Abstract:
In an embodiment, a method for aligning ink ejecting elements in an image forming device is provided. A reference pattern is printed onto a first portion of a print medium by a first ink injecting element, and an offset pattern is printed onto a second portion of the print medium by a second ink injecting element. The first portion of the print medium coincides with the second portion, and a combined pattern is formed from the reference pattern and the offset pattern. A portion of the combined pattern is scanned to generate a first response. Print medium noise, which corresponds to a thickness variation of the print medium, is removed from the first response to form a second response. The second ink ejecting element is aligned to the first ink ejecting element based on the second response.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to printing devices, and more particularly, to a method and system for aligning ink ejecting elements of an image forming device.  
       BACKGROUND OF THE INVENTION  
       [0002]     Printing devices, in particular inkjet printing devices such as printers, plotters, photocopiers or facsimile machines, typically use one or more inkjet cartridges or “pens” for printing. During a printing operation, a print medium, for example paper, is advanced (“paper-axis”) and the pen is scanned in a direction orthogonal to the paper axis (“scan-axis”). As the pen is scanned over the paper, drops of ink are shot onto the paper. The ink drop direction from the pen to the paper is referred to as the “swath axis”.  
         [0003]     Each pen includes a printhead which normally has columns of ink nozzles. The nozzles fire drops of ink onto the paper to create printed dots as the pen is scanned across the paper. Each nozzle is used to address a particular vertical column position on the paper. Horizontal positions on the paper are addressed by repeatedly firing the nozzle as the pen is scanned across the paper. Each position is referred to as a pixel.  
         [0004]     A color printer has more than one pen of different colors. The pens are mounted in stalls within a carriage assembly of the printer. To print a desired color on a specific pixel location, drops of ink are fired from a corresponding nozzle of each pen onto the specific pixel location to obtain the desired color.  
         [0005]     High resolution printing requires that drops of ink from each nozzle of the pens be precisely applied on to the paper. This requires precise alignment of the nozzles in each pen and also between the different pens in the printer. However, mechanical misalignment of the nozzles and the pens results in offsets of the drops of ink printed on the paper. The offset of the ink printed on the paper results in printed images to be distorted. Mechanical misalignment is often due to tolerances and variations of mechanical parts of the pen and the printer, which typically includes physical location of the nozzles, curvature of the platen of printhead, height of the pen from the paper, spacing between the pens, and nozzle shape.  
         [0006]     Other types of misalignment resulting in offset of the ink printed on the paper include drop placement errors due to firing timing of ink from the nozzles and directional errors due to movement of the printhead or pen (Scan Axis Directionality or “SAD” error) or movement of the paper (Paper Axis Directionality or “PAD” error).  
         [0007]     Printers normally align their inkjet pens to correct any misalignment of the pens. Usually a group of nozzles of an inkjet pen is taken to be a reference group of nozzles, and other groups of nozzles are aligned to the reference group of nozzles. Similarly for pen-to-pen alignment, one of the pens is taken to be a reference pen, and the other pens are aligned to the reference pen. For a color printer, a black color pen is usually taken to be the reference pen.  
         [0008]     A conventional method for pen alignment includes printing a reference pattern using the reference group of nozzles or the reference pen. A test pattern is subsequently printed in a predetermined relation with the reference pattern using the other groups of nozzles or pens. A user visually inspects the relation between the two patterns and determines a respective offset value to align the pens. It is also possible to scan the reference pattern and the test patterns using an optical scanner to determine the respective offset value. In this case, the pen alignment is performed automatically instead of a manual inspection of the patterns by the user.  
         [0009]     A known method for automatic pen alignment includes printing a series of test patterns on a single sheet of paper. The series of test patterns are optically readable and allow misalignment errors to be detected.  
