can heat the printhead substrate in any manner (e.g., printing pulsesdriving any resistive element, a cartridge heater, etc.) without departing from thescope of the invention.
In summary, the preferred embodiment uses a thermal model of the printheadsubstrate, having inputs of the reference temperature, the cartridgetemperature, and the printhead substrate temperature, that calculates how longthe firing chamber resistors of the printhead substrate should be driven withpackets of nonprinting pulses delivering power at the rate of Power, to theprinthead substrate between swaths to raise the printhead substratetemperature to the reference temperature.
Figure 3 shows an alternate embodiment of the invention that uses an iterativeapproach to heating the printhead substrate to the reference temperature. Thetemperature sensor measures the printhead substrate temperature. Anoutputsignal 25 of the temperature sensor is processed by either a buffer-amplifier ora data converter and goes to an error detection amplifier that compares it to areference temperature signal 36. If the printhead substrate temperature is lessthan the reference temperature, the closed-loop pulse generator will drive thefiring chamber resistor with a series of nonprinting pulses. This process isrepeated continuously during the print cycle. This and other aspects of thepresent invention are described in U.S. Patent Application 07/694,184.
As stated earlier, Figure 4A is a histogram of the distribution of print-cycletemperatures for a printhead substrate without the present invention. Theaverage print-cycle temperature, T_APCT, is 40°C. When the population ofprinthead substrates with the histogram of print-cycle temperature distributionsshown in Figure 4A adopts the present invention with the reference temperatureset at T_APCT, 40° C, these printhead substrates obtain the histogram of print-cycletemperature distributions shown in Figure 4B. It is a skewed-normaldistribution with the lower temperatures of Figure 4A avoided by use of thepresent invention. This printhead substrate made according to the preferredembodiment of the invention operates at the reference temperature of 40° Cmost of the time but it does float up to higher temperatures including amaximum temperature (i.e., the highest printhead substrate temperature) whenthe print duty cycle is high in a warm environment.
As stated earlier the preferred embodiments of the present invention, sets thereference temperature equal to T_APCT because it has the advantage ofeliminating half the temperature range and half the range of drop volumevariation due to temperature variation. Alternate embodiments could set thereference temperature equal to any temperature, such as above the maximumtemperature, equal to the maximum temperature, somewhere between T_APCT andthe maximum temperature, or below T_APCT without departing from the scope ofthe invention.
Another preferred aspect of the invention, is a darkness control knob, shown in Figure 1,that allows the user to change the reference temperature and thereby adjust thedarkness of the print or the time required for the ink to dry according topersonal preference or changes in the cartridge performance. Adjustments ofthe darkness control knob can cause the reference temperature to exceed themaximum temperature.
Raising the reference temperature has the advantage of reducing the range ofprinthead substrate temperature variation and if the reference temperatureequals the maximum temperature, the printhead substrate temperature will notvary at all. But raising the reference temperature places increased stress on theprinthead substrate and the ink and the likelihood of increased chemicalinteraction of the ink and the printhead substrate. This results in decreasedreliability of the printhead. Also, a printhead substrate with a higher referencetemperature will require more time for heating. Another disadvantage of raisingthe reference temperature is that all ink jet printer designs built to date haveshown a higher chance of misfiring at higher printhead substrate temperatures.
Figure 5A shows the drop volume range for a printhead substrate without thepresent invention. The X-axis is the volume of the drops and the Y-axis is thepercentage of drops having that volume. The peak of the distribution curve isat 52.5 pico liters. The vertical lines are the lower acceptability limit (i.e., thesmallest acceptable drops) and upper acceptability limit (i.e., the largestacceptable drop). The largest drops produced by a printhead substrate withoutthe present invention exceed the upper acceptability limit and cause thefeathering, bleeding, and block (i.e., the sleeve of a transparency film adheresto the printed area of the film and permanently changes the surface of the film)problems, as well as, the cockling and curling problems mentioned earlier.
Drop volume is a function of the printhead substrate temperature, geometricproperties of the printhead such as resistor size or nozzle diameter, and theenergy contained in a printing pulse. As shown in Figure 5A, the drop volumerange of printheads without the present invention is large. Typically, the dropsejected by previously-known printers at the cold, start-up printhead substratetemperatures are too small and produce substandard print. To produce largerdrops at the cold, start-up temperatures, the properties of a printhead withoutthe present invention, such as its geometry, must be adjusted so that the drops produced by a cold printhead substrate at power-on are large enough toproduce satisfactory print (i.e., completely formed characters of adequatedarkness). When these printhead substrates heat-up, they produce drops ofexcessively large volumes (as shown in Figure 5A) that change the saturationlevel of the graphics, make the text bloomy, and create print that does not dryquickly and results in ink that bleeds, blocks, or smears and paper that cocklesor curls. For these reasons, it is desirable to reduce the volume of the largerdrops.
Figure 5B shows the drop volume range for a printhead substrate madeaccording to the present invention. The peak of the distribution curve is at 47.5pico liters and both the lower end and the upper end of the drop distributionfits inside the
limits of acceptability. This volume distribution was obtained by using thepresent invention which keeps the printhead substrate temperature from fallingbelow the reference temperature and by skewing the entire range of dropvolumes down to lower drop volumes. This is accomplished by changing thegeometry of the printhead such as the size of the resistors and the orificediameter. Thus, an advantage of the present invention is that the largest dropscan be eliminated by skewing down the entire range of drop volumes.
