Patent Publication Number: US-9844932-B2

Title: Inkjet printing machine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims benefit of priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-014056, filed on Jan. 28, 2016 and Japanese Patent Application No. 2017-010866, filed on Jan. 25, 2017, the entire contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet printing machine that discharges ink from an inkjet head to perform printing. 
     2. Description of the Related Art 
     An inkjet printing machine that discharges an ink drop on a printing medium from a nozzle while moving an inkjet head including a plurality of nozzles, has been known in the art (See, for example, Japanese Patent Application Laid-Open No. 2005-262581). Such an inkjet printing machine is sometimes used to perform printing on a building material or on a surface of a decorative panel, which is so-called decoration. 
     Since such a building material or the like often has a heavy weight, the decoration employs a method in which the building material or the like, which is a target of the decoration, is arranged and fixed on a table and then printing is performed while an inkjet head unit is moved in a main-scanning direction. In this method, while an inkjet head is moved in the main-scanning direction, printing for pixels in a prescribed pitch width is performed. The inkjet head is then moved in a sub-scanning direction by the pitch width and printing for a next pitch width is performed. 
     However, in the inkjet printing machine that employs the above-described method of repeating a movement of the inkjet head unit in the main-scanning direction and a movement of the inkjet head unit in the sub-scanning direction, immediately after moved in the sub-scanning direction, the inkjet head unit vibrates in the moved direction. Residual vibration due to the movement may cause landing misalignment of ink drops, which brings degradation in printing quality. This conventionally requires stops of printing until the vibration immediately after the inkjet head unit is moved in the sub-scanning direction converges. As a result, there is a problem that the amount of printing per unit time is limited. To improve the productivity, recent inkjet printing machines tend to have the increased number of mounted inkjet heads along the increased number of nozzles of inkjet heads, and thus have the significantly increased weight of the inkjet head unit. With the increase in the weight of the inkjet head unit, convergence time of the vibration due to the movement of the inkjet head unit is delayed. If the printing is performed after the vibration converges, there is a problem that the productivity drops. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above discussion. One object of the present invention is to provide an inkjet printing machine employing a printing method of repeating moving an inkjet head unit in a main-scanning direction and moving the inkjet head unit in a sub scanning direction that can reduce influence of the residual vibration and improve the productivity by increasing the amount of printing per unit time while reducing the degradation of the printing quality. 
     According to one aspect of the present invention, there is provided an inkjet printing machine including: an inkjet head that includes a plurality of nozzles arranged along a sub-scanning direction; a main-scanning driver that moves the inkjet head in a main-scanning direction orthogonal to the sub-scanning direction; a sub-scanning driver that relatively moves the inkjet head and a printing medium in the sub-scanning direction; a displacement detector that detects displacement of the inkjet head in the sub-scanning direction; and a controller that performs a control for printing an image on the printing medium by alternately repeating an operation of discharging ink drops on the printing medium from the nozzles while causing the main scanning driver to move the inkjet head in the main-scanning direction, and an operation of causing the sub-scanning driver to relatively move the inkjet head and the printing medium in the sub-scanning direction, wherein the controller determines the amount of driving by the sub-scanning driver and the number of nozzles to be used among the nozzles in accordance with a maximum amplitude of vibration of the inkjet head in the sub-scanning direction, the maximum amplitude being detected by the displacement detector when the inkjet head is moved in the sub-scanning direction before starting printing, and performs printing by selecting at least one nozzle to be used in accordance with displacement of the inkjet head in the sub-scanning direction, the displacement being detected by the displacement detector while the inkjet head is moved in the main-scanning direction. 
     According to another aspect of the present invention, the controller changes the quantity of an ink drop discharged from a nozzle that is positioned at an end of the selected at least one nozzle to be used in accordance with displacement of the inkjet head in the sub-scanning direction detected by the displacement detector. 
     According to the above-described aspect of the present invention, the controller determines the amount of driving by the sub-scanning driver and the number of nozzles to be used among the nozzles in accordance with a maximum amplitude of vibration of the inkjet head in the sub-scanning direction, the maximum amplitude being detected by the displacement detector when the inkjet head is moved in the sub-scanning direction before starting printing. The controller further determines a printing pitch in the sub-scanning direction for one pass. The controller then performs printing by alternately repeating an operation of discharging an ink drop on the printing medium from the nozzles while causing the main scanning driver to move the inkjet head in the main-scanning direction, and an operation of causing the sub-scanning driver to relatively move the inkjet head and the printing medium in the sub-scanning direction by the printing pitch. 
