Abstract:
A system including a control module and a vibration generator. The control module is configured to communicate with a printhead having a plurality of nozzles to eject ink onto a print medium to create an image on the print medium. The control module is configured to detect a malfunction of a first nozzle of the plurality of nozzles. The malfunction of the first nozzle causes ink not be ejected onto a first portion of the print medium. The vibration generator, in response to the malfunction of the first nozzle, is configured to vibrate the printhead synchronously with at least one of (i) a timing of firing of the plurality of nozzles and (ii) a speed of the print medium. Ink is at least partially ejected by a second nozzle of the plurality of nozzles, which is adjacent to the first nozzle, onto the first portion of the print medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims is a continuation of U.S. patent application Ser. No. 12/017,872, filed Jan. 22, 2008, which claims the benefit of U.S. Provisional Application No. 60/886,231, filed Jan. 23, 2007. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to printing systems, and more particularly to compensating printing malfunctions by mechanical dithering of printing mechanisms. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Inkjet printers eject liquid ink through printhead nozzles to form characters and graphics on a medium such as paper. Printheads of inkjet printers are either scanning-type or page-wide array (PWA) type.  FIGS. 1A and 1B  help explain operational differences between scanning-type and PWA-type printheads.  FIGS. 2A and 2B  show arrangements of nozzles of scanning-type and PWA-type printheads, respectively.  FIGS. 3A and 3B  show exemplary inkjet printing systems that use scanning-type and PWA-type printheads, respectively. 
     Referring now to  FIGS. 1A and 1B , a scanning-type printhead  10  and a PWA-type printhead  18  are shown, respectively. In  FIG. 1A , the scanning-type printhead  10  is typically mounted on a set of rails (not shown) that are parallel to a printhead axis  12 . The scanning-type printhead  10  reciprocally slides on the rails along the printhead axis  12  across a width of paper  14 . While the scanning-type printhead  10  scans across the width of the paper  14 , the paper  14  is held stationary, and ink droplets are ejected on the paper  14  through nozzles (not shown) to print a desired image. When the scanning-type printhead  10  has completed a sweep, the paper  14  is moved along a medium axis  16  that is perpendicular to the printhead axis  12 , and the scanning-type printhead  10  begins a next sweep. During the next sweep, the scanning-type printhead  10  may print data on a new portion of the paper  14  and/or a portion where data was printed during a prior sweep. 
     In  FIG. 1B , the position of the PWA-type printhead  18  is generally fixed. The PWA-type printhead  18  is as wide as the desired print area along the width of the paper  14 . The paper  14  moves along the medium axis  16  that is perpendicular to the PWA-type printhead  18 . Typically, the paper  14  moves under the PWA-type printhead  18  only in one direction as shown. While the paper  14  moves, ink droplets are ejected through nozzles (not shown) of the PWA-type printhead  18  on the paper  14  to print a desired image. Once a portion of the paper  14  has moved under the PWA-type printhead  18  and data is printed on the portion, the PWA-type printhead  18  cannot print again on that portion of the paper  14 . 
     Referring now to  FIGS. 2A and 2B , exemplary arrangements of nozzles of the scanning-type printhead  10  and the PWA-type printhead  18  are shown, respectively. The resolution of inkjet printers depends on factors including the arrangement of nozzles on printheads. For the purposes of this discussion, only a portion  11  (shown in  FIG. 1A ) of the scanning-type printhead  10  and a portion  19  (shown in  FIG. 1B ) of the PWA-type printhead  18  are enlarged and shown in  FIGS. 2A and 2B , respectively. 
     In  FIG. 2A , nozzles  20  of the scanning-type printhead  10  are arranged in columns  22 . The scanning-type printhead  10  that can print a plurality of colors (e.g., black (BL), cyan, magenta, and yellow (YL)) may have one or more columns  22  of nozzles  20  per color. The number of columns  22  per color may vary depending on the resolution desired. For example, one column  22  per color may provide a resolution of 300 dots-per-inch (dpi). To obtain resolutions greater than 300 dpi (e.g., 600 dpi, 1200 dpi), more columns  22  may be added. For example, a resolution of 600 dpi may be obtained with two columns  22 , a resolution of 1200 dpi may be obtained with three columns  22 , and so on. 
