Abstract:
An image reproduction system with requisite technology to reduce or eliminate altogether banding effects is provided. The system includes an input tray for storing print media, an output area for holding printed media and an inkjet pen or printhead for printing information on the print media. In a preferred embodiment, the reproduction system is inkjet printer whose pen scans and ejects ink on the print media with a non-uniform dot pitch. This non-uniformity is achieved by adding visual noise in a known and predictable fashion to the uniform dot pitch.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to an inkjet printer; and more generally, to a method and system for reducing banding effects in such printers.  
         BACKGROUND OF THE INVENTION  
         [0002]    In designing a 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 of the printer is a function of its printhead resolution (i.e., the ability to resolve or separate, often visually, two image elements or details). The finer the resolution, the better the print quality.  
           [0003]    The resolution of the printhead of an inkjet printer is directly related to its nozzle pitch (i.e., distance between adjacent nozzles on the printhead). This distance is typically designed to be uniform. This generally translates into having a uniform dot pitch (i.e., distance between adjacent dots in a printout). One disadvantage of a uniform dot pitch is the inability to hide printing errors. For example, to print on a print medium, nozzles of the inkjet printer eject tiny droplets of ink, or dots, during each horizontal pass of the printhead over the print medium to form a row of dots. (Each horizontal pass of a printhead over a print medium is called a swath.) After each preceding swath, the print medium is incrementally advanced to allow room for the next row of dots. Through a succession of rows of dots, images or letters are printed on the print medium.  
           [0004]    Each dot has a uniform diameter and is placed at a uniform distance from each other on the print medium and each preceding row of dots is placed at the same distance from the succeeding row of dots. Consequently, any two adjacent rows of dots are at the same distance from each other. Hence, there is a uniform band of white space between any two adjacent rows of dots. It should be noted that dot placement in present day inkjet printers is consistent enough that this uniform band of white space is maintained even in more than one pass print mode.  
           [0005]    In any case, any small dot placement errors and dot shape variations that impinge on this band of white space create a hue or saturation shift. This hue or saturation shift is often referred to as banding since it exhibits itself as a visible band. This phenomenon is more discernible when printing graphic images.  
           [0006]    One way to address banding effects is to eliminate altogether print mechanism errors such as feed errors, nozzle to nozzle dot placement errors, shape variations and the like that foster dot placement errors and dot shape variations. To do so, however, requires the use of very sophisticated programming techniques and precisely engineering mechanical parts. This is an expensive endeavor which is often times reflected in the price of the printer thus built. Therefore, instead of attempting to eliminate these print mechanism errors, a less expensive method is to distribute these errors in such a way as to minimize their conspicuousness (i.e., to hide the errors).  
           [0007]    One method that has been used to distribute these errors is to increase the number of swath passes. That is, since sophisticated programming techniques and precisely engineering mechanical parts are not used, line feed errors, nozzle to nozzle dot placement errors, shape variations etc. do occur, albeit very minimally, at each swath pass. The higher the number of passes, therefore, the more prevalent these errors and; consequently, the less uniform the dot pitch (i.e., more and more of the dots appear to impinge on the band of white space). After a certain number of passes (this number is dependent on a particular style of printer), so much of the band of white space is impinged upon, that it ceases to exist. Banding is thus effectively eliminated.  
           [0008]    There are a few disadvantages associated with the above-described method. For example, as is obvious, the higher the number of swath passes that are used in printing a page, the slower the printer. Furthermore, the placement of the dots in the band of white space is unpredictable.  
           [0009]    Accordingly, it would be beneficial to develop a method that distributes mechanical printing errors in a known and predictable fashion over the band of white space without affecting printer speed and cost.  
         SUMMARY OF THE INVENTION  
         [0010]    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 quality of ink drops produced by an inkjet printer.  
           [0011]    The need in the art is addressed by the present invention. The present invention provides an inkjet printer with the requisite technology to reduce or eliminate altogether banding effects. The inkjet printer includes an input tray for storing print media, an output area for holding printed media and an inkjet pen or printhead for printing information on the print media. In a preferred embodiment, the inkjet pen scans and ejects ink on the print media with a non-uniform dot pitch. This non-uniformity is achieved by adding visual noise in a known and predictable fashion to the uniform dot pitch. 
       
    
    
     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 of the present invention.  
         [0018]    [0018]FIG. 6 is a schematic diagram of the nozzle circuitry associated with a given nozzle.  
