Abstract:
A system is described for compensating for misalignments in an ink jet printer having an ink jet print head cartridge that includes a heater chip. The system includes determining alignment adjustment information related to the misalignments in the ink jet printer, loading the alignment adjustment information into a volatile memory device on the heater chip, and accessing the alignment adjustment information from the volatile memory device. The system also includes generating nozzle control signals based at least in part on the alignment adjustment information. The nozzle control signals are selectively provided to resistive heating elements in the heater chip, thereby heating ink in ink chambers adjacent the heating elements and ejecting ink droplets toward a print medium. The timing of the nozzle control signals is adjusted based upon the amount of misalignment in the various components of the printer and print head. The timing adjustments are applied to groups of nozzles so that dots printed by one group are substantially vertically aligned with dots printed by another group, thereby reducing the amount of perceptible skew in the printed output.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is generally directed to ink jet printers. More particularly, the invention is directed to a system for improving print quality by compensating for misalignment or skew between various components in an ink jet printer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many ink jet printers form printed images on a print medium by ejecting droplets of ink from ink nozzles on a print head as the print head is scanned across the print medium. Ink droplets are formed and ejected from the nozzles when the ink is super-heated by resistive heating elements disposed on a heater chip in the print head. Typically, the print head rides on a carriage that scans the print head horizontally across the print medium to print a swath of the image. At the end of a swath, the print medium is advanced by the width of the swath, and the print head is again scanned across the print medium to print the next swath of the image.  
           [0003]    Typically the nozzles on the print head form an array that is aligned perpendicular to the scan direction. The length of the array generally defines the width of the swath. If the nozzle array is not perfectly perpendicular to the scan direction, visible print defects may occur at each swath-to-swath boundary in the printed image. This problem is more pronounced as nozzle counts and swath widths increase.  
           [0004]    Several factors contribute to misalignment between the nozzle array and the scan direction. These include misalignments between the heater chip and the body of the print head cartridge, and between the print head cartridge and the carriage rail.  
           [0005]    This problem has been addressed mechanically by attempting to maintain manufacturing tolerances to keep misalignments within an acceptable range. However, this approach requires expensive precision components and equipment to manufacture both the print head and the carriage. Prior attempts at electronic timing adjustments to compensate for the misalignment have proven to be cost prohibitive and size prohibitive due to large amounts of logic required per nozzle.  
           [0006]    Therefore, a system is needed for adjusting the timing of ink ejection from nozzles or groups of nozzles in a manner that reduces swath-to-swath skew to an imperceptible level, while taking into account mechanical, electrical, fluid flow, and cost restraints.  
         SUMMARY OF THE INVENTION  
         [0007]    The foregoing and other needs are met by a method for compensating for misalignments in an ink jet printer having an ink jet print head cartridge that includes a heater chip. The method includes determining alignment adjustment information related to the misalignments in the ink jet printer, loading the alignment adjustment information into a volatile memory device on the heater chip, and accessing the alignment adjustment information from the volatile memory device. The method also includes generating nozzle control signals based at least in part on the alignment adjustment information. The nozzle control signals are selectively provided to resistive heating elements in the heater chip, thereby heating ink in ink chambers adjacent the heating elements and ejecting ink droplets toward a print medium.  
           [0008]    The timing of the nozzle control signals is adjusted based upon the amount of misalignment in the various components of the printer and print head. Preferably, the timing adjustments are applied to groups of nozzles so that dots printed by one group are substantially vertically aligned with dots printed by another group, thereby reducing the amount of perceptible skew in the printed output.  
           [0009]    Preferred embodiments of the method include the steps of storing heater chip alignment information in a print head memory device on the ink jet print head cartridge, and storing print head alignment information in a printer memory device in the ink jet printer. In these embodiments, the alignment adjustment information is determined based at least in part on the heater chip alignment information stored in the print head memory device and the print head alignment information stored in the printer memory device.  
