Thermal ink jet printhead with constant operating temperature

A thermal ink jet printer is disclosed which has a printhead that is maintained at a substantially constant operating temperature during printing. Printing on demand is accomplished by the ejection of ink droplets from the printhead nozzles in response to energy pulses selectively applied to heating elements located in ink channels upstream from the nozzles which pulses vaporize the ink to form temporary bubbles. To prevent printhead temperature fluctuations during printing, especially in translatable carriage printers, the heating elements not being used to eject droplets are selectively energized with energy pulses having insufficient magnitude to vaporize the ink.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This application is a continuation-in-part application of the application 
Ser. No. 07/375,162 filed Jul. 3, 1989 now abandoned. 
This invention relates to thermal ink jet printing devices and, more 
particularly, to improved printheads which are maintained at a constant 
operating temperature so that droplet or pixel size does not vary with 
temperature. 
2. Description of the Prior Art 
Thermal ink jet printing is generally a drop-on-demand type of ink jet 
printing which uses thermal energy to produce a vapor bubble in an 
ink-filled channel that expels a droplet. A thermal energy generator or 
heating element, usually a resistor, is located in the channels near the 
nozzle a predetermined distance therefrom. The resistors are individually 
addressed with an electrical pulse to momentarily vaporize the ink and 
form a bubble which expels an ink droplet. As the bubble grows, the ink 
bulges from the nozzle and is contained by the surface tension of the ink 
as a meniscus. As the bubble begins to collapse, the ink still in the 
channel between the nozzle and bubble starts to move towards the 
collapsing bubble, causing a volumetric contraction of the ink at the 
nozzle and resulting in the separating of the bulging ink as a droplet. 
The acceleration of the ink out of the nozzle while the bubble is growing 
provides the momentum and velocity of the droplet in a substantially 
straight line direction towards a recording medium, such as paper. 
Thus, thermal ink jet devices operate by pulsing heating elements in 
contact with ink so that bubbles are nucleated, ejecting ink droplets 
toward the paper. It has been found during print tests that print quality 
is affected as the device heats up. In particular, if the device heats up 
too high (e.g., during extended high density printing), then it tends to 
lose prime, and one or more ink channels of the printhead cease to expel 
droplets. A less catastrophic defect, but still one that degrades print 
quality, is the increase in printed spot or pixel size as a function of 
device temperature. Through study of this phenomenon, it has been found 
that both the mass and velocity of the droplet increase with device 
temperature and that both the mass and velocity contribute to increased 
pixel size on the paper. For the carriage type ink jet printer with 
sufficiently high printing density, the spot size increases as the 
carriage traverses the page. Then as it pauses at the end of travel and 
reverses direction, it cools slightly, so that the next line or swath 
printed on the way back has increasing pixel sizes in the opposite 
direction. This gives rise to light and dark bands, which are most 
pronounced at the edges of the paper. Similarly, other patterns of high 
and low density printing are degraded by the increase in pixel size with 
device temperature. 
Many of the prior art devices incorporate a heat sink of sufficient thermal 
mass and of low enough thermal resistance that the device temperature does 
not rise excessively. For one example of a thermal ink jet printhead 
having a heat sink, refer to U.S. Pat. No. 4,831,390 to Deshpande et al. 
This approach has eliminated the catastrophic printing failure mode. 
However, to lower the thermal resistance to the heat sink sufficiently 
that there is no appreciable device temperature rise in the time scale of 
a carriage translation in one direction across the paper, it may be 
necessary to take packaging approaches which would increase the cost or 
otherwise constrain the printer design in an undesirable way. The 
temperature rise must be maintained such that negligible image degradation 
occurs because of thermally induced spot size nonuniformities. 
U.S. Pat. No. 4,712,930 to Maruno et al discloses a gradation thermal 
printhead and a gradation heat transfer printing apparatus which employs 
an energy controlling means for varying the voltage or pulse width of the 
signal pulse applied to a thermal printhead. The printing apparatus 
further has a power supply for the gradation thermal printhead and an 
energy controlling means for controlling the width of the pulse of the 
voltage applied to the thermal printhead in accordance with a recording 
signal. 
U.S. Pat. No. 4,536,774 to Inui et al discloses a thermal head drive 
circuit which improves printing quality by using data from previously 
printed lines to compute a corrected pulse energy for the line being 
printed. A pulse energy operator uses data from a heat accumulation state 
operator, a memory which has data on the pulse energy used in the 
previously printed lines, and from either a pulse interval detector or a 
temperature detector. 
U.S. Pat. No. 4,712,172 to Kiyohara et al discloses the use of the heating 
elements to preheat the printhead in the vicinity of the nozzles by 
subthreshold energy pulses insufficient to expel ink droplets to lower the 
viscosity of any plug of ink at the nozzles from which water has 
evaporated. Typically this preheating with subthreshold pulses is done 
when the ink jet printer is turned on or after it has sat idle for a 
period of time. 