         [0019]    [0019]FIG. 7 is a schematic diagram of a power control circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    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.  
         [0021]    I. General Overview:  
         [0022]    The present invention reduces print quality and image quality sensitivity to dot placement errors and dot shape variations in an inkjet printer by deliberately introducing low spatial frequency dot placement variations, heretofore referred to as red noise, across the printhead of the system. The red noise is introduced by sinusoidally varying the dot pitch of the printer. The sinusoidal introduction of red noise across the printhead has the effect of distributing dots in the print medium axis (i.e., axis of print medium advance) and thus fills in the band of white space between dot rows. This makes the system more robust to dot placement and shape variations.  
         [0023]    II. Detailed Operation of the Invention  
         [0024]    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 thermal printers, bubble jet printers etc. 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.  
         [0025]    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 .  
         [0026]    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. 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.  
         [0027]    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.  
         [0028]    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 .  
         [0029]    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.  
         [0030]    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 the distribution of dots in the band of white space.  
         [0031]    [0031]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. Although two columns of nozzles are displayed, more columns of nozzles may be used. 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 .  
         [0032]    [0032]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 pre-heat 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 .  
         [0033]    [0033]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 .  
         [0034]    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 .  
         [0035]    [0035]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 .  
         [0036]    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.  
         [0037]    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.  
         [0038]    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.  
         [0039]    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.  
         [0040]    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 .  
         [0041]    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 .  
         [0042]    As mentioned above, the invention includes sinusoidally varying the dot pitch of the inkjet printer. This sinusoidal variation of the dot pitch introduces a low frequency of dot placement variations. The sinusoidal introduction of the red noise is achieved by altering the location of each ink drop on the media from each nozzle. This can be accomplished, for example, by a constant nozzle pitch and varying trajectory, with constant trajectory and varying nozzle pitch or some combination thereof. Ink drops from one nozzle can be retarded (i.e., trajectory is altered such that the ink drop falls on the print media at a location lower than where they would have normally fallen on the print media) or advanced (i.e., trajectory is altered such that the ink drop falls on the print media at a location higher than where they would have normally fallen on the print media).  
         [0043]    There are many ways of altering the trajectory of the ink drops. For example, the nozzle pitch itself may be altered (e.g., each nozzle may be placed at a non-uniform location to ensure that ink droplets from the nozzle are placed at the desired location on the print media). The alteration of the trajectory may be further accomplished by systematically varying bore concentricity, resistor orifice misalignment (i.e., distance between resistor and nozzle orifice), counterbore misalignment (e.g., locating each nozzle not concentric with the bore exit) etc., or by any combination thereof. In the present invention, the trajectory of the ink drops is altered by using a non-uniform nozzle pitch.  
         [0044]    It should be noted that the term concentricity as used above refers to a geometrically relative location of the centroids of the entrance and the exit of the bore. Also, the term counterbore misalignment as used above refers to locating the counterbore (usually a circular feature) such that it is not concentric with the bore exit. The counterbore misalignment does not relate to the depth of the counterbore.  
         [0045]    In the present invention, a 3 um amplitude sine wave is superimposed over a uniform dot pitch of 42 um. A 42 um dot pitch translates into a 600 dots per inch (dpi) pitch. The period of the sine wave is chosen to be 256 to match the number of nozzles used in the print mode of interest. However, the period need not match the number of nozzles. When the period of deviation is matched with the number of nozzles used, each dot row is printed using nozzles with a variety of deviations from the average dot pitch. This ensures uniformity of tone and maximizes the benefit of the invention.  
         [0046]    As the deviation from the average dot pitch varies continuously, the distance between adjacent nozzles varies only slightly across the printhead. This is highly desirable since it will not create defects in print modes that use only a small number of passes.  
         [0047]    III. Advantages of the Invention  
         [0048]    The advantages of the present invention are many. First, it introduces visual noise into an image in a known and predictable fashion, allowing for the signature of that noise to be engineered along with the printing system. Second, the invention does not require additional passes to eliminate banding effects. For example, it has been shown that relying on line feed errors, nozzle to nozzle dot placement errors, shape variations etc. requires a twelve-pass print mode to achieve an equivalent white space saturation of the eight-pass print mode of the present invention. Hence, the present invention, in some instances, enhances the speed of printers that have decreased or eliminated banding effects. Third, since the use of the invention covers the white space creating the banding sensitivity without increasing dot size, grain is not increased.  
         [0049]    IV. Conclusion  
         [0050]    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.  
         [0051]    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.