         [0010]     A more accurate method for performing automatic pen alignment compared to the known methods is desired  
       SUMMARY OF THE INVENTION  
       [0011]     In an embodiment, a method for aligning ink ejecting elements in an image forming device is provided. A reference pattern is printed onto a first portion of a print medium by a first ink injecting element, and an offset pattern is printed onto a second portion of the print medium by a second ink injecting element. The first portion of the print medium coincides with the second portion, and a combined pattern is formed from the reference pattern and the offset pattern. A portion of the combined pattern is scanned to generate a first response. Print medium noise, which corresponds to a thickness variation of the print medium, is removed from the first response to form a second response. The second ink ejecting element is aligned to the first ink ejecting element based on the second response. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The embodiments of the invention will be better understood in view of the following drawings and the detailed description.  
         [0013]      FIG. 1  shows a flow chart for aligning a group of test nozzles to a group of reference nozzles according to an embodiment of the invention.  
         [0014]      FIG. 2A  shows a reference pattern printed by reference nozzles of a pen.  
         [0015]      FIG. 2B  shows two combined patterns printed separately using two different color pens.  
         [0016]      FIG. 3  shows a reference pattern and 3 different combined patterns for a misalignment of 0, +1 and −1.  
         [0017]      FIG. 4  shows a graphical representation of the reflectivity of the combined patterns of four different color pens.  
         [0018]      FIG. 5  shows a graphical representation of a difference in reflectivity of a cyan pen obtained from a difference between the reflectivity of the reference pattern and the combined pattern according to an embodiment of the invention.  
         [0019]      FIG. 6  shows a graphical representation of the difference in reflectivity of 4 different color pens according to an embodiment of the invention.  
         [0020]      FIG. 7  shows a flow chart for aligning a test pen to a reference pen according to an embodiment of the invention.  
         [0021]      FIG. 8  shows a combined pattern printed using two pens having different colors.  
         [0022]      FIG. 9  shows a graphical representation of the reflectivity of 3 different color pens obtained from scanning the respective combined pattern of each pen.  
         [0023]      FIG. 10  shows a graphical representation of the difference in reflectivity of 3 different color pens according to an embodiment of the invention.  
         [0024]      FIG. 11  shows a graphical representation of two responses which are obtained from the summation of a series of edge response obtained by scanning the respective combined patterns of  FIG. 2B .  
         [0025]      FIG. 12  shows a graphical representation of  FIG. 11  with both responses scaled to the same size. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     An embodiment of the invention is illustrated using a printer, in particular, an inkjet color printer. The inkjet color printer includes one or more inkjet cartridges or pens of different color. Each pen has columns of nozzles for ejecting droplets of ink onto a print medium such as paper. The colors of the pens include black, cyan, magenta and yellow. It is also possible to use other color pens for the printer.  
         [0027]     In one embodiment, one group of nozzles of an inkjet pen (referred to as test nozzles) is aligned to another group of nozzles of the same inkjet pen (referred to as reference nozzles). This is called Intra-Pen alignment. Although the Intra-Pen alignment is described as aligning a group of test nozzles to a group of reference nozzles, it is also possible to align a single test nozzle to a single reference nozzle in another embodiment.  
         [0028]      FIG. 1  shows a flow chart of an embodiment for Intra-Pen alignment. Step  101  includes printing a reference pattern on a paper using the reference nozzles of a black pen.  
         [0029]     Step  102  includes scanning the reference pattern using an optical scanner to determine a reflectivity of the reference pattern. The optical scanner uses a blue light emitting diode (LED). Other color LEDs may be used in the optical scanner in other embodiments. The shape or thickness variation of the portion of the paper which the reference pattern is printed on also affects the reflectivity of the reference pattern detected by the optical scanner.  
         [0030]     Step  103  includes printing an offset pattern on the paper using the test nozzles. The offset pattern coincides with the reference pattern to form a combined pattern. Step  104  includes scanning the combined pattern using the optical scanner to detect the reflectivity of the combined pattern.  
         [0031]     Step  105  includes obtaining a difference in reflectivity between the reference pattern and the combined pattern. The difference in reflectivity can be obtained by subtracting the reflectivity of the combined pattern from the reference pattern.  
