Patent Application
20090040257
The present invention relates to the field of thermal inkjet printhead warming and particularly relates to a technique for warming the printhead without creating interference in the operation of the printhead.
In thermal inkjet printers, print quality and jetting reliability are dependent on the temperature of the inkjet integrated circuit, the printhead. In a thermal inkjet printhead, inkjets are ejected by boiling a single bubble at intense heat for a very short duration. This process is repeated thousands of times per second for each nozzle, on the printhead resulting in an accumulation of heat that raises the temperature of the ink. Variations in ink temperature affect the shape, size and velocity of ejected drops resulting in variations of density that may be perceivable to the eye. To alleviate this problem, various printhead thermal control systems have been developed. However, these prior art thermal control systems have often created artifacts or electromagnetic noise that interferes with the operation of the printhead. Some prior art uses very narrow non-nucleating heating pulses in the heater resistors. The range of control is limited by longer pulse widths near the onset of nucleation and narrow pulse widths that are limited by the pulse generator clock resolution. A typical non-nucleating heating pulse control range might be from 70 to 250 nanoseconds. Thus, the thermal control systems that were designed to create additional reliability sometimes create problems.
In accordance with the present invention a thermal control system for a printhead is disclosed that avoids interference with the operation of the printhead. In one embodiment, a thermal inkjet apparatus is disclosed that includes a printhead body with nozzles formed in the body. Ink cavities are formed in the body that contain ink that is communicated to the nozzles to supply ink to a media, such as paper. The ink is supplied to the ink cavities by ink supply lines and heater resistors are disposed in the cavities. A firing circuit is connected to the heater resistor and it supplies a firing pulse to the heater resistors causing the heater resistors to heat sufficiently to nucleate the ink in the cavities and fire the ink out of the nozzles. In addition, a warming circuit is connected to the heater resistors and it supplies warming pulses to the heater resistors. The warming pulses heat the heater resistors sufficiently to warm the printhead but insufficiently to nucleate the ink. Thus, the warming pulses warm the printhead but do not fire ink out of the nozzles.
In a particular embodiment, the warming circuit includes a current limiting ballast resistor with at least one such current limiting ballast resistor (which may be constructed in polysilicon) connected in series with each of the heater resistors. These current limiting ballast resistors limit the flow of current through the heater resistors to a desired level. Warming switches are connected to supply warming pulses to the ballast resistors and heater resistors to heat the ballast resistors and heater resistors and thereby heat the printhead body but not nucleate the ink. Similarly the firing circuit includes a firing switch with at least one firing switch connected to each heater resistor for supplying firing pulses to the heater resistors. The current limiting ballast resistors may be located in the cavity with the heater resistors, or the ballast resistors may be located outside the cavities or in both places. In such case, two ballast resistors would be used, one inside the cavity and one outside the cavity.
In a particular embodiment, to achieve the desired warming characteristics, at least one thermal sensor is disposed to sense the temperature of the printhead body and produces a sensor signal indicating such temperature. A control circuit is connected to the thermal sensor and determines a value corresponding to the temperature of the body based on the sensor signal. The control circuit then generates control signals based on the value and transmits them to a pulse control circuit. The pulse control circuit performs multiple functions but one of its functions is to supply warming pulses to the warming circuit in response to the control signals. The pulse control circuit preferably varies the width of the warming pulse to achieve the desired warming effect. In other words, a longer warming pulse is produced if more warming effect is desired. Thus, the pulse control circuit produces control signals designating the width of a desired warming pulse based on the temperature of the body.
In one embodiment, the warming circuit includes a relatively smaller field effect transistor operating as a switch to switch the warming pulses on and off and the firing circuit includes a relatively larger field effect transistor connected to switch the firing pulses on and off. The smaller warming field effect transistor has a lesser current carrying capacity than the firing field effect transistor. The smaller warming field effect transistor is required to carry a lesser current and therefore is smaller to save space in the overall construction of the warming circuit.
When a particular heater has been fired rapidly during the preceding time periods, the ink will be relatively warm in that cavity and that portion of that printhead will likewise be relatively warm. Thus, assuming the temperature feedback system discussed above indicates that little warming is required, the pulse control system will designate a relatively small width for the warming pulse so that very little warming is produced. Also, the pulse control system monitors the firing of the heater resistors as well. If a particular heater resistor is firing during a print cycle, the control system will not initiate the creation of a warming pulse. In other words, the pulse control system will not produce a warming pulse and a firing pulse at the same time.