     For a particular color, each additional column  22  is offset (indicated by dotted line  24 ) relative to other column or columns  22  for that color. Ink droplets ejected from nozzles  20  of C mutually offset columns  22  land closer together on the paper  14  than ink droplets ejected from nozzles  20  of (C-1) columns  22  thereby increasing the resolution, where C is an integer greater than 1. The number of nozzles  20  per column  22  may vary depending on the dimensions of the scanning-type printhead  10 . The nozzles  20  may have a diameter “d.” Typically, the diameter is 1 mil (i.e., 1/1000 th  of an inch or 25.4 microns). 
     In  FIG. 2B , the nozzles  20  of the PWA-type printhead  18  are arranged in rows  26 . In some implementations, the rows  26  may be as long as the desired print area along the width of the paper  14 . Depending on the desired resolution, the PWA-type printhead  18  may have one or more rows  26  of nozzles  20  per color. For each color, the rows  26  may be offset relative to one another as indicated by dotted line  28 . 
     Referring now to  FIGS. 3A and 3B , exemplary inkjet printing systems that use the scanning-type printhead  10  and the PWA-type printhead  18  are shown. In  FIG. 3A , an exemplary inkjet printing system  50  that uses the scanning-type printhead  10  is shown. The inkjet printing system  50  comprises a printer control module  52  having a host interface  54 , a medium control system  56 , and a printhead/ink control system  58 . The printer control module  52  controls the operation of the inkjet printing system  50  via the medium control system  56  and the printhead/ink control system  58 . The printer control module  52  communicates with a host (not shown) via the host interface  54 . 
     The medium control system  56  comprises a medium control module  60 , medium axis motor  62 , a medium roller  64 , and a medium diagnostic module  66 . The medium control module  60  communicates with the printer control module  52  and controls the motion of the medium (e.g., the paper  14 ) by controlling the medium axis motor  62 . The medium axis motor  62  moves medium roller  64  that moves the paper  14  along the medium axis  16 . The medium diagnostic module  66  diagnoses any faults in the medium axis motor, detects problems with the movement of the medium roller  64 , and detects paper jams. The medium diagnostic module  66  reports error-codes to the medium control module  60 . 
     The printhead/ink control system  58  comprises a printhead control module  68 , a printhead axis motor  70 , the scanning-type printhead  10 , a printhead diagnostic module  72 , an ink control module  74 , and ink supply  76 . The printhead control module  68  communicates with the printer control module  52  and controls the motion of the scanning-type printhead  10  by controlling the printhead axis motor  70 . The printhead axis motor  70  moves the scanning-type printhead  10  along the printhead axis  12 . Additionally, the printhead control module  68  generates nozzle firing signals that fire or activate the nozzles  20  of the scanning-type printhead  10 . The printhead control module  68  controls the firing or activation of the nozzles  20  by controlling the timing of the nozzle firing signals. 
     The printhead diagnostic module  72  diagnoses any problems in the scanning-type printhead  10  and reports errors including any malfunctioning nozzles to the printhead control module  68 . The ink control module  74  communicates with the printer control module  52  and controls the supply of ink to the scanning-type printhead  10  from the ink supply  76 . 
       FIG. 3B  illustrates an exemplary inkjet printing system  100  that uses the PWA-type printhead. The inkjet printing system  100  comprises a printer control module  102  having a host interface  104 , a medium control system  106 , and a printhead/ink control system  108 . The printer control module  102  controls the operation of the inkjet printing system  100  via the medium control system  106  and the printhead/ink control system  108 . The printer control module  102  communicates with a host (not shown) via the host interface  104 . 