           [0010]    In another aspect, the invention provides an ink jet printer for forming printed images on a print medium based on print data. The printer includes a carriage that is movable in a first direction relative to the print medium, and an ink jet print head cartridge mounted on the carriage. The print head cartridge includes a cartridge housing that is mechanically coupled to the carriage, where the cartridge housing is oriented with respect to the carriage according to a print head alignment angle. The cartridge also includes an ink jet heater chip oriented with respect to the cartridge housing according to a heater chip alignment angle. The ink jet heater chip has an array of resistive ink-heating elements, and a heater chip memory device for receiving alignment adjustment information. The print head cartridge further includes a print head memory device for storing heater chip alignment information related to the heater chip alignment angle. An array of ink-ejection nozzles is provided on the print head cartridge corresponding to the array of ink-heating elements.  
           [0011]    The printer includes a printer controller having a printer memory device for storing print head alignment information related to the print head alignment angle. The printer controller incorporates control electronics that are electrically coupled to the heater chip memory device, the print head memory device, and the printer memory device. The control electronics access the print head memory device to retrieve the heater chip alignment information, access the printer memory device to retrieve the print head alignment information, determine the alignment adjustment information based at least in part on the heater chip alignment information and the print head alignment information, and provide the alignment adjustment information to the heater chip memory device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the drawings, which are not to scale, wherein like reference characters designate like or similar elements throughout the several drawings as follows:  
         [0013]    [0013]FIG. 1 depicts misalignments between an ink jet heater chip, an ink jet print head cartridge, a printer carriage, and a carriage rail in an ink jet printer;  
         [0014]    [0014]FIG. 2 is a functional block diagram of an ink jet printer which electronically compensates for misalignments between various components in the printer according to a preferred embodiment of the invention;  
         [0015]    [0015]FIG. 3 is a functional block diagram of an ink jet printer which electronically compensates for misalignments between various components in the printer according to an alternative embodiment of the invention;  
         [0016]    [0016]FIG. 4 depicts memory devices, logic circuits, and nozzle groups used in electronically compensating for misalignments between various components in a printer according to a preferred embodiment of the invention;  
         [0017]    [0017]FIG. 5 depicts a logic circuit for adjusting the timing of nozzle select signals according to a preferred embodiment of the invention;  
         [0018]    [0018]FIG. 6 is a functional flow diagram of a method for compensating for misalignments between various components in an ink jet printer according to a preferred embodiment of the invention; and  
         [0019]    [0019]FIG. 7 is a functional flow diagram of a method for compensating for misalignments between various components in an ink jet printer according to an alternative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 illustrates the problem addressed by the present invention. As shown in FIG. 1, an ink jet print head cartridge  12  is attached to a carriage  11  which rides along a rail  13 . Due to mechanical imperfections in various mating surfaces of the carriage  11  and the print head  12 , the print head  12  and the carriage  11  may be misaligned. The misalignment between the carriage  11  and the print head  12  may be characterized by a print head alignment angle φ PH . Due to mechanical imperfections in the attachment of the carriage  11  to the rail  13 , the carriage  11  and the rail  13  may also be misaligned. The misalignment between the carriage  11  and the rail  13  may be characterized by a carriage alignment angle φ C . On the print head  12  is an ink jet heater chip  14  which contains an array of ink heating elements associated with an array of ink ejection nozzles  15 . The heater chip  14 , and consequently the array of nozzles  15 , may be misaligned relative to the print head  12  as indicated by the heater chip alignment angle φ HC .  
         [0021]    [0021]FIG. 1 also depicts a pair of images  11  and  12  printed by the print head  12  during two passes of the print head  12  across a print medium. The upper portion of each image I 1  and I 2  is printed as part of a first print swath SW 1 , and the lower portion of each image I 1  and I 2  is printed as part of a second print swath SW 2 . Image I 1  is printed with no compensation for the various misalignments between the carriage  11 , print head  12 , and heater chip  14 . Due to the various misalignments, the dots formed by the ink droplets are not vertically aligned. Rather, the dots are skewed from vertical according to a misalignment or skew angle that is the sum of φ C , φ PH , and φ HC . Due to this skew, there is a substantial discontinuity where the upper and lower portions of the image I 1  meet.  