U.S. Pat. No. 4,791,435 to Smith et al discloses a thermal ink jet 
printhead having temperature sensors to provide the input needed to 
estimate the printhead temperature, so that the printhead may be kept at 
the desired predetermined time by slowing down the printing, if it is too 
hot to cool it off, or adds warming pulses too short to expel droplets, if 
it is too cold. All decisions and actions are made preceding a printing 
operation. 
U.S. Pat. No. 4,910,528 to Firl et al discloses the use of a temperature 
sensor to measure the printhead temperature and a microcomputer to 
determine the pattern of droplets to be printed, so that prior to the 
commencement of printing, the number of droplets required to print the 
printed swath is known and used to predict the temperature at the end of 
swath. If the predicted printhead temperature exceeds a maximum value, the 
start of printing can be delayed or the printing mode can be modified. If 
the predicted printhead temperature is below a minimum value, the heating 
elements are pulsed with non-droplet ejecting current pulses or the sensor 
can be used as a supplementary heater to warmup the printhead before the 
start of printing. In conjunction with the current temperature of the 
printhead as sensed by a sensor thereon, the future printing demand is 
utilized to predict the printhead temperature at the end of the printing 
of a swath of information and the printing modified to ensure that the 
temperature limits are not exceeded. 
U.S. Pat. No. 4,719,472 to Arakawa discloses the use of a separate heater 
and temperature sensor to heat and monitor the temperature of the ink in 
the reservoir to adjust the viscosity of the ink. 
U.S. Pat. No. 4,490,728 to Vaught et al discloses the use of a two part 
electrical pulse to the heating elements of a thermal ink jet printer. The 
pulses comprise a precursor pulse insufficient to vaporize the ink 
following by a nucleation pulse to expel an ink droplet. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide an improved thermal 
ink jet printhead which maintains itself at a substantially constant 
operating temperature while printing. 
It is another object of the invention to maintain the operating temperature 
of the printhead constant during a printing mode by supplying supplemental 
heat thereto by applying non-vapor producing energy pulses to at least 
some of the heating elements that are not ejecting ink droplets. 
In the present invention, a thermal ink jet printhead of the type having an 
ink supply manifold and a plurality of parallel ink channels with each 
having a nozzle and a heating element is improved by means for maintaining 
the printhead at a substantially constant operating temperature. In the 
printing mode, the printhead ejects ink droplets on demand by the 
selective energization of the heating elements with energy pulses having 
sufficient magnitude to vaporize instantaneously the ink in contact with 
the energized heating element, so that temporary vapor bubbles are formed 
which eject the ink droplet. The improvement comprises counting the pulses 
which expel droplets to determine the heat energy applied to the printhead 
and energization of predetermined heating elements with a sufficient 
quantity of energy pulses insufficient in magnitude to vaporize the ink at 
times when the heating elements are not being energized for the ejection 
of ink droplets to provide supplemental heat, as necessary, to maintain 
the printhead at a substantially constant operating temperature without 
the need of continually sensing the printhead temperature. Alternatively, 
the supplemental heat may be supplied by energizing one or more additional 
heaters on the printhead which are provided solely to supply heat and 
which are not used to vaporize ink to bring about droplet ejection. 
In another embodiment, all of the heating elements are pulsed with 
subthreshold electrical pulses, which are insufficient in magnitude to 
vaporize ink during the standby mode. During the printing mode, those 
heating elements not being used to eject droplets are pulsed with 
subthreshold pulses. 
A more complete understanding of the present invention can be obtained by 
considering the following detailed description in conjunction with the 
accompanying drawings, wherein like parts have the same index numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An enlarged, schematic isometric view of the front face 29 of a typical 
thermal ink jet printhead 10, showing an array of droplet emitting nozzles 
27, is depicted in FIG. 1. Referring also to FIG. 2, discussed later, the 
lower electrically insulating substrate or heating element plate 28 has 
the multi-layered, thermal transducers 36, including the heating elements 
34, and addressing electrodes 33 patterned on surface 30 thereof, while 
the upper substrate or channel plate 31 has parallel grooves 20 which 
extend in one direction and penetrate through the upper substrate front 
face edge 29. The other end of grooves terminate at slanted wall 21. The 
internal recess 24, which is used as the ink supply manifold for the 
capillary filled ink channels 20, has an open bottom 25 for use an an ink 
fill hole. The surface of the channel plate with the grooves are aligned 
and bonded to the heater plate 28, so that a respective one of the 
plurality of heating elements 34 is positioned in each channel, formed by 
the grooves and the lower substrate or heater plate. Ink enters the 
manifold formed by the recess 24 and the lower substrate 28 through the 
fill hole 25 and, by capillary action, fills the channels 20 by flowing 
through an elongated recess 38 formed in the thick film insulative layer 
18. The ink at each nozzle forms a meniscus, the surface tension of which, 
together with the slight negative pressure of the ink supply, prevents the 
ink from weeping therefrom. The addressing electrodes 33 on the lower 
substrate or channel plate 28 terminate at terminals 32. The upper 
substrate or channel plate 31 is smaller than that of the lower substrate 
in order that the electrode terminals 32 are exposed and available for 
wire bonding 15 to the electrodes 14 on the daughter board 19, on which 
the printhead 10 is permanently mounted. Layer 18 is a thick film 
passivation layer, discussed later, sandwiched between upper and lower 
substrates. This layer is etched to expose the heating elements, thus 
placing them in a pit 26, and is etched to form the elongated recess 38 to 
enable ink flow between the manifold 24 and the ink channels 20. In 
addition, the thick film insulative layer is etched to expose the 
electrode terminals. 