     The medium control system  106  comprises a medium control module  110 , medium axis motor  112 , a medium roller  114 , and a medium diagnostic module  116 . The medium control module  110  communicates with the printer control module  102  and controls the motion of the medium (e.g., the paper  14 ) by controlling the medium axis motor  112 . The medium axis motor  112  moves medium roller  114  that moves the paper  14  along the medium axis  16 . The medium diagnostic module  116  diagnoses any faults in the medium axis motor, detects problems with the movement of the medium roller  114 , and detects paper jams. The medium diagnostic module  116  reports error-codes to the medium control module  110 . 
     The printhead/ink control system  108  comprises a printhead control module  118 , the PWA-type printhead  18 , a printhead diagnostic module  122 , an ink control module  124 , and ink supply  126 . The printhead control module  118  communicates with the printer control module  102  and controls the PWA-type printhead  18 . The printhead control module  118  generates nozzle firing signals that fire or activate the nozzles  20  of the PWA-type printhead  18 . The printhead control module  118  controls the firing or activation of the nozzles  20  by controlling the timing of the nozzle firing signals. 
     The printhead diagnostic module  122  diagnoses any problems in the PWA-type printhead  18  and reports errors including any malfunctioning nozzles to the printhead control module  118 . The ink control module  124  communicates with the printer control module  102  and controls the supply of ink to the PWA-type printhead  18  from the ink supply  126 . 
     SUMMARY 
     A system comprises a control module that communicates with a printhead having nozzles, that detects a malfunctioning nozzle, and that generates control signals when the malfunctioning nozzle is detected. A vibration generator selectively vibrates the printhead along a first axis of a print medium based on the control signals. The first axis is selected from a group consisting of parallel and perpendicular to a second axis of motion of the print medium. 
     In other features, the printhead is selected from a group consisting of a scanning-type printhead and a page-wide array (PWA) type printhead. The first axis is perpendicular to the second axis when the printhead is a page-wide array (PWA) type printhead. The first axis is parallel to the second axis when the printhead is a scanning-type printhead. When the printhead vibrates, the nozzles from opposite sides of a line that bisects the malfunctioning nozzle and that is perpendicular to the first axis move toward the line. Ink is selectively ejected from the nozzles on the print medium during printing. When the printhead vibrates, the ink from the nozzles that are on opposite sides of the line and that are adjacent to the line impact portions of the print medium that do not receive the ink from the malfunctioning nozzle. When the printhead is a page-wide array (PWA) type printhead, the print medium moves unidirectionally and perpendicularly to the first axis under the printhead. 
     In other features, when the printhead is a scanning-type printhead, the printhead moves perpendicularly to the second axis over the print medium. The vibration generator vibrates the printhead by a distance that is proportional to a diameter of the nozzles. The vibration generator is selected from a group consisting of a piezoelectric crystal, a cam/follower, an electromagnet, a solenoid, and an electric motor with a counterbalance. The vibration generator is mounted externally to the printhead. Timing of the control signals is based on one of speed of the print medium, speed of the printhead, and timing of firing the nozzles during printing. The vibration generator is integrated with the printhead. 
     A method comprises detecting a malfunctioning nozzle of a printhead having nozzles; generating control signals for the printhead when the malfunctioning nozzle is detected; and selectively vibrating the printhead along a first axis of a print medium based on the control signals. The first axis is selected from a group consisting of parallel and perpendicular to a second axis of motion of the print medium. 
     In other features, the method includes selecting the printhead from a group consisting of a scanning-type printhead and a page-wide array (PWA) type printhead. The first axis is perpendicular to the second axis when the printhead is a page-wide array (PWA) type printhead. The first axis is parallel to the second axis when the printhead is a scanning-type printhead. The method includes moving the nozzles from opposite sides of a line that bisects the malfunctioning nozzle and that is perpendicular to the first axis toward the line when the printhead vibrates. The method includes rejecting ink from the nozzles that are on opposite sides of the line and that are adjacent to the line on portions of the print medium that do not receive the ink from the malfunctioning nozzle when the printhead vibrates. The method includes moving the print medium unidirectionally and perpendicularly to the first axis under the printhead when the printhead is a page-wide array (PWA) type printhead. The method includes moving the printhead perpendicularly to the second axis over the print medium when the printhead is a scanning-type printhead. 