Considering the above discussion, it will be appreciated that a method is taught for warming a thermal inkjet printhead that has a heater resistor responding to firing pulses during firing cycles to nucleate ink and fire the ink from a nozzle. The method includes supplying a warming pulse to the heater resistor to warm the heater resistor and to thereby warm the printhead and further includes pulse width modulating the warming pulse to produce a pulse having a width that is sufficient to warm the heater as desired but insufficient to nucleate the ink. In this method, only one pulse width modulating warming pulse is produced during one firing cycle for one heater resistor. Further, a warming pulse is not supplied to a heater resistor that is receiving a firing pulse during a particular firing cycle.
In one embodiment of this method, the temperature of the printhead is monitored and warming pulses are supplied only as needed to raise the temperature of the printhead to a desired temperature. In this respect, the warming pulses may be modulated to have a width that will create the desired warming effect, or the warming pulses may be omitted entirely if the printhead is sufficiently warm. In this method, the current is intentionally limited in the warming pulse to a current level that is lower than the firing pulse so that the warming pulse will not nucleate the ink.
An exemplary embodiment is disclosed in the detailed description and the figures in which:
Referring now to the drawings in which like reference characters designate like or corresponding parts throughout the several views, there is shown in
The inkjet printhead 20 includes a plurality of nozzles 22 for ejecting ink out of the printhead and onto a print media. Thermal sensors 24 and 28 are mounted on the printhead 20 to sense the temperature of the printhead and sensor lines 26 and 30 connect the sensors 24 and 28 back to the thermal control circuit 14. The lines 26 and 30 may be regarded as part of the thermal control circuit 14. In operation, the temperature sensors 24 and 28 generate sensor signals that are applied back to the thermal control circuit 14 that in turn supplies control signals to the pulse control circuit 14 that are based on the temperature of the printhead 20. The thermal control circuit 14 and the pulse control circuit 18 produce a pulse width modulated signal that is applied through line 32 to the printhead and causes the printhead to produce warming pulses that warm the printhead. If the printhead is below a desired temperature, the thermal control circuit 14 will produce warming pulses having greater widths or durations so that the warming pulses have a greater warming effect on the printhead. On the other hand, if the printhead is very near the correct operating temperature, but is slightly cooler then it should be, the thermal control circuit 14 and the pulse control circuit 18 will produce warming pulses having durations (a width) that are much smaller and will have a small warming effect. The printhead 20 is rapidly firing the nozzles 22 one time per firing cycle. During a firing cycle, the pulse control circuit 18 will either produce a firing pulse to nucleate ink and eject it from a particular nozzle or the circuit 18 will produce a warming pulse to warm the printhead 20 in the vicinity of a particular nozzle that is not being fired during the firing cycle, or the circuit will produce no pulses and nothing happens.
The operation and effect of the warming pulse and the firing pulse for each nozzle may best be understood by reference to
Referring now to
Referring to
Referring again to junction 76, it is also connected to line 82 which is connected to ground 90′ through a firing field effect transistor (RPP) 92. The firing pulse is applied to transistor 92 through line 94. Line 94 is connected to an and gate 96 that represents an addressing system. When all three signals (PFIRE, A, EA) on lines 98, 100 and 102 are “on”, the and gate will cause an “on” signal to be applied to line 94 which will turn on the field effect transistor 92 (a switch) and a firing pulse will flow from power 70 through heater resistor 72 and through transistor 92 to ground 90. The and gate 96 is illustrated with three control lines or address lines attached to it. It will be understood that various different types of control and address systems may be used and that the current embodiment shows three control lines only as an illustration and there is no intent to limit this particular embodiment to any particular number of address lines or control lines. As previously mentioned, a warming pulse and a firing pulse are not generated at the same time. Thus, line 88 will not turn on at the same time as line 94. The circuit shown in
Depending on the particular design, turning “on” a line, such as lines 88 or line 94, would require the voltage on the line to go from a lower state to a higher state or from a higher state to a lower state depending on the design. By referring to “on” conditions and “off” conditions, it is intended to generalize the condition of
Referring now to
The foregoing description of preferred embodiments for this 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. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