A cross sectional view of FIG. 1 is taken along view line 2--2 through one 
channel and shown as FIG. 2 to show how the ink flows from the manifold 24 
and around the end 21 of the groove 20 as depicted by arrow 23. The ink 
droplets (not shown) are ejected by control circuitry 48, drivers 49, and 
power supply 52 in response to receipt of data to be printed. The encoder 
50 monitors when the printhead is in the printing region and the optional 
microprocessor 60 counts the droplet ejecting electrical pulses applied to 
each of the heating elements 34. As is disclosed in U.S. Pat. No. 
4,638,337 to Torpey et al, a plurality of sets of bubble generating 
heating elements 34 and their addressing electrodes 33 are patterned on 
the polished surface of a (100) silicon wafer. Prior to patterning the 
multiple sets of printhead electrodes 33, the resistive material 34 that 
serves as the heating elements, and the common return 35, the polished 
surface of the wafer is coated with an underglaze layer 39 such as silicon 
dioxide, having a thickness of about 2 micrometers. The resistive material 
may be a doped polycrystalline silicon which may be deposited by chemical 
vapor deposition (CVD) or any other well known resistive material such as 
zirconium boride (ZrB.sub.2). The common return and the addressing 
electrodes are typically aluminum leads deposited on the underglaze and 
over the edges of the heating elements. The common return ends or 
terminals 37 and addressing electrode terminals 32 are positioned at 
predetermined locations to allow clearance for wire bonding to the 
electrodes 14 of the daughter board 19, after the channel plate 31 is 
attached to make a printhead. The common return 35 and the addressing 
electrodes 33 are deposited to a thickness of 0.5 to 3 micrometers, with 
the preferred thickness being 1.5 micrometers. 
In the preferred embodiment, the lower substrate or heating element plate 
28 is silicon with an underglaze layer 39 of thermal oxide or other 
suitable insulative layer such as silicon dioxide. Polysilicon heating 
elements 34 are formed and an insulative overglaze layer (not shown) is 
deposited over the underglaze layer and heating elements thereon. This 
overglaze layer may be either silicon dioxide, thermal oxide, or reflowed 
polysilicon glass (PSG). The thermal oxide layer is typically grown to a 
thickness of 0.5 to 1.0 micrometer to protect and insulate the heating 
elements from the conductive ink. Reflowed PSG is usually about 2 
micrometers thick. The overglaze layer is masked and etched to produce 
vias therein near the edges of the heating elements for subsequent 
electrical interface with the aluminum (Al) addressing electrode 33 and Al 
common return electrode 35. In addition, the overglaze layer in the bubble 
generating region of the heating element 34 is concurrently removed. If 
other resistive material such as hafnium boride or zirconium boride is 
used for the heating elements, then other suitable well known insulative 
materials may be used. 
The next process step in fabricating the thermal transducer is to deposit a 
pyrolytic silicon nitride layer 17 directly o the exposed polysilicon 
heating elements, followed by the deposition of about one micrometer thick 
tantalum layer 12 for cavitational stress protection of the pyrolytic 
silicon nitride layer 17. 
The pyrolytic silicon nitride serves two very useful functions. First, it 
has very good thermal conductivity, so that it produces a thermally 
efficient resistor structure when deposited directly in contact with the 
resistor. Secondly, it is one of few materials that is resistant to Ta 
etches. 
The multi-layered, thermal transducer structure is completed with either 4 
wt % CVD PSG or preferably, plasma nitride lead passivation. Either of 
these materials can be selectively etched off the Al bonding pads and 
resistor area. 
For electrode passivation, a two micrometer thick phosphorous doped CVD 
silicon dioxide film 16 is deposited over the entire heating element plate 
or wafer surface, including the plurality of sets of heating elements and 
addressing electrodes. The passivation film 16 provides an ion barrier 
which will protect the exposed electrodes from the ink. Other ion barriers 
may be used, such as, for example, polyimide, plasma nitride, as well as 
the above-mentioned phosphorous doped silicon dioxide, or any combinations 
thereof. An effective ion barrier layer is achieved when its thickness is 
between 1000 angstroms and 10 micrometers, with the preferred thickness 
being 1 micrometers. The passivation film or layer 16 is etched off of the 
terminal ends of the common return and addressing electrodes for wire 
bonding later with the daughter board electrodes. This etching of the 
silicon dioxide film may be by either the wet or dry etching method. 
