PATENT ABSTRACT
A thermal inkjet printer is provided. The printer has a sensor that detects the operating temperature of its printhead. If the temperature of the printhead is below the printhead&#39;s normal operating temperature when the printer is going to start to print an image or document, the operating temperature of the printhead is set at a temperature higher than its normal temperature. This is to ensure that the drop-volume of the printer stays at an optimum level when the printer is beginning to start to print the image or document after a period of non-use. Shortly after the printer has started the printing task, the operating temperature of the printhead is reduced to its normal operating temperature.

PATENT DESCRIPTION
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to a thermal inkjet printer; and more generally, to an optimum initial operating temperature for a thermal inkjet printer.  
         BACKGROUND OF THE INVENTION  
         [0002]    In designing a thermal inkjet printer, it is important to provide as economically and simply as possible a relatively high output quality at a relatively high speed. The output quality and relative speed of a thermal inkjet printer are often times a function of the startup operating temperature of the printhead, especially after a period of non-use.  
           [0003]    For example, conventional thermal inkjet printers contain multiple inkjet nozzles. Associated with each nozzle is a heating resistor and a drive transistor. The nozzle includes a nozzle chamber within which the heating resistor is located. To fire ink from the nozzle chamber, the drive transistor outputs a firing pulse to the heating resistor. The firing pulse is a current pulse of a magnitude sufficient enough to heat up the resistor and thus the ink to an ejection temperature. The ink then ejects from the chamber toward a print media sheet. To determine when any given nozzle is to fire, a controller circuit is used.  
           [0004]    Typically, existing printers use a single print head operating temperature throughout the duration of printing a document. If this temperature is set too high, then a variety of longer term reliability issues can occur such as ink plugs in the nozzles, material degradation in the print head, or ejection of overly concentrated colorant from evaporation of the ink vehicle thought the nozzles. If this temperature is set too low, then there can be significant initial short term reliability issues with getting the print head to reliably fire when first called upon to do so. What is needed is high initial ejection reliability of high initial operating temperatures combined with the improved long term reliability afforded by lower operating temperatures for the duration of an image.  
           [0005]    In certain printers, to maximize reliable ink drop ejections, the ink is pre-heated. However, to pre-heat the ink when the printer is not is use would result in a waste of energy and ink as the ink will thicken or be reduced through evaporation. Furthermore, because of ink evaporation, pre-heating the ink during a long period of non-use may damage the printhead. For all these obvious reasons, therefore, the resistors are not pre-heated if the printer is not in use.  
           [0006]    It is well known in the industry that one of the problems associated with thermal inkjet printers concerns the amount of ink ejected or deposited from the printhead during the formation of each ink drop. The quantity of deposited ink, commonly referred to as the “drop-volume” of the printhead, is dependent on the temperature of the printhead. If the printhead is cool, it will deposit less ink in each droplet. Missing, weak or low drop-volume results in poor quality images that appear faint or washed out. Consequently, when a printer has gone through a period of non-use or the printhead is cool, a certain amount of firing time is required to allow the printhead to reach its optimum drop-volume. This is usually accomplished by having the nozzles spit or eject low drop-volume ink droplets into a spittoon. Obviously, this scheme fosters ink wastage and a longer printing time.  
           [0007]    Therefore, what is needed is a method to facilitate a thermal inkjet printer to reach its optimum drop-volume from a period of non-use as quickly as possible while minimizing ink wastage.  
         SUMMARY OF THE INVENTION  
         [0008]    To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a printing system for improving the edge sharpness, color uniformity, banding and faint or washed out appearance of ink drops produced by an inkjet printer.  
           [0009]    The need in the art is addressed by the present invention. The present invention provides a thermal inkjet printer with the requisite technology to increase or reduce its operating temperature. The printer uses a sensor to detect the operating temperature of its printhead. If the temperature of the printhead is below the printhead&#39;s default or normal operating temperature when the printer is going to start to print an image or document, the operating temperature of the printhead is set at a temperature higher than its default or normal temperature.  
           [0010]    This is to ensure that the drop-volume of the printer stays at an optimum level when the printer is starting to print the image or document after a period of non-use. Shortly after the printer has started the printing task, the operating temperature of the printhead is reduced to its default normal operating temperature. The higher temperature depends on the probability of successful ejection of the nth drop. Satisfactory image quality depends on all drops to have the proper volume, velocity and directionality.  