     In other features, the method includes vibrating the printhead by a distance that is proportional to a diameter of the nozzles. The method includes selecting a vibration generator from a group consisting of a piezoelectric crystal, a cam/follower, an electromagnet, a solenoid, and an electric motor with a counterbalance. The method includes mounting a vibration generator externally to the printhead. The method includes adjusting timing of the control signals based on one of speed of a print medium, speed of the printhead, and timing of firing the nozzles during printing. The method includes integrating a vibration generator with the printhead. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  depicts the motion of a scanning-type printhead across a printing medium according to the prior art; 
         FIG. 1B  depicts a layout of a page-wide array (PWA) type printhead across a printing medium according to the prior art; 
         FIG. 2A  depicts an exemplary arrangement of nozzles of a scanning-type printhead according to the prior art; 
         FIG. 2B  depicts an exemplary arrangement of nozzles of a PWA-type printhead according to the prior art; 
         FIG. 3A  is a functional block diagram of an exemplary inkjet printing system that uses a scanning-type printhead according to the prior art; 
         FIG. 3B  is a functional block diagram of an exemplary inkjet printing system that uses a PWA-type printhead according to the prior art; 
         FIG. 4A  depicts an exemplary printout generated by an inkjet printer using a scanning-type printhead with one or more nozzles malfunctioning; 
         FIG. 4B  depicts an exemplary printout generated by an inkjet printer using a PWA-type printhead with one or more nozzles malfunctioning; 
         FIG. 5A  depicts compensating nozzles that compensate errors caused by a malfunctioning nozzle during printing according to the present disclosure; 
         FIG. 5B  depicts blurring of a blank line accomplished by vibrating a PWA-type printhead according to the present disclosure; 
         FIG. 6A  is a functional block diagram of a system for compensating errors caused by malfunctioning nozzles by vibrating a PWA-type printhead using a piezoelectric crystal according to the present disclosure; 
         FIG. 6B  is a functional block diagram of an exemplary printhead control module of the system of  FIG. 6A  according to the present disclosure; 
         FIG. 7A  is a functional block diagram of a system for compensating errors caused by malfunctioning nozzles by vibrating a PWA-type printhead using a cam/follower according to the present disclosure; 
         FIG. 7B  is a functional block diagram of an exemplary printhead control module of the system of  FIG. 7A  according to the present disclosure; 
         FIG. 8  is a flowchart of a method for compensating errors caused by malfunctioning nozzles by vibrating a PWA-type printhead using a piezoelectric crystal according to the present disclosure; and 
         FIG. 9  is a flowchart of a method for compensating errors caused by malfunctioning nozzles by vibrating a PWA-type printhead using a cam/follower according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module, circuit and/or device refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     In inkjet printers, nozzles  20  of printheads can malfunction due to various reasons. For example, nozzles  20  may be defective when manufactured. Nozzles  20  may not fire (i.e., may not eject ink) due to ink drying in the nozzles  20 . Media-debris may clog the nozzles  20  over time thereby preventing the nozzles  20  from firing. Occasionally, nozzles  20  may fire but the ink may eject in the wrong direction instead of ejecting perpendicularly to the medium on which data is printed. Malfunctioning nozzles  20  may adversely affect print quality. 
     Referring now to  FIGS. 4A and 4B , blank lines may occur on a printout when one or more nozzles  20  of printheads malfunction. In  FIG. 4A , horizontal lines  150  may occur when one of the nozzles  20  of the scanning-type printhead  10  malfunctions. In  FIG. 4B , blank vertical lines  152  occur when one or more nozzles  20  of the page-wide array (PWA) type printhead  18  malfunction. The blank horizontal and vertical lines  150 ,  152  may be only about 1 mil thick and yet may be visible to human eyes. 