Alternatively, the electrode passivation may be accomplished by plasma 
deposited silicon nitride (Si.sub.3 N.sub.4). 
Next, a thick film type insulative layer 18 such as, for example, 
Riston.RTM., Vacrel.RTM., Probimer 52.RTM., or polyimide, is formed on the 
passivation layer 16 having a thickness of between 10 and 100 micrometers 
and preferably in the range of 25 to 50 micrometers. The insulative layer 
18 is photolithographically processed to enable etching and removal of 
those portions of the layer 18 over each heating element (forming recesses 
26), the elongated recess 38 for providing ink passage from the manifold 
24 to the ink channels 20, and over each electrode terminal 32, 37. The 
elongated recess 38 is formed by the removal of this portion of the thick 
film layer 18. 
In thick film layer 18, the pit 26 is formed having walls 42 that exposes 
each bubble generating area of the multi-layered thermal transducer 36 and 
walls 41 defining an elongated recess 38 to open the ink channels to the 
manifold. The recess walls 42 inhibit lateral movement of each bubble 
generated by the pulsed heating element which lie at the bottom of 
recesses 26, and thus promote bubble growth in a direction normal thereto. 
Therefore, as disclosed in U.S. Pat. No. 4,638,337, the blowout phenomena 
of releasing a burst of vaporized ink which causes an ingestion of air is 
avoided. 
The passivated addressing electrodes are exposed to ink along the majority 
of their length and any pinhole in the normal electrode passivation layer 
16 exposes the electrode 33 to electrolysis which would eventually lead to 
operational failure of the heating element addressed thereby. Accordingly, 
an added protection of the addressing electrode is obtained by the thick 
film layer 18, since the electrodes are passivated by two overlapping 
layers, passivation layer 16 and a thick film layer 18. 
As disclosed in U.S. Pat. Nos. Re. 32,572 and 4,638,337 and incorporated 
herein by reference, the channel plate is formed from a (100) silicon 
wafer to produce a plurality of upper substrates 31 for the printhead. The 
heating element plate 28 is also obtained from a wafer or wafer sized 
structure (not shown) containing a plurality thereof. Relatively large 
rectangular through recesses and a plurality of sets of equally, spaced 
parallel V-groove recesses are etched in one surface of the wafer (not 
shown). These recesses will eventually become the ink manifolds 24 and ink 
channels 20 of the printheads. The channel plate and heating element plate 
containing wafers are aligned and bonded together, then diced into a 
plurality of individual printheads. One of the dicing cuts produces end 
face 29, opens one end of the elongated V-groove recesses 20 producing 
nozzles 27. The other ends of the V-groove recesses 20 remain closed by 
end 21. However, the alignment and bonding of the above-mentioned wafers 
places the ends 21 of each set of channels 20 directly over elongated 
recess 38 in the thick film insulative layer 18 as shown in FIG. 2, 
enabling the flow of ink into the channels from the manifold 24 as 
depicted by arrow 23. 
The temperature of the improved printhead is held substantially constant, 
even though it is not ejecting droplets at the extreme ends of the 
carriage translation. As disclosed in U.S. Pat. No. 4,571,599 to Rezanka 
and shown in FIG. 3, a typical multicolor thermal ink jet printer 11 is 
shown containing several disposable ink supply cartridges 22, each with an 
integrally attached printhead 10 of the present invention. The cartridge 
and printhead combination are removably mounted on a translatable carriage 
40. Curing the printing mode, the carriage reciprocates back and forth on, 
for example, guide rails 43 parallel to the recording medium 44 as 
depicted by arrow 45. The end-to-end travel distance of the carriage and 
printheads is shown as distance B. The recording medium, such as, for 
example, paper, is held stationary while the carriage is moving in one 
direction and, prior to the carriage moving in a reverse direction, the 
paper is stepped in the direction of arrow 46 a distance equal to the 
height of the swath of data printed thereon by the printheads 10 during 
traversal in one direction across the paper. The width of the recording 
medium is the printing zone or region during the carriage traversal and is 
indicated as distance A. To enable printing by all of the plurality of 
printheads and to accommodate printhead priming and maintenance stations 
(not shown), the overall travel distance B is larger than the printing 
region A. Thus, an encoder 50 (see FIGS. 2 and 5) must be used to monitor 
when the printheads are within the printing region. The droplets are 
ejected on demand from the nozzles 27 in front face 29 of the printheads 
along the trajectories 47 to the paper. The front face of the printhead is 
spaced from the paper a distance of between 0.01 and 0.1 inch, with the 
preferred distance being about 0.02 inches. The stepping tolerance for the 
paper and the linear deviation of the printheads are held within 
acceptable limits to permit contiguous swaths of information to be printed 
without gaps or overlaps. 