           [0011]    The present invention as well as a more complete understanding thereof will be made apparent from a study of the following detailed description of the invention in connection with the accompanying drawings and appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention can be further understood by reference to the following description and attached drawings that illustrate the preferred embodiment. Other features and advantages will be apparent from the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.  
         [0013]    [0013]FIG. 1 depicts a block diagram of an inkjet printer connected to a workstation.  
         [0014]    [0014]FIG. 2 illustrates particular aspects of the printer and the workstation.  
         [0015]    [0015]FIG. 3 is a perspective view of the inkjet printer.  
         [0016]    [0016]FIG. 4 depicts a thermal inkjet printhead and a printhead controller.  
         [0017]    [0017]FIG. 5 illustrates one of a plurality of nozzles used in the present invention.  
         [0018]    [0018]FIG. 6 is a schematic diagram of a nozzle circuitry associated of the present invention.  
         [0019]    [0019]FIG. 7 is a schematic diagram of the power control circuit  648 .  
         [0020]    [0020]FIG. 8 illustrates a chart of temperature versus time of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    In the following description of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration a specific example in which the invention may be practiced. Other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
         [0022]    I. General Overview  
         [0023]    The present invention ensures reliable ejection of an optimum ink drop-volume as quickly as possible after a period of printer non-use. This is done by momentarily setting the temperature of the printhead at a temperature much higher than its standard operating temperature. After a certain period of time, the operating temperature of the printhead is reduced to its default or normal or standard operating temperature. In the present invention, the default or normal operating temperature of the printhead is 55 degrees Celsius and the higher temperature is 75 degrees Celsius.  
         [0024]    II. Detailed Operation of the Invention  
         [0025]    With reference now to the figures, FIG. 1 depicts a block diagram of an inkjet printer  110  connected to a workstation  120 . This invention may also be implemented in other types of printers, such as bubble jet printers. Further, although the invention is described in the context of printers, it may also be used in conjunction with other image reproduction systems such as copiers, scanners and the like.  
         [0026]    As is well known in the field, the workstation  120  has at least one processor  210  to process data, including printing data. The workstation  120  also has a system memory  220  (e.g., RAM) that holds data that is to be immediately used by the processor  210  and a storage system  230  (e.g., ROM, hard disk, floppy disk, CD-ROM etc.) to store application programs. One such application program is a printer driver that is used to control the printer  110 .  
         [0027]    The printer  110  itself has a processor  250 , a volatile memory  260  (e.g., RAM) and a non-volatile memory  270  (e.g., ROM, flash etc.). The processor  250  is used to control all moving mechanical parts of the printers as well as to heat up and to fire the nozzles. Just as in the case of the workstation  120 , the volatile memory  260  is used to hold data for the immediate use of the processor  250 . The non-volatile memory  270  is used to store, among other programs, the present invention.  
         [0028]    However, before delving into the present invention, a brief description of an inkjet printer is needed. FIG. 3 is a perspective view of the inkjet printer  110 . The printer  110  has an input tray  310  containing sheets of print medium which pass through a printing zone and along a print medium advance direction  320 , past an exit  330  into an output tray  340 . Electronics control  350  for commanding the processor  250  to perform various functions are included.  
         [0029]    A movable carriage  360  holds print cartridges  22 ,  24 ,  26  and  28  which respectively hold yellow (Y), magenta (M), cyan (C) and black (B) inks and dispense these inks upon command from the processor  250 . The back of the carriage  360  has multiple bushings (not shown) which ride along a slide rod  370 , enabling bidirectional movement of the carriage along the rod  370 .  
         [0030]    The carriage  360  thus moves along a carriage scanning direction  2 , above a sheet of print medium upon which an image is being formed by print cartridges  22 - 28 . The position of the carriage  360 , as it traverses the print medium back and forth, is determined by an encoder strip  380 . This very accurate positioning device enables selective firing of the various ink nozzles on each print cartridge at the appropriate times during each carriage scan to form the image.  
         [0031]    With each scan or swath pass of the carriage  360 , the print medium is advanced incrementally in the direction  320  along the print medium axis. These incremental advances allow for an image or document to be printed on a media sheet.  