     The scanning-type printhead  10  may be able to print over the blank horizontal line  150 . The PWA-type printhead  18 , however, cannot print over the blank vertical line  152 . Specifically, the scanning-type printhead  10  moves across the width of the medium (e.g., the paper  14 ) and can repeat a pass over a portion having a missing horizontal line  150 . Accordingly, the scanning-type printhead  10  can compensate for a malfunctioning nozzle  20  by passing another working nozzle  20  over the portion that has the missing horizontal line  150 . The scanning-type printhead  10  may be unable to compensate, however, if the inkjet printer operates in a fast mode where the scanning-type printhead  10  prints data on a portion of the paper  14  only once and does not repeat a pass over that portion. 
     Unlike the scanning-type printhead  10 , the PWA-type printhead  18  is fixed in position. Additionally, the paper  14  typically moves under the PWA-type printhead  18  only in one direction, thereby leaving no opportunity to compensate for a malfunctioning nozzle  20 . As a result, one or more missing vertical lines  152  caused by one or more malfunctioning nozzles  20  may persist uncorrected. 
     The present disclosure relates to compensating errors caused by malfunctioning nozzles  20  of printheads when the printheads do not or cannot repeat a pass over the portion of the medium having missing data. The disclosure uses the PWA-type printhead  18  as an example since the PWA-type printhead  18  is stationary and cannot repeat a pass over the portion of the paper  14  having missing data. Although the disclosure uses the PWA-type printhead  18  as an example, the teachings of the disclosure can be applied to the scanning-type printhead  10 . For example, the teachings of the disclosure can be applied to the scanning-type printhead  10  when the scanning-type printhead  10  prints in a mode where the scanning-type printhead  10  does not repeat a pass over the portion of the paper  14  having missing data. 
     Specifically, errors caused by malfunctioning nozzles of the PWA-type printhead  18  can be compensated for by mechanical dithering of the PWA-type printhead  18 . Dithering is an intentionally applied noise or interference that is used to randomize errors. Mechanical dithering of the PWA-type printhead  18  can be in the form of intentionally applied vibrations to the PWA-type printhead  18 . The PWA-type printhead  18  may be vibrated along the printhead axis  12  (i.e., perpendicular to the medium axis  16 ) by a predetermined distance during printing. For example, the predetermined distance may be approximately equal to the diameter “d” of the nozzles  20 . The vibrations may vibrate the PWA-type printhead  18  by the predetermined distance along the width of the paper  14  (i.e., perpendicular to the direction of motion of the paper  14 ). 
     Referring now to  FIGS. 5A and 5B , the vibrations may drop ink from one or more nozzles  20  that are adjacent to malfunctioning nozzles on the portions of the paper  14  that do no receive ink from the malfunctioning nozzle  20 . In  FIG. 5A , a malfunctioning nozzle  20 - 1  and a plurality of compensating nozzles  20 - 2  are shown. The compensating nozzles  20 - 2  are adjacent to a line  16  that passes through the malfunctioning nozzle  20 - 1  and that is parallel to the medium axis  16 . Depending on the resolution being used, the compensating nozzles  20 - 2  may be located in the same row  26  as the malfunctioning nozzle  20 - 1  and in other rows  26  of the same ink color. Additionally, the compensating nozzles  20 - 2  may be located in rows  26  of other ink colors. 
     In  FIG. 5B , an enlarged view of a portion  154  (shown in  FIG. 4B ) of a blank vertical line  152  is shown as an example. The vibrations may mix the layering of ink drops ejected by the compensating nozzles  20 - 2  on portions of the paper  14  having the missing vertical line  152  caused by the malfunctioning nozzle  20 - 1 . Mixing the layering of the ink drops may sufficiently blur the vertical line  152  as shown. The blurred or partly blank lines may be less visible or invisible to human eyes than a totally blank line. When viewed normally (i.e., without enlargement), the human eye may not notice the remaining blank portions, if any, of the vertical line  152 . 
     When one or more nozzles  20  malfunction, printheads may be vibrated randomly. Alternatively, the printheads may be vibrated synchronous to the process of printing. For example, the vibrations of the PWA-type printhead  18  may be synchronized to the timing of firing of odd and even numbered rows  26  of nozzles  20 . The PWA-type printhead  18  may be moved in a first direction when nozzles  20  of even numbered rows are fired. The PWA-type printhead  18  may be moved in a second direction that is opposite to the first direction when nozzles  20  of even numbered rows are fired. Alternatively, the timing for generating the vibrations may be synchronized to the speed of the paper  14 . The scanning-type printhead  10  may be vibrated synchronously to the firing of nozzles or to the speed of the paper  14  and/or the speed of the scanning-type printhead  10 . 