Each cartridge 40 contains a different colored ink, one black and one to 
three additional cartridges of different selected colors. The combined 
cartridge and printhead is removed and discarded after the ink supply in 
the cartridge has been depleted. In this environment, some of the nozzles 
do not eject droplets during one complete carriage traversal and, 
generally, none of the nozzles eject droplets as the printheads move 
beyond the edge of the paper. While at this end of a carriage traversal, 
there is a small dwell time while the paper is being stepped one swath in 
height in the direction of arrow 46. Thus, as discussed above, the 
printhead of the prior art printers cool down. However, the printheads of 
the present invention are kept at a constant operating temperature by the 
application of electrical or energy pulses to the heating element not 
ejecting droplets having insufficient magnitude to vaporize the ink. This 
supplemental heat keeps the operating temperature of the printhead 
constant. The number of unused heating elements, the pulse widths, and/or 
the power of the supplemental pulses control the printhead temperature 
while it is in the printing mode. 
In the preferred embodiment of FIG. 5A, discussed later, a zero data 
detector 54 enables all heating elements of the printhead to be pulsed 
with non-droplet ejecting or subthreshold pulses to maintain the operating 
temperature of the printhead substantially constant. Periodically, the 
ambient printer temperature is checked by a temperature sensor 55 located 
within the printer (not shown) and in the vicinity of the printhead 10 for 
a reference temperature which the logic controller uses to control the 
compensating energy applied by subthreshold pulses. Optionally, the 
temperature of the printhead could be used instead of the ambient printer 
temperature. This reference temperature is checked at startup, when 
entering the printing mode, and at the conclusion of printing a 
predetermined number of full pages, rather than sensing the printhead 
temperature continually or frequently such as during or after each swath 
of printed information as required by the prior art. Thus, this invention 
does not need to continually check the printhead temperature or even check 
for a reference temperature more frequently than after printing more than 
one page. A pulse count look up table 51 in response to the pulse counter 
61, which counts the droplet ejecting pulses required by the data to be 
printed, determines the umber and width of the nondroplet ejecting 
(subthreshold) pulses in conjunction with the subthreshold pulse width 
controller 56 and enables the logic controller to apply the required 
subthreshold pulses having the appropriate pulse width to the heating 
elements not ejecting droplets. 
Optionally, a microprocessor 60 counts the droplet ejecting pulses per 
heating element per unit of time, so that if the number of heating 
elements used and/or the rate of droplets expelled are not within 
predetermined values, supplemental heat is applied to the printhead by 
subthreshold pulsing of the least used heating elements. Subthreshold 
pulses are not capable of vaporizing the ink, so that droplets are not 
ejected. A consequence of using supplemental heat to keep the temperature 
of the printhead constant during printing is that the average device 
temperature will be higher than it would be otherwise. However, this is an 
advantage, if the temperature is kept below a predetermined maximum 
temperature, whereat the printhead begins to fail. This maximum 
temperature is about 70.degree. C. when the inks used comprise ethylene 
glycol and a water base, but varies with different ink formulations and 
ink channel geometries. Below 70.degree. C., the drop velocity becomes 
more uniform as the temperature is increased. At 20.degree. C., some ink 
channels of the printhead having water based ink formulations have been 
observed to have marginally acceptable droplet velocities. The droplet 
velocity increases to a highly satisfactory range with a moderate increase 
in printhead temperature. The ideal operating point depends on ink and 
device parameters, but in the present case would appear to be roughly 
30.degree. C. to 50.degree. C. An additional advantage of operating at 
elevated temperature is that the ink viscosity decreases, so that refill 
times of the channels may be decreased, enabling higher printing 
frequencies. The printhead 10 has a heat sink 71 with a predetermined heat 
dissipating capacity, so that the heat added to the printhead by the 
droplet ejecting pulses and the subthreshold pulses will be dissipated at 
a known rate and taken into account by the pulse count look up table 51 
and/or the optional microprocessor 60. 