         [0032]     Step  106  includes aligning the test nozzles to the reference nozzles based on the difference in reflectivity between the reference pattern and the combined pattern. The aligning of the test nozzles to the reference nozzles will be described in detail later.  
         [0033]     After aligning the test nozzles to the reference nozzles, it is determined at step  107  whether all the nozzles in the inkjet pen are aligned. If not all the nozzles are aligned to the reference nozzles, steps  101  to  106  are repeated to align another group of test nozzles. If all the nozzles are aligned, the aligning of the nozzles of the pen is complete. Steps  101  to  106  may be repeated to align the nozzles of another pen.  
         [0034]     Although the scanning of the combined pattern is described to be performed directly after printing each combined pattern, it is also possible to print all the combined patterns for all the groups of test nozzles and pens, and subsequently, reverse the paper to scan all the combined patterns.  
         [0035]      FIG. 2A  shows the reference pattern  201  printed by the reference nozzles. The reference pattern  201  includes a series of blocks  202  of evenly spaced vertical lines  203 . The offset pattern (not shown separately) is also a series of blocks of evenly spaced lines. The number of blocks of the reference pattern and the offset pattern is determined by a desired alignment range of the nozzles.  
         [0036]     The centre block of the offset pattern is intended to align completely with the centre block of the reference pattern  201 . Two blocks of the offset pattern which are adjacent to the centre block of the offset pattern are shifted by one column with respect to the respective corresponding adjacent blocks of the reference pattern  201 . The two adjacent blocks of the offset pattern are shifted in a direction away from the centre block. Additionally, two further adjacent blocks of the offset pattern are shifted by two columns with respect to the respective corresponding further adjacent blocks of the reference pattern  201  in the direction away from the centre block.  
         [0037]     The combined patterns  204 ,  210  for two different color pens are shown in  FIG. 2B . The combined centre block  205  shows a completely aligned pattern in the case when there is no misalignment of the nozzles of the pens. The two blocks  206  adjacent to the centre block  205  show an offset of 1 column between the corresponding adjacent blocks of the reference pattern and the offset pattern. The two further adjacent blocks  207  show an offset of 2 columns between the corresponding further adjacent blocks of the reference pattern and the offset pattern.  
         [0038]     It can be seen that when a block of the offset pattern is completely aligned with a block of the reference pattern, the corresponding combined pattern is the least dense compared to the case when a block of the offset pattern is offset from a block of the reference pattern. This is because the lines of the two blocks completely overlap each other, and hence, have a maximum amount of white space between them. When the offset between the blocks increases, the amount of overlapping of the lines, and hence, the amount of white space decreases. This also results in an optical density of the combined block to increase. The increase in the optical density of the combined blocks translates to a decrease in reflectivity.  
         [0039]      FIG. 3  illustrates a few examples of combined patterns. The reference pattern  221  printed by the reference nozzles comprises five blocks of evenly spaced lines. The offset pattern is printed by the test nozzles on the reference pattern  221  to form the combined pattern  222 . The reference pattern  221  is represented by solid vertical lines  223  and the offset pattern is represented by dotted vertical lines  224 .  
         [0040]     When the test nozzles are completely aligned with the reference nozzles, the centre block  225  of the combined pattern  222  is the best aligned block. However, when the test nozzles have misalignment of one column to the right (misalignment value of +1), the adjacent block  226  to the left of the centre block  225  becomes the best aligned block of the combined pattern  227 . In this case, the test nozzles are aligned to be reference nozzles by offsetting the test nozzles by one column to the left (offset value of −1).  
         [0041]     When the test nozzles have misalignment of one column to the left (misalignment value of −1), the adjacent block  228  to the right of the centre block  225  becomes the best aligned block of the combined pattern  229 . In this case, the test nozzles are aligned to the reference nozzles by offsetting the test nozzles by one column to the right (offset value of +1).  