     Printheads may be vibrated using different vibration-generating devices. For example, the printheads may be vibrated using piezoelectric crystals, cam/followers, electromagnets, solenoids, and electric motors with a counterbalance. Piezoelectric crystals may be best suited to generate vibrations of the order of the diameter of the nozzles  20  without disturbing the fixed position of the PWA-type printhead  18 . 
     Before a detailed discussion is presented, a brief description of drawings is presented.  FIGS. 6A and 6B  show a system for vibrating the PWA-type printhead  18  using a piezoelectric crystal.  FIGS. 7A and 7B  show a system for vibrating the PWA-type printhead  18  using a cam/follower.  FIGS. 8 and 9  show methods for compensating for errors caused by malfunctioning nozzles  20  of the PWA-type printhead  18  by using a piezoelectric crystal and a cam/follower, respectively. 
     Referring now to  FIGS. 6A and 6B , a system  200  for vibrating the PWA-type printhead  18  using a piezoelectric crystal  202  is shown. In  FIG. 6A , the system  200  includes the PWA-type printhead  18 , the piezoelectric crystal  202 , the printer control module  102 , the printhead diagnostic module  122 , the medium control module  110 , and a printhead control module  204 . The piezoelectric crystal  202  may be integrated with the PWA-type printhead  18  or may be mounted externally at a suitable mount-point adjacent to the PWA-type printhead  18 . 
     The printhead control module  204  communicates with the printer control module  102  and controls the PWA-type printhead  18 . The printhead control module  204  generates nozzle firing signals that fire or activate the nozzles  20  of the PWA-type printhead  18 . The printhead control module  204  controls the firing or activation of the nozzles  20  by controlling the timing of the nozzle firing signals. The printhead control module  204  may output the nozzle firing commands directly to the printhead  18  as shown at  251  and/or indirectly through the printhead diagnostic module  122 . The printhead diagnostic module  122  reports error codes to the printhead control module  204  when one or more of the nozzles  20  of the PWA-type printhead  18  malfunction. The printhead control module  204  may drive the piezoelectric crystal  202  randomly or synchronously. The piezoelectric crystal  202 , in turn, may vibrate the PWA-type printhead  18 . 
     The PWA-type printhead  18  may be vibrated synchronous to the firing of the nozzles  20 . The printhead control module  204  may generate control signals that drive the piezoelectric crystal  202  based on the nozzle firing signals that fire the nozzles  20 . Alternatively, the printhead control module  204  may generate the control signals that drive the piezoelectric crystal  202  based on timing data of the paper motion received from the medium control module  110  via the printer control module  102 . 
       FIG. 6B  illustrates the printhead control module  204  in greater detail. The printhead control module  204  comprises a control module  206 , a nozzle firing module  207 , an error lookup module  208 , a mode select module  210 , a synchronization module  212 , and a pulse generator module  214 . The control module  206  generates the nozzle firing signals. The nozzle firing module  207  selectively fires nozzles  20  based on the nozzle firing signals. The error lookup module  208  receives error codes related to malfunctioning nozzles  20  of the PWA-type printhead  18  from the printhead diagnostic module  122 . The error lookup module  208  communicates the error codes to the control module  206 . The mode select module  210  receives information from the printer control module  102  related to whether the PWA-type printhead  18  should be vibrated in a random mode or a synchronous mode. The mode select module  210  communicates the information to the control module  206 . 
     The synchronization module  212  receives timing information related to the paper motion from the printer control module  102  when the PWA-type printhead  18  is to be vibrated synchronously to the paper motion. When using the scanning-type printhead  10 , the synchronization module  212  may also receive timing information related to the motion of the scanning-type printhead  10  if the scanning-type printhead  10  is to be vibrated synchronous to the motion of the scanning-type printhead  10 . The synchronization module  212  communicates the timing information to the control module  206 . 
