Abstract:
An apparatus and method for controlling temperature fluctuations between printhead dies in a multiple printhead die printer. By reducing temperature variations, changes in image intensity that are attributable to temperature variations are reduced.

Description:
This application is a continuation of Ser. No. 09/231,331 filed Jan. 13, 1999 now U.S. Pat. No. 6,322,189. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to printheads with multiple printhead dies and, more specifically, to temperature control among the multiple printhead dies to improve print quality. 
     BACKGROUND OF THE INVENTION 
     Several types of printing devices are known in the art and they include laser, dot matrix, mechanical actuated ink jet and thermal actuated ink jet printers and the like. The present invention is particularly applicable to ink jet printers and, more specifically, to thermal actuated ink jet printers. Nonetheless, it should be recognized that the effects of temperature on ink and print quality may be an issue in all types of printers (because of the coefficient of expansion of ink and other materials, among other reasons) and thus, the present invention is applicable to all printers. 
     Ink jet printheads are known that include a semiconductive substrate or “die” on which are formed a plurality of firing chambers. Ink and control signals are provided to the firing chambers for controlled expulsion of ink. In order to achieve faster printing rates, the present invention contemplates providing a plurality of these dies in a side by side arrangement or the like (thereby creating a larger ink expulsion area), and such an arrangement is termed an array or module (hereinafter referred to as an “array”). 
     When multiple dies are placed side by side to form a printhead array, however, print quality issues can arise. A principal concern stems from the performance of two neighboring dies that are operating at different temperatures. The concern usually manifests itself as a sudden change in image intensity at the interface between the dies. The change in image intensity is caused by different sized ink drops being expelled by the neighboring die because ink drop volume varies with die temperature. Thus, a need exists to provide a printhead array in which the printhead dies or the like are maintained at a more uniform temperature and thus produce ink drops of more uniform volume. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a multiple printhead arrangement that creates ink drops having an approximately uniform volume. 
     It is another object of the present invention to provide a multiple printhead arrangement in which the operating temperature of each printhead is controlled. 
     It is also an object of the present invention to provide a multiple printhead arrangement in which each of the printheads operate at approximately the same temperature. 
     These and related objects of the present invention are achieved by use of a multiple printhead apparatus with temperature control and method as described herein. 
    
    
     The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a plurality of printhead dies arranged in an array in accordance with the present invention. 
     FIG. 2 is a schematic diagram of an analog implementation of a temperature control circuit in accordance with the present invention. 
     FIG. 3 is a schematic diagram of a digital implementation of a temperature control circuit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a side view of a plurality of printhead dies (generally referred to herein as “printheads”)  11 - 13  arranged in an array  10  in accordance with the present invention is shown. While three printheads are shown in FIG. 1, it should be recognized that the present invention is applicable to any number of printheads greater than one. Each printhead includes at least one firing chamber  41  with an ink expulsion mechanism  42  such as a resistor (thermal actuation) or a piezo-electric actuator (mechanical actuation). 
     A heating element such as a resistive heating element or the like  43  is also preferably provided in each printhead. If the heating element is implemented as a resistive heating element, it may be achieved as a resistor or transistor. Suitable heating elements are generally known in the art. 
     It should be recognized that the heating element represented by reference numeral  43  illustrates the provision of a heating source within a printhead that can heat the ink to a desired temperature. If the ink expulsion mechanism  42  is a thermal actuated mechanism, it is possible that the expulsion mechanism can serve the function of ink expulsion and ink warming. Thus, expulsion mechanism  42  would provide the functions represented by reference numeral  43 . This can be achieved, for example, by sending a signal pulse that is of sufficient duration to heat ink in firing chamber  41  to a desired temperature, but not long enough to expel ink, or by sending a reduced current signal. 
     Each printhead is coupled to a shared temperature signal conductor  30 . In an analog embodiment (discussed first), it is possible for the temperature signal conductor to be a single line that propagates a voltage representative of a temperature level. In a digital embodiment (discussed further below), the temperature signal conductor is preferably a bus driven by tri-state buffer drivers. 
     Temperature control logic or circuit  50  is preferably provided in each printhead and is coupled to the temperature signal conductor. Among other functions, each control circuit is capable of sensing the signal on conductor  30  and comparing this signal with the temperature of its printhead. Depending on the outcome of this comparison, the control logic either increases the temperature of the printhead, sends a signal to other printheads to increase their temperature or does neither. Analog and digital implementations are now presented. 
     In an analog embodiment, conductor  30  is preferably analog signal line and each control circuit is configured to sense a voltage on conductor  30  that is indicative of temperature. If a given printhead is cooler than the bus temperature, than the heating element associated with that printhead is enabled. If the printhead is hotter than the bus temperature by a predefined temperature, Δ, then a voltage signal representative of the hotter temperature (minus Δ) is driven onto conductor  30  by circuit  50  of that printhead. If the printhead temperature detected at logic  50  is not greater than Δ degrees above the temperature on line  30 , then no action is taken. 
