Method and apparatus for controlling an ink-jet print head temperature

An ink-jet print head having a dual function thermal controller is disclosed. In a thin-film print head apparatus, a buried resistive layer is located generally circumscribing the other active elements of the print head, viz., the drop generators and the firing logic. During printing operations, the buried resistive layer is used to sense print head temperature. When the print head temperature falls beneath a predetermined minimum limit, the buried resistive layer is activated to act as a heater for the entire print head. Alternatively, the heater can be cycled at predetermined intervals.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to thermal ink-jet printing, more
 particularly to free-ink ink-jet pens and, more specifically to a dual
 function thermal control mechanism for ink-jet print heads.
 2. Description of Related Art
 The art of ink-jet technology is relatively well developed. Commercial
 products such as computer printers, graphics plotters, copiers, and
 facsimile machines employ ink-jet technology for producing hard copy. The
 basics of this technology are disclosed, for example, in various articles
 in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4
 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August
 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)
 editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub
 in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr,
 Academic Press, San Diego, 1988).
 In the art, it is known to provide a print head having an orifice plate
 that operates in combination with subjacent heating elements, such as
 resistors. Thermal excitation of ink is used to eject droplets through
 tiny nozzles in the orifice plate onto an adjacent print medium. The
 combination of a nozzle with an orifice, an ink manifold, and a firing
 resistor is sometimes referred to simply as a "drop generator" or an
 "ejector." Generally, the print head is scanned across the print medium
 and dot matrix manipulation is performed to create a graphics or
 photographic images or alphanumeric characters from patterns of individual
 ink droplets at particular locations that can be described as a linear
 matrix array of picture elements ("pixels").
 The ink-jet print head mechanism itself may have a self-contained reservoir
 (referred to in the art as "on-axis") for storing ink and providing
 appropriate amounts of ink to the print head during a printing cycle.
 These self-contained, disposable mechanisms are often referred to as "pint
 cartridges."
 If a refillable type "pen" rather than a print cartridge is employed in the
 hard copy apparatus, ink is generally supplied from a remote, refillable
 or replaceable, offboard ("off-axis")ink reservoir which is coupled by an
 ink conduit to a relatively permanent pen body and print head mechanism.
 Alternatively, such a "free-ink" ink-jet printing mechanisms have also
 been designed to have a print head mechanism and a detachable, on-board,
 reservoir that can be refilled or replaced as needed. The ink-jet pen and
 particularly the print head element is thus expected to have a longer life
 than a disposable cartridge.
 Early in the development of thermal ink-jet printing it was discovered that
 the preheating of ink in the vicinity of the ink drop firing resistors has
 many advantages, as explained for example in U.S. Pat. No. 4,490,728
 (Vaught et al., 1984, assigned to the common assignee of the present
 invention and incorporated herein by reference). The electrical pulse to
 each resistor comprises a "precursor pulse" and a "nucleation pulse." The
 precursor pulse preheats the ink in the vicinity of the resistor to a
 temperature below the boiling temperature of the ink so as to preheat the
 ink while avoiding vapor bubble nucleation within the local ink supply.
 Subsequently occurring nucleation pulses very quickly heat the resistor to
 near the superheat limit of the ink, causing an ink droplet to be ejected
 through the nozzle. Thus, temperature sensing, or monitoring, of the print
 head mechanism also became an important operational parameter.
 Various means have been invented to accomplish a preheating function in
 thermal ink-jet print heads. See e.g., U.S. Pat. Nos. 4,704,620;
 4,899,180; 4,910,528 (Firl et al., assigned to the common assignee of the
 present invention); 5,107,276; and, also assigned to the common assignee
 of the present invention: 5,109,234; 5,144,336; 5,168,284; 5,235,346;
 5,418,558 (Firl et al.); 5,428,376; and 5,475,405. Each of these
 techniques has its advantages and disadvantages.