     Based on the timing information of the nozzle firing signals and the information received from the error lookup module  208 , the mode select module  210 , and the synchronization module  212 , the control module  206  generates control signals. The control signals are input to the pulse generator module  214 . Based on the control signals, the pulse generator module  214  generates pulses that drive the piezoelectric crystal  202 . Depending on the mode selected, the pulses may drive the piezoelectric crystal  202  randomly or synchronously. Based on the pulses received, the piezoelectric crystal  202  may vibrate randomly or synchronously in a direction shown by the arrow  203  during printing. 
     The vibrations generated by the piezoelectric crystal  202  vibrate the PWA-type printhead  18  by the predetermined distance across the width of the paper  14  along the printhead axis  12 . The vibration of the PWA-type printhead  18  may mix the layering  156  of ink drops ejected by compensating nozzles  20 - 2 . The mixing of the layering  156  of the ink drops may sufficiently blur the vertical lines  152  so as not to be visible to the human eyes. 
     Referring now to  FIGS. 7A and 7B , a system  250  for vibrating the PWA-type printhead  18  using a cam/follower  252  is shown. In  FIG. 7A , the system  250  includes the PWA-type printhead  18 , the cam/follower  252 , the printer control module  102 , the printhead diagnostic module  122 , the medium control module  110 , a printhead control module  254 , and a cam driver module  256 . The cam/follower  252  may be mounted adjacent to the PWA-type printhead  18  at a suitable mount-point. 
     The printhead control module  254  generates nozzle firing signals that fire or activate the nozzles  20  of the PWA-type printhead  18 . The printhead control module  254  controls the firing or activation of the nozzles  20  by controlling the timing of the nozzle firing signals. The printhead control module  254  may output the nozzle firing commands directly to the printhead  18  as shown at  253  and/or indirectly through the printhead diagnostic module  122 . The printhead diagnostic module  122  reports error codes to the printhead control module  254  when one or more of the nozzles  20  of the PWA-type printhead  18  malfunction. The printhead control module  254  may generate control signals to drive the cam/follower  252  randomly or synchronously. The cam/follower  252 , in turn, vibrates the PWA-type printhead  18 . 
     The PWA-type printhead  18  may be vibrated synchronous to the firing of the nozzles  20 . The printhead control module  254  may generate control signals that drive the cam/follower  252  based on the nozzle firing signals that fire the nozzles  20 . Alternatively, the printhead control module  254  may generate the control signals that drive the cam/follower  252  based on timing data of the paper motion received from the medium control module  110  via the printer control module  102 . 
       FIG. 7B  illustrates the printhead control module  254  in greater detail. The printhead control module  254  comprises a control module  258 , the nozzle firing module  207 , the error lookup module  208 , the mode select module  210 , the synchronization module  212 , and a pulse generator module  260 . The control module  258  generates the nozzle firing signals. The nozzle firing module  207  selectively fires nozzles  20  based on the nozzle firing signals. The error lookup module  208  receives error codes related to malfunctioning nozzles  20 - 1  of the PWA-type printhead  18  from the printhead diagnostic module  122 . The error lookup module  208  communicates the error codes to the control module  258 . The mode select module  210  receives information from the printer control module  102  related to whether the PWA-type printhead  18  should be vibrated in a random mode or a synchronous mode. The mode select module  210  communicates the information to the control module  258 . 
     The synchronization module  212  receives timing information related to the paper motion from the printer control module  102  when the PWA-type printhead  18  is to be vibrated synchronously to the paper motion. When using the scanning-type printhead  10 , the synchronization module  212  may also receive timing information related to the motion of the scanning-type printhead  10  if the scanning-type printhead  10  is to be vibrated synchronous to the motion of the scanning-type printhead  10 . The synchronization module  212  communicates the timing information to the control module  258 . 