     Referring to FIG. 2, a schematic diagram of temperature control circuit  50  in accordance with the present invention is shown. Circuit  50  preferably includes a first comparator  51  that is coupled to an auxiliary heater  52  and receives inputs from a temperature sensor  53  and line  30 . Circuit  50  also contains a second comparator  61  that receives inputs from the temperature sensor (minus Δ via level shifter  63 ) and line  30 . The output of comparator  61  controls a field effect transistor  64  (preferably a PFET) or the like. 
     The comparators  51  and  61  (and the other components herein) are preferably formed within the semiconductive substrates of the printhead dies. The comparators preferably perform functions similar to commercially available LM308 devices or the like. 
     The output of comparator  51  functions as the enable for auxiliary heater  52 . The auxiliary heater may be implemented in a variety of manners which include, but are not limited to, incorporating the thermal ink expulsion mechanisms (as discussed above), formed as or supplemental to heating element  43 , or as otherwise known in the art. 
     The temperature sensor  53  is preferably implemented using a material having a resistance that varies with temperature or through band gap and junction techniques or as otherwise known in the art. Level shifter  63  is preferably implemented with a resistor and constant current source. A voltage drop of Δ may be implemented with resistive divider networks or the like. 
     In operation, comparator  51  compares the printhead temperature signal to the temperature signal on line  30 . When the printhead temperature signal is lower than the temperature control line signal, auxiliary heater  52  is enabled by comparator  51 . While the primary function of comparator  51  is to control heating of the printhead, the primary function of comparator  61  is to control the driving of an elevated or new highest temperature signal on to line  30 . If the printhead temperature signal is greater by Δ from the line temperature signal, then gate  64  is switched such that line  30  is driven by V DD  or the like until line  30  (detected through the immediate feed back loop) reaches a level that causes comparator  61  to switch off, i.e., open circuit, the driving force. 
     A voltage signal driven on to line  30  is received at the control circuits of the other printheads. A comparison similar to that discussed immediately above is undertaken by each of the control circuits of the multiple printheads and if appropriate the auxiliary heating elements for those printheads are enabled to raise printhead temperatures such that they are approximately equal to the temperature indicated on line  30 . In this manner, it is possible to create an environment in which adjacent printheads and more importantly ink within those printheads is provided at approximately the same temperature. As a result, there is significantly less variation in image intensity between the multiple printheads. 
     The use of a threshold temperature range, Δ, before an elevated or new temperature signal is driven on to line  30  prevents a positive feedback scenario in which printheads are continually heated until they reach a temperature that is too hot for proper operation. It should be recognized that conventional techniques for printhead temperature protection do exist and if a printhead threshold temperature is achieved, the printheads are simply deactivated (no firing signals are sent until they cool off). Exemplary voltage and temperature parameter include a voltage range of 1-4 V that corresponds to temperature from 20 to 100° C. Δ may be approximately 150 mV and the shut-off temperature is approximately 100° C. 
     Referring to FIG. 3, a schematic diagram of a digital implementation of a temperature control circuit  150  in accordance with the present invention is shown. The circuit of FIG. 3 is referred to with reference numeral  150 , and is intended as a substitute for circuit  50  of FIGS. 1 and 2. 
     Circuit  150  includes a comparator  151 , auxiliary heater  152 , temperature sensor  153 , and level shifter  163 , that are analogous in function to corresponding components in FIG.  2 . Circuit  150  also includes control logic  170 , a buffer driver  172 , register circuit  173  and sensed temperature register  155 . In operation, temperature is sensed by sensor  153 , converted to a digital representation by A/D converter  154  and stored in register  155 . Bus temperature is loaded from bus  30  (preferably an 8 bit bus, plus control) into register circuit  173  from which it is propagated through level shifter  163  to comparator  151 . Bus  30  in the digital implementation may be a shared bus, for example, part of the system bus (with time domain multiplexing), or a dedicated bus. Level shifter  163  subtracts an appropriate Δ and if the sensed temperature held by register  155  is less than the bus temperature minus Δ, then the auxiliary heater  152  is enabled. 
     Control logic  170  preferably includes an ID register  179  for unique identification. The control logic is preferably coupled to the control logic of the other printhead dies through control lines associated with bus  30  or through other control signal lines indicated by phantom lines  181 . The control logic control lines permit time domain multiplexing or other bus arbitration/utilization scenarios to be implemented. In a time domain multiplexing scenario, the temperatures of the other printhead dies are sequentially gated into register circuit  173  and looked at by control logic  170 . Each new temperature that is gated in is compared to the preceding value and the hottest temperature is preferably retained. During the bus control interval for the printhead of FIG. 3, control logic  170  enables driver  172  which drives the temperature signal from register  155  via conductor  178  onto the bus. Control logic  170  also outputs an enable signal via conductor  176  to comparator  151  which is active when the output of comparator  151  is valid. It should be recognized that while control logic  170  is represented as being formed within a particular printhead die in FIG. 3, the control logic and related logic could alternatively be provided on an off-die processor or elsewhere. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.