Figure 6 shows thetemperature sense resistor 22 that the preferredembodiment of the invention uses.Temperature sense resistor 22 measuresthe average temperature of aprinthead substrate 20 since it wraps around allnozzles 24 ofprinthead substrate 20. The temperature of the ink in the dropgenerators is the temperature of greatest interest, but this temperature isdifficult to measure directly buttemperature sense resistor 22 can measure itindirectly. The silicon is thermally conductive and the ink is in contact with thesubstrate long enough that the temperature averaged around the head is veryclose to the temperature of the ink by the time the printhead ejects the ink.
Printheadsubstrate temperature sensor 22 is inexpensive to manufacturebecause it does not require any processing steps or materials that are notalready a part of the manufacturing procedure for thermal ink jet printheads.However, it must be calibrated using standard calibration techniques, anaccurate thermistor located in the printer box, and a known temperaturedifference between the printhead substrate and printer box. Other possibilitiesfor calibrating printheadsubstrate temperature sensor 22 include laser trimmingof the resistor.
The preferred embodiment of the invention heats the printhead substrate byusing packets of nonprinting pulses. The power delivered by these packetsequals 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 areejected from the printhead) times the energy in each nonprinting pulse. Thispower parameter is used to create the thermal model shown in Figure 2. Thenumber of nozzles and the frequency of the nonprinting pulses are constantand set by other aspects of the printhead design. Alternate embodiments ofthe invention can vary the frequency of the nonprinting pulses and pulse somebut not all of the nozzles without departing from the scope of the invention.
In the preferred embodiment of the invention, the nonprinting pulses have thesame voltage as the printing pulses so that the various time constants in thecircuit are the same for printing pulses and nonprinting pulses. The pulse widthand energy delivered by printing pulses are adjusted according thecharacteristics of each particular printhead. The width of nonprinting pulses isequal to or less than .48 times the width of the printing pulse so that it has littlechance of ever ejecting ink from the printhead. In the preferred embodimentof the invention, the printing pulses have a width of 2.5µsec. and thenonprinting pulses have a width of .6µsec.
The preferred embodiment of the invention changes the reference temperature with changes in resolution that are caused by a change in print speed. At thestandard print speed, the resolution is 300 dots per inch along the paper feedaxis and 600 dots per inch across the width of the paper in the carriage scandirection which translates into twice the number of dots across the width of thepaper. Figure 7A shows the coverage of dots in 300 x 600 dot per inch print.If the print speed is doubled, the printhead operates the same way but theresolution becomes 300 x 300 dots per inch. Figure 7B shows the coverageof dots when the resolution is reduced to 300 x 300 dots per inch print. Holesopen up between the dots. At the lower resolution modes, the present inventionincreases the reference temperature to T_LDref, shown in Figure 2, so that theprinthead ejects drops with a larger volume that produces larger dots thatbetter fill in the empty space between the dots as shown in Figure 8.
The increase in temperature between T_ref and T_LDref depends on how dropvolume increases with temperature, the pl/°C rating, and the dot size versusdrop volume. If the printhead experiences .5pl change per degree C, thenswitching from T_ref = 40°C to T_LDref = 55° C produce a drop volume change of7.5pl. Even though the reference temperature is increased, the pulse width andvoltage remain the same.
The foregoing description of the preferred embodiment of the present inventionhas been presented for the purposes of illustration and description. It is notintended to be exhaustive nor to limit the invention to the precise formdisclosed. Obviously many modifications and variations are possible in light ofthe above teachings. The embodiments were chosen in order to best explainthe best mode of the invention. Thus, it is intended that the scope of theinvention to be defined by the claims appended hereto.

Claims

A method for reducing the range of variation in dropvolume of a thermal ink jet printhead, the methodcomprising:
measuring the temperature of the printhead substrate(20);
comparing the measured temperature with a referencetemperature; and
heating the printhead substrate (20) to the referencetemperature, the reference temperature being such that theheating of the printhead substrate (20) thereto will serveto reduce the range of drop volume variation;
characterised in that:
the printhead is structuredsuch that the maximum drop volume is approximately60 picolitres.
A method as claimed in claim 1, furthercomprising the step of heating the printhead substrate (20)by driving a firing chamber resistor on the printheadsubstrate with nonprinting pulses.
A method as claimed in claim 1 or claim 2, whereinthe printhead substrate (20) is heated to the referencetemperature during the print cycle.
A method as claimed in any preceding claim, furthercomprising setting the reference temperature equal to amaximum temperature of the printhead substrate (20).
A method as claimed in any one of claims 1 to 3,further comprising setting the reference temperature belowthe average print-cycle temperature.
A method as claimed in any one of claims 1 to 3,further comprising setting the reference temperature betweenan average print-cycle temperature and a maximum temperatureof the printhead substrate (20).
A method as claimed in any one of claims 1 to 3,further comprising setting the reference temperature equalto approximately an average print-cycle temperature of theprinthead substrate (20).
A method as claimed in any preceding claim, furthercomprising increasing the reference temperature when theprint resolution of the printhead substrate (20) decreases.
A method as claimed in any preceding claim, furthercomprising using a thermal model of the printhead substrate(20) to estimate how much energy the nonprinting pulsesshould deliver to the firing chamber resistors.
A method as claimed in any preceding claim, furthercomprising the step of varying the reference temperature inresponse to a user input.
A thermal ink jet printhead containing ink of predetermined characteristics, said printhead comprising:
means for measuring the temperature of the printheadsubstrate (20);
means for comparing the measured temperature with areference temperature; and
means for heating the printhead substrate (20) to thereference temperature;
the reference temperature being such that the heatingof the printhead substrate (20) thereto will serve to reducethe range of drop volume;
characterised in that the printhead is so structured in relation to the predeterminedcharacteristics of the ink that the maximum drop volume is approximately 60picolitres.
A thermal ink jet printhead as claimed in claim 11,further comprising means permitting a user toselect the reference temperature thereby to control thedarkness of the print.