         [0042]      FIG. 4  shows a graphical representation of the reflectivity of the combined patterns of the four different color (black, yellow, magenta and cyan) pens  300 ,  301 ,  302 ,  303 . The reflectivity of the paper  304  is also shown in the same graph. The best aligned block (having the highest reflectivity) for the black pen  300  and the yellow pen  301  can be detected easily by the optical scanner. However, the best aligned block for the magenta pen  302  and the cyan pen  303  is obscured by the variation in the paper reflectivity due to the variation of the paper thickness. Another reason that the detectability of the reflectivity of the magenta and cyan pens  302 ,  303  is difficult is due to the use of the blue LED in the optical scanner. The color of blue is very close to the color of magenta and cyan, making the magenta and cyan ink difficult to detect.  
         [0043]     By obtaining the difference in reflectivity between the reference pattern and the combined pattern, the variation in the paper reflectivity is removed.  FIG. 5  shows a graphical representation of the reflectivity of the reference pattern printed by the cyan pen  401 , the reflectivity of the combined pattern printed by the cyan pen  402 , the difference in reflectivity  403  between the combined pattern  402  and the reference pattern  401 , and the reflectivity variation due to paper  404 .  
         [0044]     It can be seen that the difference in reflectivity  403  between the combined pattern  402  and the reference pattern  401  of the cyan pen is a slight V-shape curve. The shape of the curve can be seen more evidently by scaling the curve.  
         [0045]      FIG. 6  shows a graphical representation of the difference in reflectivity between the combined pattern and the reference pattern of the black, yellow, magenta and cyan color pens  405 ,  406 ,  407 ,  408 . The block corresponding to the best aligned block can now be detected for each pen. The best aligned block for each pen corresponds to the lowest value of the difference in reflectivity.  
         [0046]     It is also possible to represent the difference in reflectivity of one or more pens using a suitable curve. A suitable curve would be a second order polynomial curve. The block corresponding to a minimum point of the second order polynomial curve is determined as the best aligned block for the pen.  
         [0047]     Although it has been described that the reflectivity variation of the paper is removed by obtaining the difference in reflectivity between the combined pattern and the reference pattern, it is also possible to remove the reflectivity variation of the paper by first scanning the paper reflectivity and removing it subsequently.  
         [0048]     In another embodiment, the test nozzles of each pen are aligned to the reference nozzles of the same pen by first scanning the paper to determine the reflectivity variation of the paper. The reference pattern and the offset pattern are printed by the reference nozzles and the test nozzles, respectively, to form the combined pattern. A difference in reflectivity between the combined pattern and the paper is obtained. Finally, the test nozzles are aligned to the reference nozzles. based on the obtained difference in reflectivity between the combined pattern and the paper.  
         [0049]     In another embodiment, one inkjet pen (referred to as test pen) is aligned to another inkjet pen (referred to as reference pen). This type of alignment is called Inter-Pen or Pen-to-Pen alignment.  
         [0050]      FIG. 7  shows a flow chart of an embodiment for Inter-Pen alignment. Step  701  includes printing a reference pattern on the paper using the reference pen. Step  702  includes scanning the reference pattern using the optical scanner to determine the reflectivity of the reference pattern. Step  703  includes printing an offset pattern on the paper by the test pen. The offset pattern coincides with the reference pattern to form a combined pattern. Step  704  includes scanning the combined pattern using the optical scanner to detect the reflectivity of the combined pattern.  
         [0051]     Step  705  includes obtaining a difference in reflectivity between the reference pattern and the combined pattern. The difference in reflectivity can be obtained by subtracting the reflectivity of the combined pattern from the reference pattern. Step  706  includes aligning the test pen to the reference pen based on the difference in reflectivity between the reference pattern and the combined pattern. The test pen is aligned to the reference pen in the same way of aligning the test nozzles to the reference nozzles of an inkjet pen.  
         [0052]     Step  707  includes determining whether all the inkjet pens are aligned. If not all the pens are aligned to the reference pen, steps  701  to  706  are repeated for another pen as the test pen.  
         [0053]     Similarly, it is also possible to print all the combined patterns for all the pens, and subsequently, reverse the paper to scan all the combined patterns.  