 It has been found, however, that there is a need for a mechanism allowing a
 preheating of the print head in a solid state fabrication ink-jet print
 head such that the prior art's complicated and chip area consuming logic
 are no longer required to accomplish the preheating function.
 SUMMARY OF THE INVENTION
 In its basic aspects, the present invention provides a thermal ink-jet
 print head, including: a plurality of drop generators; combinatorial print
 head driver logic, connected to each of the drop generators, for receiving
 printing data and driving selected drop generators to fire ink drops based
 upon the printing data; and a mechanism for thermally controlling
 temperature of the print head, mounted in relation to both the drop
 generators and the combinatorial print head driver logic such that the a
 mechanism for thermally controlling temperature is selectively a passive
 thermal sensor of average print head temperature and an active heater of
 the print head when the print head temperature falls below a predetermined
 minimum operating temperature limit.
 The present invention also provides for a thermal ink-jet pen, including: a
 housing, having an ink accumulation chamber; a print head mounted on the
 housing; circuitry for connecting the print head to a source of data and
 power; an ink inlet port for coupling the accumulation chamber to a supply
 of ink; a regulator coupled to the ink inlet port for controlling both
 flow of ink into the ink accumulation chamber and gauge pressure at the
 print head; the print head including a plurality of drop generators,
 combinatorial driver logic, connected to each of the drop generators, for
 receiving printing data and selectively driving drop generators based upon
 the printing data, and mechanisms for thermally controlling temperature of
 the print head, mounted adjacent both the drop generators and the
 combinatorial driver logic, wherein the mechanisms for thermally
 controlling temperature is selectively a passive thermal sensor of average
 print head temperature and an active heater of the print head when the
 print head temperature falls below minimum temperature limit.
 The present invention also provides for a method for controlling
 temperature of a thermal ink-jet print head, the method including the
 steps of providing a temperature controller device including a resistor
 element substantially encompassing the print head; using the resistor
 element as a passive device, measuring temperature of the print head and
 transmitting a signal indicative of average print head temperature; when
 the signal indicative of average print head temperature falls below a
 predetermined minimum temperature for operation of the print head, using
 the temperature controller device to activate the resistor element as an
 active device to heat the print head to a predetermined operational
 temperature.
 The present invention also provides a thermal ink-jet print head,
 including: a plurality of drop generators; combinatorial print head driver
 logic, connected to each of the drop generators, for receiving printing
 data and driving selected drop generators to fire ink drops based upon the
 printing data; and a thermally control temperature of the print head,
 including an integrally mounted print head resistor, mounted in relation
 to both the drop generators and the combinatorial print head driver logic
 such that the print head resistor is selectively a passive thermal sensor
 of average print head temperature and an active heater of the print head
 when the print head temperature falls below a predetermined minimum
 operating temperature limit and a reference resistor connected to the
 print head resistor.
 It is an advantage of the present invention that it eliminates the
 necessity of complex preheating algorithms for ink-jet pen drop
 generators.
 It is another advantage of the present invention that it provides for a
 simple solid state fabrication of an ink-jet pen print head mechanism.
 It is a further advantage of the present invention that it provides dual
 functionality to a print head sensing element.
 It is still another advantage of the present invention that an ink-jet
 print head is heated with constant low power rather than short high power
 pulses, lengthening product life.
 It is yet another advantage of the present invention that heating of a
 print head is provided by separate mechanisms other than an ink drop
 firing resistor, lengthening product life.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Reference is made now in detail to a specific embodiment of the present
 invention, which illustrates the best mode presently contemplated by the
 inventors for practicing the invention. Alternative embodiments are also
 briefly described as applicable.
 FIG. 1 shows an ink-jet hard copy apparatus; in this exemplary embodiment,
 it depicts a computer peripheral printer 101. A housing 103 encloses the
 electrical and mechanical operating mechanisms of the printer 101.