In FIG. 4, one embodiment of this invention is shown in which, for example, 
a 48 jet or channel printhead is used, printing up to two channels at a 
time. In this example of an energy compensating pulse scheme, the 48 
channels are being pulsed 2 at a time and channels 1, 2, 3, 24, 25, 26 and 
48 are assumed to have printed. The shorter pulses during the compensation 
cycle are provided so that the total energy dissipated in the time 
interval associated with a group of 48 pixels is constant. Since the 
carriage is moving continuously, it is necessary to finish printing all 48 
jets in a fraction of the time it takes to get from one pixel to the next, 
or the dot or pixel pattern will be too jagged. AT 2 kHz operation, we 
have 500 .mu.sec to get from one pixel position to the next, while at 3 
kHz, we would have 333 .mu.sec. By comparison, the printing cycle is 
composed of 24 intervals of 5 .mu.sec (120 .mu. sec total), during which 
up to two channels will be fired or energized at a time using about 3 
.mu.sec duration pulses. The energy dissipated during the printing cycle 
in one set of up to 48 pixels is E.sub.p =n P t.sub.p, where n is the 
number of channels fired (0 to 48), P is the power per print pulse, and 
t.sub.p is the pulse width (3.mu.sec in our example). The maximum energy 
dissipated is E.sub.max =NP t.sub.p, where N=48 in our example. For 
strictly constant energy input, m short pulses would be added (none of 
which is sufficient for bubble nucleation) during what is normally a "rest 
period", so that E.sub.p +E.sub.c =n P t.sub.p +m P t.sub.c =E.sub.max =N 
P t.sub.p, where E.sub.c is the compensating energy and t.sub.c is the 
pulse width or duration of the compensating pulse. For example, if t.sub.c 
=t.sub./ /4 (0.75 .mu.sec), then n+m/4=48, and during period s of time 
when printing is not occurring (n=0), there would be 192 of the short 
pulses required. For 2 kHz operation the energy compensating cycle would 
be 350 .mu.sec (allowing 120 .mu.sec printing cycle and 30 .mu.sec setup 
times). By pulsing up to 2 heaters at a time during the energy 
compensating cycle, as would be done during the printing cycle, there will 
be 96 pulse intervals, so that the short pulses would be on for 0.75 
.mu.sec and off for 2.9 .rho.sec. Other cases of interest are shown in 
Table 1, assuming 120 .mu.sec printing cycle and 30 .mu.sec setup times. 
Selection criteria are that bubbles not be nucleated during t.sub.c, but 
that the driver transistors be fast enough. 
TABLE 1 
______________________________________ 
Energy Compensating Pulse Widths (.mu.sec) 
t.sub.p /t.sub.c 
t on 2 kHz t off 
3 kHz t off 
______________________________________ 
4 .75 2.9 1.2 
3 1.0 3.9 1.5 
2 1.5 5.8 2.3 
______________________________________ 
A variety of method or embodiments may be devised for implementing the 
logic for the energy compensation pulses. One method would be to count the 
pulses during the printing cycle and decrement a counter for the 
compensation cycle accordingly. Referring to FIG. 5A, this method does not 
keep track of which heating elements were fired, unless the optional 
microprocessor 60 is used, and would simply cycle through the heating 
elements not being used to eject droplets until enough compensating pulses 
were fired. The pulse counter 61, zero data detector 54 and logic 
controller 58 of the control circuitry 48 receive data to be printed in 
the form of digitized data signals. The encoder 50 provides signals 
indicative of the location of the printhead 10, relative to the printing 
region A of FIG. 3, to the logic controller 58 and subthreshold pulse 
width controller 56. The pulse counter 61 determines how many jet or 
heating elements are being fired during a particular time interval. Jets 
fired have a pulse width given by the ejection pulse controller 62. In the 
event that the zero data detector 54 indicates that no jets are to be 
fired (i.e., no droplets are to be ejected as when a new page of printing 
has not begun, or the printhead has reached the end of a line, or during 
white space within a line), it indicates to the logic controller 58 that 
subthreshold pulse firing may occur. The pulse count look up table 51 
compares the number of droplet ejection pulses which have recently been 
first or are about to be fired, and indicates to the subthreshold pulse 
width controller 56 how many and how wide the subthreshold pulses should 
be to bring the printhead 10 to the desired operating point. 
In the preferred embodiment, the power supply 52 provides a constant 
voltage V.sub.o to the common return electrode 35. The heating elements 34 
are pulsed with this voltage through drivers 49 which are connected to the 
printhead addressing electrodes 33 and to ground. Thus, the electrical 
pulses applied to the heating elements or resistors 34 have a constant 
amplitude and the width is varied to eject a droplet or provide only 
supplemental heat with pulse widths insufficient to vaporize ink. Clock 53 
provides the timing for the logic controller 58. The control circuitry 48 
may optionally contain a look up table 57 (shown in dashed line) which 
receives input signals representative of the ambient temperature from 
temperature sensor 55 located within the printer (not shown) in the 
vicinity of the printhead or optionally thereon. Based upon the 
temperature sensor, the subthreshold pulse width controller signals the 
logic controller for supplemental heat generating electrical pulses 
insufficient to eject droplets. 
An optional dedicated heater 59 on the printer, but not shown in FIG. 2, 
could also be used to provide the required supplemental heat to the 
printhead instead of pulsing the heating elements, as is well known in the 
art. 
An optional microprocessor 60 keeps track of which heating elements have 
not been fired very often and employs those heating elements which have 
not been used often to do the threshold pulsing, in order to average out 
the overall number of pulses for each heating element for lifetime 
purposes. This is accomplished by counting the number of droplet-ejecting 
pulses each heating element received during a predetermined time period, 
such as, for example, during the printing of a swath of information. This 
count per heating element could be stored and averaged or simply erased 
after each printed swath or printed page. 