         [0022]    Image I 2  is printed with compensation applied according to a preferred embodiment of the invention. As described in more detail below, the invention adjusts the timing of ejection of ink droplets for groups of the nozzles  15  to minimize the visually perceptible effect of the skew.  
         [0023]    Shown in FIG. 2 is a functional block diagram of a preferred embodiment of an ink jet printer  10  which implements skew control to cure the problem depicted in image I 1  of FIG. 1. The printer  10  includes the print head  12  containing the heater chip  14 . As described in more detail below, the heater chip  14  includes logic circuits, resistive heating elements, and driver devices for driving the heating elements. The heater chip  14  also includes a memory device  16 , such as volatile random access memory registers, for storing skew adjustment data. Although the memory  16  of the preferred embodiment is volatile memory, it will be appreciated that the memory  16  could also be a non-volatile memory device. The print head  12  preferably includes non-volatile memory  18  for storing skew adjustment information related to the skew angle φ HC .  
         [0024]    Within the printer  10  is a printer controller  20  that receives print data, such as from a host computer, formats the print data for each print swath, and provides the print data to the print head  12 . The controller  20  includes control electronics  22  that, among other things, format the print data and calculate skew adjustment data, as described below. The controller  20  preferably also includes non-volatile memory  24  for storing skew adjustment information related to the skew angles φ PH  and φ C . It will be appreciated by those skilled in the art that printer controller  20 , including its control electronics  22  and non-volatile memory  24 , may alternatively be locally or remotely associated with the host computer.  
         [0025]    According to a preferred embodiment of the invention as depicted in the block diagram of FIG. 2 and the flow diagram of FIG. 6, during or after the manufacture of the print head  12 , a measurement is made to characterize the alignment angle φ HC  between the heater chip  14  and the print head  12 . A value, such as an angular value corresponding to the heater chip alignment angle φ HC , is then stored in the nonvolatile memory device  18  on the print head  12  (step  100 ). Similarly, during the manufacture of the printer  10 , measurements are made to characterize the misalignment angle φ C  between the rail  13  and the carriage  11 , and the misalignment angle φ PH  between the carriage  11  and the print head  12 , respectively. Values, such as angular values corresponding to the carriage and print head alignment angles φ C  and φ PH , are then stored in the nonvolatile memory device  24  in the printer controller  20  (step  102 ).  
         [0026]    In the preferred embodiment, when the printer  10  is powered on, the controller  20  accesses the data stored in the print head memory device  18  related to the heater chip alignment angle φ HC  (step  104 ), and accesses the data stored in the printer memory device  24  related to the carriage and print head alignment angles φ C  and φ PH  (step  106 ). The controller  20  then determines the skew adjustment data based on the heater chip alignment angle φ HC , the carriage alignment angle φ C , and the print head alignment angle φ PH  (step  108 ).  
         [0027]    In an alternative embodiment of the invention, user feedback is utilized to determine an optimum value of misalignment compensation to be applied. According to this embodiment, as depicted in FIGS. 3 and 7, the printer  10  prints a plurality of test images on a test page  26  (step  200 ). For each test image, a different value of alignment adjustment is applied, corresponding to different amounts of angular misalignment between the heater chip  14  and the rail  13 . The user  28  observes the test images printed on the test page  26  (step  202 ), and selects at least one of the test images as most visually appealing in comparison with the other test images (step  204 ). The user  28  then enters the selection of the most appealing test image into the host computer  30 , preferably by entering a number in a dialog box corresponding to the selected test image.  
         [0028]    Based on the selected test image, the host computer  30  determines the value of alignment adjustment that was applied while printing the selected test image (step  206 ). This optimum value of alignment adjustment is then stored in a printer memory device (step  208 ), preferably the nonvolatile memory device  24  associated with the printer controller  20 . Since it is preferably stored in nonvolatile memory, this alignment adjustment value is available each time the printer  10  is powered on. Thus, the test page procedure need not be performed each time the printer  10  is turned on, but is preferably performed each time a new print head  12  is installed in the printer  10 .  