         [0054]      FIG. 8  shows an example of a combined pattern printed by the reference pen and the test pen. The solid vertical lines  801  represent the reference pattern, and the dotted vertical lines  802  represent the offset pattern. The centre block  803  of the offset pattern is intended to completely align the centre block of the reference pattern. When the test pen is completely aligned to the reference pen, the centre block  803  corresponds to the best aligned block of the combined pattern.  
         [0055]      FIG. 9  shows a graphical representation of the reflectivity of the combined patterns of the different color test pens: yellow  901 , magenta  902  and cyan  903 . The reflectivity of the paper  904  is also shown in the same graph. The best aligned block for the magenta pen  902  and the cyan pen  903  are obscured by the reflectivity variation of the paper  904 , but can be detected easily after obtaining the difference in reflectivity in step  705  described earlier.  
         [0056]      FIG. 10  shows a graphical representation of the difference in reflectivity for each test pen: yellow pen  911 , magenta pen  912  and cyan pen  913 . It can be seen that the shape of the difference in reflectivity for each pen is evident, and the best aligned block of the combined patterns corresponding to each test pen can be determined. The difference in reflectivity for each pen may also be represented using a suitable curve, in particular, a second order polynomial curve.  
         [0057]     Similarly, it is also possible to remove the reflectivity variation of the paper by first scanning the paper reflectivity, and removing it subsequently. In another embodiment, the test pen is aligned to the reference pen by first scanning the paper to determine the reflectivity variation of the paper. The reference pattern and the offset pattern are printed by the reference pen and the test pen, respectively, to form the combined pattern. A difference in the reflectivity between the combined pattern and the paper is obtained. Finally, the test pen is aligned to the reference pen based on the obtained difference in reflectivity between the combined pattern and the paper.  
         [0058]     In the above-described embodiments, the optical scanner scans the reference patterns and the combined patterns to detect the optical density of the patterns. Therefore, the scanning of the reference patterns and the combined patterns is not limited by the resolution of the optical scanner, which is normally at 600 dpi (dots per inch). Accordingly, the patterns can be printed at a high resolution such as at 1200 dpi or even 2400 dpi on coated media, and alignment of the nozzles and pens can be performed at the printed resolutions without any extrapolations.  
         [0059]     Furthermore, the embodiments as described above allow a high resolution alignment process to be implemented using a low-cost printer. This is because only a low-cost single-color LED optical sensor instead of a multi-color LED optical sensor is needed in the optical scanner of the printer for the high resolution alignment process.  
         [0060]     The accuracy of the alignment process described in the above embodiments can be further improved by printing the reference patterns and the offset patterns over the same area several times. This increases the optical density of the patterns, and hence, results in greater contrast between the reflectivity of the patterns and the paper. Also, the patterns can be printed over a large portion of the paper to average out the reflectivity variations due to the thickness variation of the paper.  
         [0061]     The causes of misalignment between inkjet pens include carriage mounting, vibration due to carriage movement, carriage speed, manufacturing tolerance and printhead seating. Such misalignments could be large. Accordingly, a pre-alignment stage is performed to pre-align the test nozzles/pen to the reference nozzles/pen in an embodiment to increase the alignment range for aligning the test nozzles/pen to the reference nozzles/pens. Therefore, large misalignments of the nozzles/pens can be detected and corrected.  
         [0062]     In an embodiment for pre-aligning the cyan pen to the black pen, the combined pattern  204  printed by the black pen and the combined pattern  210  printed by the cyan pen, as shown in  FIG. 2B , are scanned in a first step. It is to be noted in this case that the Intra-Pen alignment is normally performed prior to the Inter-Pen alignment. Therefore, the combined patterns  204 ,  210  of the black and cyan pens are already printed on the paper. If the combined patterns  204 ,  210  are not printed, they can be printed on the paper prior the pre-alignment stage.  
         [0063]     The scanning of the combined patterns  204 ,  210  detects the edges of the blocks in the patterns to form a series of pulses for each pen. Each pulse corresponds to a block in the combined pattern. In a second step, all the pulses for each pen are summed to form a “super-bar”.  FIG. 11  shows a graph depicting two super-bars corresponding to the black pen  921  and the cyan pen  922 .  