Alternately, as shown in FIG. 5B, a device 63 for determining the logical 
complement of the printing data is given to the subthreshold pulse width 
controller 67 so that those heating elements which are not fired to eject 
droplets are automatically pulsed with subthreshold pulses. This ensures 
that each heating element experiences the same number of pulses for 
lifetime purposes, although some experience a greater number of droplet 
ejection pulses. 
The decisions made by the pulse width controller 56 in the control 
circuitry 48 of FIG. 5A is shown in the flow chart of FIG. 6. When the 
printing mode is activated, the ink channels are primed and the heating 
elements are all pulsed with electrical current pulses having sufficient 
magnitude or average power to vaporize the ink in contact therewith and 
eject nozzle clearing droplets in an ink collection recess or absorbent 
material forming part of a maintenance station (not shown). After a 
predetermined number of droplets are ejected from each nozzle, the 
printhead warmup is continued with application of subthreshold electrical 
pulses to the heating elements. By subthreshold, it is meant those pulses 
having insufficient energy or average power to vaporize ink and expel ink 
droplets. 
Upon receipt of digitized data to be printed, the location of the printhead 
is checked to see if it is within the printing region A as shown in FIG. 
3. If not, the printhead is pulsed with subthreshold pulses to provide 
supplemental heating while it is moved into proper position for printing. 
Once the printhead is in the printing region, droplets are ejected and 
propelled to a recording medium 44. The pulse counter 61 counts the number 
of pulses which eject droplets and the logic controller 58 determines the 
pulses per clock time unit, that is the printing rate or density, and 
compares this rate or density with a minimum value required to maintain 
the operating temperature of the printhead within the appropriate 
temperature range. 
Optionally, the microprocessor 60 identifies which nozzles were fired; i.e. 
used to expel droplets. If the printing density is sufficient to maintain 
the printhead operating temperature sufficiently constant, printing is 
continued without supplemental heating. If not, the number and width of 
subthreshold pulses required are determined by the logic controller and 
those heating elements not being used to eject droplets are pulsed with 
the subthreshold pulse. If desired, the subthreshold pulses can be applied 
only to those heating elements which have not ejected a droplet during the 
time period for which the droplet rate or density was measured. For 
example, at intermediate points along a swath of printed droplets or at 
the end of a printed swath or both. 
Thus, the operating temperature of the printhead of the present invention 
is maintained substantially constant within the appropriate temperature 
without the need for continually measuring the printhead temperature and 
modifying the printing speed to cool it down or add heat to boost the 
temperature until the printhead sensor reads the desired value. 
A temperature sensor 55 within the printer is used periodically during 
standby or initial start-up of printing, but constant reference to it is 
not required. The decisions made by the control circuitry 48A of FIG. 5B 
are shown in the flow chart of FIG. 7. AS in the flow chart of FIG. 6, the 
ink channels are primed and the heating elements pulsed to eject nozzle 
clearing droplets when the printing mode is activated. After a 
predetermined number of droplets are ejected from each nozzle, the 
printhead warmup is continued with the application of subthreshold pulses 
to the heating elements. 
Upon receipt of data to be printed, the location of the printhead is 
checked to see if it is within the printing region by the encoder 50. If 
not, the printhead is pulsed with subthreshold pulses to provide 
supplemental heating while it is being moved into the proper position for 
printing. Once the printhead is in the printing region, droplets are 
ejected and propelled to the recording medium 44. The logical complement 
63 identifies those heating elements not being used to eject droplets, and 
in response to the logical complement input, the subthreshold pulse width 
controller 67 and ejection pulse controller 62 via logic controller 58 
apply respective pulses to each heating element. In this arrangement all 
of the heating elements are fired or pulsed with either droplet ejecting 
pulses or subthreshold pulses during the actual printing operation. Thus, 
when no data is to be printed, only subthreshold pulses are applied to the 
heating elements. The subthreshold pulse width is determined by the 
ambient temperature sensor 55 and the known heat transfer rate from the 
heat sink 71. 
This invention does not restrict itself to the case of E.sub.p +E.sub.c 
=E.sub.max. For one thing, E.sub.c should probably be somewhat less than 
E.sub.max -E.sub.p because no heat is being carried off by ejected drops 
during the compensation cycle. In addition, it is not necessary to keep 
the printhead temperature exactly constant. It may be found that an upper 
limit of compensation less than E.sub.max is satisfactory. The advantage 
of using less energy compensation is that it would be easier to maintain a 
thermal equilibrium which did not approach the upper operating temperature 
for a longer period of time. 