 Operation is administrated by an electronic controller (usually a
 microprocessor-controlled, printed circuit board, FIG. 3, element 311;
 such controllers 311 are known in the art and typically also provide other
 functions for the hard copy apparatus in which they are employed, such as
 control of the print head carriage (FIG. 1, 109), movement of a print
 media through the printer 101, and the like), connected by appropriate
 cabling to a computer (not shown). Cut-sheet print media 105, loaded by
 the end-user onto an input tray 107, is fed by an internal paper-path
 transport mechanism (not shown; e.g., a motor and paper driver rollers) to
 an internal printing station where images or alphanumeric text are
 printed. A carriage 109, mounted on a slider 111, scans the print medium.
 An encoder 113 is provided for keeping track of the position of the
 carriage 109 at any given time and feeding back positional information to
 the controller 311. A set 115 of ink-jet pens (or print cartridges)
 117A-117D are releasable mounted in the carriage 109 for easy access. In
 pen-type hard copy apparatus, separate, replaceable or refillable, ink
 reservoirs (not shown; relatively large volume--with respect to pen
 size--disposable, ink cartridges) are located within the housing 103 and
 appropriately coupled to the pen set 115 via ink conduits (not shown).
 Once a printed page is completed, the print medium 105 is ejected onto an
 output tray 119.
 FIG. 2 shows an exemplary ink-jet pen 201. A shell, or housing, 203
 includes appropriate bosses and datums 204 for mounting the pen 201 in the
 carriage 109 (FIG. 1). The cartridge housing 203 also contains an
 internal, ink accumulation chamber, or accumulator, 205. Ink from the ink
 reservoir is supplied to the accumulation chamber 205 via a suitable ink
 conduit coupled to a mechanism mounted on and through the cartridge
 housing 203 as an ink inlet port 207. A pressure regulator (not shown) is
 mounted within the accumulation chamber 205 for regulating the flow of ink
 from the reservoir to a print head 219 and for maintaining the appropriate
 print head back pressure (gauge pressure relative to ambient atmospheric
 pressure). In the state of the art, it is known that the print head 219,
 having an array 213 of orifices 215 (and respective subjacent nozzles and
 a manifold that fluidically couple the print head 219 to the ink
 accumulator chamber 205 can be fabricated as a thin-film device (that is
 fabricated integrated circuit techniques; see FIG. 5, infra). The print
 head 219 can be fabricated as part of a flexible circuit 211 (e.g., tape
 automated bonding, TAB) that wraps about appropriate faces of the pen
 cartridge housing 203 such that the print head 219 will be appropriately
 positioned as the pen 201 is scanned across a print media. For printing
 data signals and power, the flexible circuit 211 provides electrical
 contacts 217 for interconnecting the on-board, print head driver logic
 (FIG. 3, element 313) to the printer controller 311.
 With ink supplied from an off-board, replaceable or refillable reservoir,
 it is intended that the pen 201 have an extended life; that is, a much
 larger throughput volume of ink will be used in conjunction with the
 free-ink pen 201 than would be with a unitary, disposable, print cartridge
 having a self-contained ink reservoir.