         [0032]    [0032]FIG. 4 depicts a thermal inkjet printhead  410  and a printhead controller  411 . The printhead  410  includes a plurality of nozzles  412  and is part of an inkjet pen (not shown) used for printing ink onto a media sheet. Note that although two columns of nozzles, many more can be used and would be well within the scope of the present invention. Along with the nozzles, a temperature sensor  428  is shown. The temperature sensor is used to measure the temperature of the printhead  410 . The printhead controller  411  is connected to printhead  410  and monitors the temperature sensor  428 .  
         [0033]    [0033]FIG. 5 illustrates one of a plurality of nozzles used in the present invention. As shown in FIG. 5, each nozzle includes a nozzle chamber  516  for holding ink  511  and a heating resistor  518 . In operation, the heating resistor  518  receives a firing pulse from drive transistor  520  causing the heating resistor  518  to heat up the ink  511  in the chamber  516  to ejection temperature in order to eject the ink through orifice  524 . For each nozzle, there is a corresponding nozzle chamber  516 , heating resistor  518 , drive transistor  520  and heating transistor  526 . Although two transistors are used (one to preheat and one to drive resistor  518 ), the use of one transistor is perfectly within the scope of the present invention. In that case, the one transistor can fire less pulse current to pre-heat resistor  518  and more pulse current to drive resistor  518 .  
         [0034]    [0034]FIG. 6 is a schematic diagram of the nozzle circuitry associated with a given nozzle  412 . The heating resistor  518  is coupled to a nozzle voltage source  640  at one contact point and to the drains of the drive transistor  520  and warming transistor  526  at another contact point. The drive transistor  520  is formed by one or more power field effect transistor (FET) devices  642 . In the embodiment illustrated six FETs  642   a - 642   f  formed the drive transistor  520 . The warming transistor  526  is formed by a smaller FET device  644 .  
         [0035]    The drains of the FET devices  642  and  644  are coupled in common to the heating resistor  518  via an interconnect  643 . The sources of the devices  642  and  644  are coupled in common to ground  646 . The gates M 1 -M 6  of the FET devices  642   a - 642   f  are coupled to a power control circuit  648  which receives the firing control signal  532 . The gate M 7  of the warming transistor device  644  is coupled to the printhead controller  411  for receiving the warming control signal  530 .  
         [0036]    [0036]FIG. 7 is a schematic diagram of the power control circuit  648 . The power control circuit  648  is formed by a set of current booster circuits. A firing control signal is received from the printhead controller  411 . The signal is boosted to generate a signal  750  input to the gates M 1 -M 6  of the drive transistor devices  642 . In the illustrated embodiment, the power control circuit includes eight FET devices  752 - 766  and an inverter  768 .  
         [0037]    The firing control signal  532  is active when a logic low is received at the power control circuit  648 . The logic low is inverted at inverter  768  resulting in a logic high signal  750  output from the power control circuit  648  into the gates M 1 -M 6  of the drive transistor devices  642 . Referring again to FIG. 6, the gates M 1 -M 6  allow current flow through the devices  642 . Specifically, current flows from the nozzle voltage source  640  through the heating resistor  518  into the drains  72   a - 74   f  to ground  46 . When an inactive signal (e.g., a logic high) is received at power control circuit  648 , signal  750  is a logic low. Thus, the junction from drain to source at drive transistor devices  642   a - 642   f  is closed.  
         [0038]    When an active signal level is received at the warming transistor device  644 , gate M 7  enables current flow through the device  644 . Specifically, current floes from the nozzle voltage source  640  through the heating resistor  518  into the drain  82  and out through the source  84  of the warming transistor  644  to ground  646 . When an inactive signal level is received at the gate M 7  of the warming transistor device  644 , the junction from drain  82  to source  84  is closed.  
         [0039]    The warming control signal  530  and the firing control signal  532  are separate signals having separate signal paths. To generate a warming pulse, the firing control signal  532  is inactive and the warming control signal is active. Thus, a small current flows from the nozzle voltage source  640  through the heating resistor  518  into the drain  82  and out the source  84  of the warming transistor  644  to ground  646 . The current flowing through the heating resistor  518  is based upon the size of the transistor device  644 . Such current is insufficient to cause the nozzle  412  to fire. Warming transistor device  644  is used as a switching device turning the current flow through the device  644  on or off. The current magnitude for a warming pulse may be between 2.0 and 3.5 mA; and the nozzle voltage around 21 volts.  