         [0029]    Based on the optimum value of alignment adjustment stored in the memory  24 , when a printing task is initiated, the printer controller  20  calculates skew adjustment information that includes compensation for the misalignment (step  210 ). Preferably, this skew adjustment information is loaded into the volatile memory device  16  on the ink jet heater chip  16  (step  212 ).  
         [0030]    The skew adjustment information determined during the user feedback procedure depicted in FIG. 3 preferably takes into account misalignments between the rail  13  and the carriage  11 , between the carriage  11  and the print head  12 , and between the print head  12  and the heater chip  14 . Thus, the procedure determines one alignment adjustment value to compensate for all of these misalignment components. Since this embodiment requires only one nonvolatile memory device to store the skew adjustment information, that memory device could be the device  24  located in the printer body or could be the device  18  located on the print head  12 .  
         [0031]    Depicted in FIG. 4 are the memory registers  16 , nozzle select logic circuits NS, and print enable logic circuits PE provided on the heater chip  14  to select and enable particular heating elements to cause ejection of ink from selected ones of  320  nozzles  15  which are preferably divided into eight nozzle groups NG 1 -NG 8 . Within each nozzle group NG 1 -NG 8  of the preferred embodiment are two nozzle blocks NB D , where there are preferably twenty nozzles  15  per nozzle block NB D . As shown in FIG. 4, the selection and activation of particular heating elements is based upon signals provided on M number of address lines AM, D number of print data lines PD, and N number skew adjust data lines SN. In the preferred embodiment of the invention, there are five address lines A 1 -A 5  (M=5), sixteen print data lines P 1 -P 16  (D=16), and twenty-four skew adjust data lines S 1 -S 24  (N=24). It should be appreciated, however, that the invention is not limited to any particular number of address lines, print data lines, skew adjustment data lines, nozzle blocks, nozzle groups, or nozzles.  
         [0032]    The memory device  16  of FIG. 4 preferably consists of eight 3-bit data registers R 1 -R 8 , with one corresponding to each of eight nozzle groups NG 1 -NG 8 . Each of the eight registers R 1 -R 8  is loaded from X number of the N number skew adjust data lines SN, and the skew adjustment data is stored in the registers R 1 -R 8  until the printer power is turned off (step  110  of FIG. 6). In the preferred embodiment of the invention, X is equal to three. As shown in FIG. 4, the skew adjustment data bits from the registers R 1 -R 8  are provided to the nozzle select logic NS where they are used to modify the address data provided on the address lines AM.  
         [0033]    The nozzle select logic NS preferably includes eight nozzle select logic circuits NS 1 -NS 8 , an exemplary one of which, NS 1 , is depicted in detail in FIG. 5. In the preferred embodiment of the invention, each of the other circuits NS 2 -NS 8  are identical in structure and function to circuit NS 1 . As shown in FIGS. 4 and 5, the three bits of skew adjust data S 1 -S 3  are loaded from the memory register R 1  (step  112  of FIG. 6), and the three bits of address data on the address lines A 3 -A 5  are received (step  114 ) and logically added to the three skew adjustment data bits (step  116 ) in an addition logic circuit  32  to provide adjusted address bits SA 3 -SA 5 . The address bits on the address lines A 1 -A 2  and the adjusted address bits SA 3 -SA 5  are then provided to the decode circuit  34  (step  118 ). The decode circuit  34  decodes the five address bits A 1 , A 2 , SA 3 , SA 4 , and SA 5  to set a logic high signal on one of twenty nozzle select lines NSL 1   1 -NSL 1   20  (step  120 ).  
         [0034]    Note that in this embodiment, the carry information from the addition operation is lost. Because the carry information is lost, the data manipulation in the controller  20  is somewhat complicated, but straightforward in its implementation. Other implementations of this logic will be apparent to those skilled in the art, such as those in which the address data or skew adjust data are not encoded, or are partially encoded.  