         [0064]     In a third step, the two super-bars are scaled to a same scale for easy comparison. The scaled super-bars are shown in  FIG. 12 . It can be seen from the graph of  FIG. 12  that there is an offset between the scaled superbars of the black pen  923  and the cyan pen  924 . The amount of misalignment between the two pens is determined based on the offset between the two scaled superbars  923 ,  924 , and the cyan pen is pre-aligned to the black pen accordingly. After pre-aligning the cyan pen to the black pen, the alignment process as described by the flowchart of  FIG. 7  is performed to align the cyan pen to the black pen.  
         [0065]     Aliasing effects may affect the accuracy of the alignment process described in the above embodiments. Assuming that the lines of each block of the reference patterns and offset patterns are spaced 10-column apart, a determined misalignment of 1 column of the test nozzles/pen may in fact be 11 columns, 21 columns, 31 columns and so on. This is called aliasing effect. Aliasing effects are normally assumed to be negligible. The detection of aliasing effect according to an embodiment can be illustrated with an example for aligning two pens.  
         [0066]     The reference pattern and offset pattern are printed with the lines in each block evenly spaced at 10-column apart. The misalignment of the test pen is assumed to be determined as +1. The test pen is offset by a value of −1accordingly to be aligned to the reference pen. A new reference pattern and a new offset pattern are then printed with the lines in each block evenly spaced at 11-column apart to form a new combined pattern. The misalignment of the test pen is again determined based on the new combined pattern.  
         [0067]     If the actual misalignment of the test pen is +1, the test pen would be completely aligned to the reference pen when being offset by −1. Hence the misalignment determined based on the new combined pattern will be zero. However, if the actual misalignment of the test pen is +11, the test pen would still be misaligned from the reference pen by +10 even when offset by −1. Hence, when the new reference pattern is printed (e.g. at column number 0, 11, 22, etc), the new offset pattern would be 1 column to the left of the new reference pattern (i.e. at column number −1, 10, 21, etc). Therefore, the misalignment determined based on the new combined pattern would be −1. Similarly in the case when the actual misalignment of the test pen is +21, the misalignment determined based on the new combined pattern would be −2. Accordingly, the aliasing effects can be detected based on the misalignment determined from the new combined pattern.  
         [0068]     In an embodiment, a further reference pattern and a further offset pattern are printed by the test nozzles/pen and the reference nozzles/pen, respectively, to form a further combined pattern. The lines in the blocks of the further reference pattern and the further offset pattern are evenly spaced at a distance different from that of the reference pattern and the offset pattern. An offset value is determined based on the further combined pattern. The determined offset value is used to determine the misalignment of the test nozzles/pen from the reference nozzles/pen, and hence, the amount of offset required to align the test nozzles/pen.  
         [0069]     Different types of print media have different ink absorption characteristics. The quality of paper also affects its ability to hold ink. A good quality paper is able to retain ink well, and ink printed on a poor quality paper may diffuse on the paper. Therefore, thinner lines are normally used for printing on poor quality paper as compared to printing on good quality paper to prevent the diffusion of ink to fill up the gaps between the lines.  
         [0070]     In an embodiment, a most misaligned combined pattern with varying line thickness is printed on a paper. The most misaligned combined pattern is scanned using an optical scanner to determine a threshold thickness value when the gaps between the lines are filled up, that is when a reading from the optical scanner becomes constant. On a white paper, when the white space (or the gap) between the lines are large, the reading from the optical scanner is high. However, when the white space decreases (due to the use of thicker lines), the readings from the optical scanner decreases. When all the white spaces are filled up, the reading from the optical scanner becomes constant. The thickness of the lines when the reading of the optical scanner decreased to a constant value is the determined threshold thickness value. Based on the determined threshold thickness value, the optimal line thickness for printing on the paper is determined accordingly.  
         [0071]     Although the present invention has been described in accordance with the embodiments as shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.