 FIG. 3 depicts a simplified block diagram of the electronics of a thermal
 ink-jet printer that employs the print head 219 thermal control techniques
 of the invention. In addition to other hard copy apparatus functions, a
 controller 311 receives print data input (usually supplied by a computer
 to the controller; e.g., a graphical image on a video display to be
 printed) and processes the print data to provide print control information
 to the print head driver circuitry 313. The print head driver circuitry
 313 in the present invention is simple combinatorial logic for
 multiplexing the drop generators to the input data. A controlled voltage
 power supply 315 provides the print head driver circuit 313 with a
 controlled supply voltage, V.sub.S whose magnitude is controlled by the
 controller 311. The print head driver circuit 313, as controlled by the
 controller 311, applies driving voltage pulses, V.sub.P (also referred to
 as energizing or firing pulses) to a thin-film ink-jet print head 219 that
 includes ink drop firing ink drop firing resistors 317. Since the actual
 voltage across a heater resistor cannot be readily measured, turn-on
 energy for a heater resistor 317 will be with reference to the voltage
 applied to the contact pads of the print head associated with the heater
 resistor. The resistance associated with a heater resistor 317 will be
 expressed in terms of pad-to-pad resistance of a heater resistor 317 and
 it's interconnect circuitry (the resistance between the print head contact
 pads associated with a specific heater resistor). The relationship between
 the pulse voltage V.sub.P and the supply voltage V.sub.S will depend on
 the characteristics of the driver circuitry. For example, the print head
 driver 313 can be modeled as a substantially constant voltage drop
 V.sub.D, and for such implementations the pulse voltage V.sub.P is
 substantially equal to the supply voltage V.sub.S reduced by the voltage
 drop V.sub.D of the driver circuit:
EQU V.sub.P =V.sub.S -V.sub.D (Equation 1).
 If the print head driver 313 is better modeled as having a resistance
 R.sub.D, then the pulse voltage V.sub.P is expressed as:
EQU V.sub.P =V.sub.S (R.sub.P /(R.sub.D +R.sub.P)) (Equation 2),
 where R.sub.P is the pad-to-pad resistance associated with a heater
 resistor 317.
 The controller 311 provides pulse width and pulse frequency parameters to
 the print head driver circuitry 313 which then produces appropriate drive
 voltage pulses V.sub.P multiplexed to specific ink drop firing resistors
 317 in accordance with input data. In accordance with the present
 invention, separate preheating and nucleation pulses are not needed. Thus,
 the print head driver 313 (FIGS. 3 and 4) can be a simplified
 combinatorial logic; that is, logic that based on the DATA input shifted
 in merely needs to provide a FIRE pulse or NOT FIRE switching function to
 the ink drop firing resistors 317. Note again that all of the extra
 drivers and control circuits required by the prior art--such as for an
 integrated, disposable, print cartridge--for precursor pulse warming is
 eliminated.
 A rudimentary electromechanical schematic of the print head 219 in
 accordance with the present invention is shown in FIG. 4. It should be
 understood that the print head 219 is fabricated using integrated circuit
 techniques and that in the practical state of the art, hundreds of
 components are incorporated into the print head. Each of the nozzle
 orifices 215, 215', 215" has a respective, subjacent, thin-film, firing
 resistor R.sub.F1, R.sub.F2, RF.sub.3 which can be selectively turned on
 and off by related respective transistors Q.sub.1, Q.sub.2, and Q.sub.3
 based upon the output of the control logic 301. Again, as taught by Vaught
 et al. and Firl et al., supra, thermal control is known to be provided in
 the art by sending precursor, or preheating, pulse to each of the firing
 resistors R.sub.F1 -R.sub.FN individually; this naturally requires extra
 onboard logic and control, expensive and complex hardware (resistors not
 actually firing need to be preheated for the next data cycle). Moreover,
 since the pen 201 is to have an extended life, the use of such precursor
 pulse warming is impractical since it shortens the life of firing
 resistors. Thermal control is determined by adding a separate thermal
 sensor (e.g., as taught by Hock et al., supra) for sampling print head
 temperature.
 The print head 219 of the present invention can be fabricated using known
 thin-film construction technology (analogous to the manufacture of
 integrated circuits) and structured as shown in FIG. 5. A silicon
 substrate 601 forms a base, or platform, for the electrical circuitry and
 orifice plate, i.e., the drop generator constructs. In the same
 metallization layer in which firing resistors 317, R.sub.F1 -R.sub.FN, are
 formed, a single, thin-film, metal layer 501, comprising the thermal
 controller 321, is formed as a metallization layer circumnavigating the
 print head 219. Both the firing resistors 317 and the metal layer 501 are
 provided with electrical leads 605, 607, respectively. An ink manifold 609
 is formed to bring ink 611 from the accumulator 205 (FIG. 2) into each
 drop generator. The nozzle plate 213 itself completes the structure. The
 thermal controller 321, including metal layer 501, has a dual function: a
 print head temperature sensor and a resistive, print head heater.