     Based on the timing information of the nozzle firing signals and the information received from the error lookup module  208 , the mode select module  210 , and the synchronization module  212 , the control module  258  generates control signals. The control signals are input to the pulse generator module  260 . Based on the control signals, the pulse generator module  260  generates pulses and inputs the pulses to the cam driver module  256 . The cam driver module  256  drives the cam/follower  252 . Depending on the mode selected, the cam driver module  256  may operate the cam/follower  252  randomly or synchronously. The cam/follower  252  may move in a direction shown by the arrow  203  during printing. 
     The movement generated by the cam/follower  252  may vibrate the PWA-type printhead  18  along the printhead axis  12  by the predetermined distance. The vibration of the PWA-type printhead  18  may mix the layering  156  of ink drops ejected by compensating nozzles  20 - 2 . Mixing the layering  156  of the ink drops may sufficiently blur the vertical lines  152  so as not to be visible to the human eyes. 
     Referring now to  FIG. 8 , a method  300  for compensating printing errors caused by malfunctioning nozzles  20 - 1  of the PWA-type printhead  18  using the piezoelectric crystal  202  is shown. The method  300  begins in step  302 . The printhead control module  204  determines whether the printhead diagnostic module  122  detected one or more malfunctioning nozzles  20 - 1  in step  304 . If false, the method  300  waits. If true, the printhead control module  204  determines whether to vibrate the PWA-type printhead  18  randomly or synchronously with paper motion in step  306 . 
     If the printer control module  102  communicates to the printhead control module  204  that the PWA-type printhead  18  is to be vibrated randomly, the pulse generator module  214  generates pulses in step  308  that operate the piezoelectric crystal  202  so as to vibrate the PWA-type printhead  18  randomly. If, however, the printer control module  102  communicates to the printhead control module  204  that the PWA-type printhead  18  is to be vibrated synchronously, the control module  206  uses the timing information of the nozzle firing signals and/or obtains the timing information related to the paper motion from the printer control module  102  in step  310 . Using the timing information, the pulse generator module  214  generates pulses in step  312  that operate the piezoelectric crystal  202  so as to vibrate the PWA-type printhead  18  synchronously. 
     The pulses generated by the pulse generator module  214  operate the piezoelectric crystal  202  that vibrates the PWA-type printhead  18  randomly or synchronously by the predetermined distance in step  314 . The vibration of the PWA-type printhead  18  mixes the layering  156  of ink drops ejected by compensating nozzles  20 - 2  in step  316 , thereby compensating errors caused by the malfunctioning nozzles. The method  300  ends in step  318   
     Referring now to  FIG. 9 , a method  350  for compensating printing errors caused by malfunctioning nozzles  20 - 1  of the PWA-type printhead  18  using the cam/follower  252  is shown. The method  350  begins in step  352 . The printhead control module  254  determines whether the printhead diagnostic module  122  detected one or more malfunctioning nozzles  20 - 1  in step  304 . If false, the method  350  waits. If true, the printhead control module  254  determines whether to vibrate the PWA-type printhead  18  randomly or synchronously with paper motion in step  356 . 
     If the printer control module  102  communicates to the printhead control module  254  that the PWA-type printhead  18  is to be vibrated randomly, the pulse generator module  260  generates pulses in step  358  that operate the cam/follower  252  so as to vibrate the PWA-type printhead  18  randomly. If, however, the printer control module  102  communicates to the printhead control module  254  that the PWA-type printhead  18  is to be vibrated synchronously, the control module  258  uses the timing information of the nozzle firing signals and/or obtains the timing information related to the paper motion from the printer control module  102  in step  360 . Using the timing information, the pulse generator module  260  generates pulses in step  362  that operate the cam/follower  252  so as to vibrate the PWA-type printhead  18  synchronously. 
     The pulses generated by the pulse generator module  260  operate the cam/follower  252  that vibrates the PWA-type printhead  18  randomly or synchronously by the predetermined distance in step  364 . The vibration of the PWA-type printhead  18  mixes the layering  156  of ink drops ejected by compensating nozzles  20 - 2  in step  366 , thereby compensating errors caused by the malfunctioning nozzles. The method  350  ends in step  368 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.