Energy compensation will be required whenever printing is occurring or 
about to occur. In particular, energy compensation should continue at its 
maximum rate during carriage pauses at the end of travel. It should also 
occur just preceding starting to print. Warmup time should not be 
objectionably long, but in a one page per minute printer 1-4 seconds 
should be satisfactory for a large part of the temperature rise occurs 
within 3 seconds. The heat sinking should be designed so that the device 
temperature is raised for the most part within a few seconds, and then 
rises much slower after that. Energy compensation could also be applied 
for the longer term heating effects, e.g., by decrementing a counter a 
certain number of pulses for each line printed. The other heat sink 
requirement is that the device temperature remain in the optimal range 
(e.g., 40.degree. C. .+-.10.degree. C.). 
Energy compensation may also be controlled by modifying the pulse width, 
t.sub.c, depending on the number of channels fired during the printing 
cycle. In one embodiment, short compensating pulses are fired only during 
the compensation cycle. In another embodiment, short pulses are fired 
during the printing cycle as well, with the pulse width widened for those 
channels where printing is desired. The minimum pulse width increment 
would be determined by the fastest clock in the system, which might 
typically be 10-20 MHz. Another way to control the energy compensation is 
to modify its pulse power, but this is more difficult to implement. It has 
been assumed here that the compensation energy is provided by the same 
heating elements responsible for printing, but this is not a requirement. 
One or more special heating elements (not shown) for supplying only 
supplemental heat may be formed anywhere on the heating element plate 28, 
preferably in a location where they do not contact the ink. 
The advantages of this inventive compensating pulsing scheme are as 
follows: 
1. It may be implemented without temperature sensors or extra heating 
elements being on the printhead. 
2. It is capable of making thermally induced spot size variation and 
banding negligible. 
3. Thermal packaging to obtain a lower thermal resistance path from device 
to heat sink becomes less critical. 
4. Peak power required for bubble formation is reduced, since spot size 
increases with device temperature as well as print pulse condition. 
5. Operation at elevated temperature will improve uniformity of drop 
velocity, thus improving the yield of good performing devices. 
6. Operation at elevated temperature is expected to decrease ink viscosity 
within the device, and improve channel refill times. 
An additional feature that might prove useful is a temperature sensor on 
the printhead that measures the absolute temperature. The energy 
compensation scheme could then be modified, for example, through the use 
of lookup tables to provide the desired device temperature independent of 
ambient temperature or length of time the printer has been operating. 
Although the above description was cast in terms a carriage type ink jet 
printer, this invention is equally applicable to a page width or partial 
page printer. The subthreshold pulses would keep all of the subunits or 
modules making up the page width printhead at the same temperature, so 
that they would produce droplets having the same volume and the printed 
spot size would be uniform. By applying subthreshold temperature 
compensating pulses in relation to the density of printing by each module, 
they all could be maintained within the desired operating temperature 
without the need of individual temperature sensors on each printhead 
subunits, but only one within the pagewidth printhead structural bar. 
In one embodiment, a printhead is composed of a plurality of fully 
functional, small individual printhead subunits. Each subunit could be 
used individually as a carriage type printhead capable of being scanned 
across a recording medium to print a swath of pixels or dots of ink. 
Referring to FIGS. 8A and 8B, a plurality of the printhead subunits 66 are 
mounted on a structural bar 68 which could either be translated across a 
recording medium, (not shown) to print partial pages (e.g., one large 
swath of information) or be fixed for page width printing where the 
recording medium is moved thereby at a constant velocity. In FIG. 8A, the 
subunits are alternately mounted on opposite sides of the bar with spaces 
between subunits on the same side of the bar. A single temperature sensor 
69 mounted on the bar is used to establish a reference temperature for 
determining the number and/or width of the subthreshold pulses applied to 
the heating elements of each printhead subunit 66. The control circuitry 
48C or 48D either uses the logic complement (not shown) of the data to be 
printed to apply subthreshold pulses (those pulses having a magnitude 
insufficient to vaporize ink) to all heating elements in each subunit not 
ejecting droplets or counts the droplet ejecting pulses and through a 
lookup table determines the number and pulse width of the subthreshold 
pulses of predetermined heating elements not ejecting droplets. A 
microprocessor 60B could be optionally used to count the number of 
droplets ejected by each heating element in each subunit and apply 
subthreshold pulses to the heating elements least used to eject droplets. 
The droplet ejecting or subthreshold pulses are applied by the control 
circuitry via the drivers 49. The temperature sensor provides a reference 
temperature of the structural bar 68 or ambient temperature which is only 
used at startup and then periodically, but infrequently, as a reference 
parameter. The primary control of the operating temperature is by 
monitoring the heat energy applied to the printhead subunits in the form 
of droplet ejecting or subthreshold pulses per unit of time after the 
reference or ambient temperature has been established. Thus the desired 
operating temperature of each subunit is maintained within the same 
desired operating temperature without the need of individual temperature 
sensors on each printhead subunit. 
Many modifications and variations are apparent from the forgoing 
description of the invention, and all such modifications and variations 
are intended to be within the scope of the present invention.