 The electrical equivalent circuit describing the operation of the dual
 function resistor 501 is demonstrated in FIG. 6. The thermal controller
 thin-film resistor 501 has a known nominal resistance at a given
 temperature, e.g. R.sub.25C. Resistance is always given in terms of a
 tolerance, e.g. .+-.15%, and a temperature coefficient, e.g.
 .+-.0.35%.degree. C. Thus, during operation, true resistance of the
 thermal controller 321 is:
EQU R.sub.TC =R.sub.25C +0.0035 (T-25) (Equation 3).
 The mechanism for thermally controlling temperature further includes a
 reference resistor R.sub.R connected to the metal layer 501, forming a
 voltage divider therewith such that a voltage tapped between an externally
 mounted, precision, reference resistor R.sub.R 325 (FIGS. 3 and 6) and the
 resistor 501 element is indicative of the average temperature of the print
 head. The resistance of the reference resistor, R.sub.R, is known.
 Therefore, the output of the thermal controller 321 in the active mode is:
EQU V.sub.TCout =R.sub.TC (V.sub.S)/(R.sub.R +R.sub.TC) (Equation 4)
 when transistor S.sub.2 is ON and transistor S.sub.1 is OFF. Periodically
 sampling V.sub.TCout --e.g., every five milliseconds--is therefore an
 equivalent to determining the average print head temperature. A
 predetermined lower limit operating temperature can be compared and, when
 V.sub.TCout indicates that the print head temperature is below the lower
 limit tolerance, switching transistor S.sub.1 can be turned on and power
 applied to the resistor R.sub.TC 501. Power can be applied either for a
 predetermined fixed time period or until V.sub.TCout is raised to a
 predetermined voltage equivalent to the proper print head operating
 temperature. A sampling period is determined experimentally for each print
 head design; sampling too often would waste controller bandwidth and too
 infrequently would lead to undesirable print head temperature excursions.
 Thus, the temperature controller 321 can be operated by periodic sampling,
 cyclic activation, or by comparison to set temperature thresholds or a
 range of temperatures, and using the temperature controller accordingly
 based upon a comparison match criteria.
 Referring back to FIG. 3, the analog output of the thermal controller 321
 is sent to an analog-to-digital (A/D) converter 323 which provides a
 corresponding digital signal to the controller 311. In the passive mode of
 operation, the digital output of the A/D converter 323 comprises quantized
 samples of the analog output of the thermal controller 321 acting in its
 passive temperature sensor mode. Therefore, the output of the AND
 converter 323 is indicative of the temperature of the print head 219 as
 detected by the thermal controller 321. When the detected temperature
 falls beneath a predetermined operating temperature, e.g., twenty-five
 degrees Centigrade, 25.degree. C., the controller will turn on the thermal
 controller 321 such that it acts as an active print head heater.
 Thus the present invention provides a thermal ink-jet pen with a print head
 having an on-board thermal controller having substantial advantages over
 the prior art. The foregoing description of the preferred embodiment of
 the present invention has been presented for purposes of illustration and
 description. It is not intended to be exhaustive or to limit the invention
 to the precise form disclosed. Obviously, many modifications and
 variations will be apparent to practitioners skilled in this art.
 Similarly, any process steps described might be interchangeable with other
 steps in order to achieve the same result The embodiment was chosen and
 described in order to best explain the principles of the invention and its
 best mode practical application to thereby enable others skilled in the
 art to understand the invention for various embodiments and with various
 modifications as are suited to the particular use contemplated. It is
 intended that the scope of the invention be defined by the claims appended
 hereto and their equivalents.