         [0035]    In an alternative embodiment of the invention, the circuit  32  of FIG. 5 is a subtraction logic circuit for logically subtracting the three bits of skew adjust data S 1 -S 3  from the three bits of the address data on address lines A 3 -A 5 . As with the previously-described embodiment, the difference data bits SA 3 , SA 4 , and SA 5  are combined with the address bits A 1  and A 2  in the decode circuit  34  to select one of the twenty nozzle select lines NSL 1   1 -NSL 1   20 . With this embodiment, the nozzle timing adjustment is in the opposite direction from that of the previous embodiment, but the overall effect is the same. Note that the borrow information is lost from the subtraction operation.  
         [0036]    Referring again to FIGS. 4 and 5, the print data, which is preferably fully decoded, is provided on the sixteen print data lines P 1 -P 16  to the print enable logic block PE, where the data lines P 1 -P 16  are distributed to the corresponding sixteen print enable logic circuits PE 1 -PE 16  (step  122  of FIG. 6). The nozzle select lines NSL 1   1 -NSL 20  are provided to the print enable logic circuits PE 1 , and PE 2 , the nozzle select lines NSL 2   1 -NSL 2   20  are provided to the print enable logic circuits PE 3  and PE 4 , and so forth. In the print enable logic block PE 1 , bits on the nozzle select lines NSL 1   1 -NSL 1   20  are logically ANDed with data on the print data line P 1  to generate nozzle control signals on lines NC 1   1 -NC 1   20  (step  124 ). Similarly, in the print enable logic block PE 2 , the bits on the nozzle select lines NSL 1   1 -NSL 1   20  are logically ANDed with data on the print data line P 2  to generate nozzle control signals on lines NC 2   1 -NC 2   20 . The twenty nozzle control signals on the lines NC 1   1 -NC 1   20  are provided to the nozzle block NB 1  to control twenty heating elements, and the twenty nozzle control signals on the lines NC 2   1 -NC 2   20  are provided to the nozzle block NB 2  to control another twenty heating elements (step  126 ). The forty nozzles in the nozzle blocks NB 1  and NB 2  comprise the nozzle group NG 1 .  
         [0037]    Thus, in the preferred embodiment, three skew adjust data bits, such as on adjust data lines S 1 , S 2 , and S 3 , are used to adjust the timing of the forty nozzle control signals in a single nozzle group, such as NG,. The number of bits of skew adjustment data per group determines the timing adjustment step size. For example, a single bit cuts the normal nozzle timing in half, two bits cuts it by a factor of four, three bits by a factor of eight, and so on. In the preferred embodiment of the invention, nozzle addressability in the horizontal (scan) axis is 300 dots per inch (dpi), and there are three skew adjustment bits (X=3) per nozzle group NG, which provides for 2400 dpi (or about 10 micron) adjustment steps. Thus, the eight nozzle groups NG 1 -NG 8  of the preferred embodiment provide a total adjustment range of about 80 microns ({fraction (1/300)} inch).  
         [0038]    Since the skew adjust data may change which nozzle is selected within a nozzle block, the timing of the print data must be adjusted accordingly. The adjustment of the print data timing preferably takes place in the printer control electronics  22  (FIGS. 2 and 3). In an alternative embodiment, the skew adjustment data is provided to the host computer  30  (FIG. 3), and the adjustment of the print data preferably takes place therein.  
         [0039]    Some print head heater chips have a center-fed ink via with columns of nozzles on either side of the via. For such heater chips, the invention may be used to independently control the timing of each nozzle column. For example, an entire nozzle column could be treated as a nozzle group, and the adjustment data may be used solely for the purpose of controlling timing to account for the horizontal separation between columns.  
         [0040]    It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings that modifications and/or changes may be made in the embodiments of the invention. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of preferred embodiments only, not limiting thereto, and that the true spirit and scope of the present invention be determined by reference to the appended claims.