         [0040]    To generate a firing pulse, the warming control signal  530  is inactive and the firing control signal is active. Thus, current flows from the nozzle voltage source  640  through the heating resistor  518  into the drains  72   a - 72   f  and out of the source  74   a - 74   f  to ground  646 . The current flowing through the heating resistor  518  is based upon the number and size of the transistor devices  642   a - 642   f . Such current is enough to cause a nozzle  412  to fire. The current magnitude for a firing pulse may be around 300 mA and the nozzle voltage source around 21 volts.  
         [0041]    Obviously, other voltage and current levels may be used in alternative embodiments. Furthermore, to fire a nozzle  412  both a firing signal  532  and a warming signal  530  may be active so that current flows from the nozzle voltage source  640  through the heating resistor  518  and through all the devices  642  and device  644  to ground  646 .  
         [0042]    When both the firing control signal  532  and the warming control signal  530  are inactive, current does not flow through the devices  642  and  644 . Consequently, current does not flow through the heating resistor  518 .  
         [0043]    Returning back to FIG. 4 and FIG. 5, when a given nozzle  412  is to be fired, the controller  411  sends a firing control signal  532  to drive transistor  520  for such nozzle  412 . Further, as the controller  411  monitors temperature sensor  428 , if it detects that the temperature of the printhead falls below a threshold temperature, the controller  411  generates a warming control signal  530  for one or more nozzles  412  to bring the printhead temperature back to the operating temperature. In the present invention, the printhead operating temperature is around 55 degrees Celsius.  
         [0044]    When the printer is not in use, the printhead temperature will fall below the operating temperature of 55 degrees Celsius. It will continue to fall until it reaches ambient temperature, which often is room temperature (around 25 degrees Celsius). When a printhead starts at that temperature, it often requires a certain number of spits before optimum drop-volume can be reliably achieved. In an experiment, it was shown that if the printhead temperature is brought to the 55 degrees Celsius operating temperature from a period of non-use, at least 10 spits (this number depends on the printer) were needed before the optimum drop-volume was achieved. It was also shown that if the printhead temperature is brought to 75 degrees Celsius, zero spits was needed to obtain the optimum drop-volume. Thus, 75 degrees Celsius seems to be an ideal start-up temperature for the printhead.  
         [0045]    However, having the printhead operate continually at that high of a temperature can foster reliability issues such as material incompatibility. Furthermore, the higher temperature may foster faster water evaporation (in the case of a water based ink) through the nozzles which ultimately may cause ink plugs. Thus, after the initial start-up temperature of 75 degrees Celsius, the temperature of the printhead should be reduced to the optimum 55 degrees Celsius operating temperature. In that experiment it was shown that if the temperature of the printhead was reduced to 55 degrees Celsius after 5 to 500 ink droplets (this number depends on the inkjet printer), no problems with reliability issues or ink plugs ensued.  
         [0046]    In the present invention, therefore, the printhead controller  411  of FIG. 4 is designed to bring the initial temperature of the printhead  414  momentarily to 75 degrees Celsius and then to reduce the printhead operating temperature to 55 degrees Celsius. The 75 degrees Celsius temperature allows for a more efficient ink ejection (i.e., grams of ink per uJ of energy). This efficient ink ejection eliminates ink plugs and chamber bubbles. Consequently, the time for nozzle recovery is significantly reduced.  
         [0047]    [0047]FIG. 8 illustrates a chart of temperature versus time of the present invention. Dashed line  810  is the control temperature line and solid line  820  is the actual printhead temperature line. Note that the control time for the higher temperature can vary anywhere from 10 msec to 1 sec. In this figure, the higher temperature is set at 75 degrees Celsius and the default or normal operating procedure is set at 55 degrees Celsius, but both temperatures can vary. This variation may be dependent upon a particular printer.  
         [0048]    IV. Conclusion  
         [0049]    The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Therefore, the foregoing description should not be taken as limiting the scope of the invention defined by the appended claims.  
         [0050]    The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. As an example, the above-described inventions can be used in conjunction with inkjet printers that are not of the thermal type, as well as inkjet printers that are of the thermal type. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.