     The controller performs printing by selecting at least one nozzle to be used in accordance with displacement of the inkjet head in the sub-scanning direction while the inkjet head is moved in the main-scanning direction. This enables a region of use nozzles, which is to be formed by selected nozzles for the printing pitch, to be moved following the displacement due to the vibration occurring when the inkjet head  41  relatively moves in the sub-scanning direction. As a result, the productivity is improved without causing degradation in the printing quality. 
     According to the above-described aspect of the present invention, the controller changes the quantity of an ink drop discharged from a nozzle that is positioned at an end of the selected at least one nozzle included in a region of use nozzles in accordance with displacement of the inkjet head in the sub-scanning direction due to vibration. It is therefore possible to deal with displacement smaller than the diameter of the nozzle, which cannot be absorbed only by selecting nozzles to be used, by changing the size of an ink drop at the end. As a result, degradation of the printing quality can be reduced further. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a configuration of an inkjet printing machine according to a first embodiment of the present invention. 
         FIG. 2  is a perspective view of major parts of a shuttle unit of the inkjet printing machine shown in  FIG. 1 . 
         FIG. 3  is a control block diagram of the inkjet printing machine shown in  FIG. 1 . 
         FIG. 4  is a schematic view of a nozzle face of an inkjet head shown in  FIG. 2 . 
         FIG. 5A  is a diagram illustrating operations of inkjet heads inside the shuttle unit shown in  FIG. 2  in respective steps, which are viewed from a side of a housing. 
         FIG. 5B  is a diagram illustrating operations of the inkjet heads inside the shuttle unit shown in  FIG. 2  in the respective steps, which are viewed from the front of the housing. 
         FIG. 6  is a diagram illustrating a positional relationship between the inkjet head of a head unit and openings. 
         FIG. 7A  is a diagram illustrating a state in which displacement detecting sensors shown in  FIG. 1  detect displacement of the inkjet head. 
         FIG. 7B  is a diagram illustrating a state in which the displacement detecting sensors shown in  FIG. 1  detect displacement of the inkjet head. 
         FIG. 7C  is a diagram illustrating a state in which the displacement detecting sensors shown in  FIG. 1  detect displacement of the inkjet head. 
         FIG. 8  is a diagram illustrating nozzles of an inkjet head shown in  FIG. 2 , which are viewed from the top of the inkjet head. 
         FIG. 9  is a diagram illustrating respective nozzle positions while an inkjet head shown in  FIG. 2  moves in a main-scanning direction, which are viewed from the top of the inkjet head. 
         FIG. 10A  is a selection diagram illustrating a positional relationship among use nozzles, which is viewed from the top of the inkjet head. 
         FIG. 10B  is a selection diagram illustrating a positional relationship among changed use nozzles, which is viewed from the top of the inkjet head. 
         FIG. 11A  is a diagram illustrating a dot pattern viewed from the top of the inkjet head when print processing is performed by selecting nozzles to be used. 
         FIG. 11B  is a diagram illustrating a dot pattern viewed from the top of the inkjet head when print processing is performed by selecting nozzles to be used. 
         FIG. 11C  is a diagram illustrating a dot pattern viewed from the top of the inkjet head when print processing is performed by selecting nozzles to be used. 
         FIG. 12  is a flowchart illustrating operations of the inkjet printing machine shown in  FIG. 1 . 
         FIG. 13A  is a diagram illustrating dots formed by ink drops discharged from use nozzles according to the first embodiment, which are viewed from the top of the inkjet head. 
         FIG. 13B  is a diagram illustrating dots formed by ink drops discharged from use nozzles according to a second embodiment, which are viewed from the top of the inkjet head. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. In the drawings, the same or similar reference symbol is attached to the same or similar structural element. 
     The following embodiments present examples of an apparatus and the like for realizing the technical concept of the present invention. The technical concept of the present invention regarding the material, the shape, the structure, the arrangement, and the like of various structural components is not limited to these embodiments. Various modifications can be made in the technical concept of the present invention within the scope of claims. 
     First Embodiment 
       FIG. 1  is a schematic perspective view of a configuration of an inkjet printing machine according to a first embodiment of the present invention.  FIG. 2  is a perspective view of major parts of a shuttle unit of the inkjet printing machine shown in  FIG. 1 .  FIG. 3  is a control block diagram of the inkjet printing machine shown in  FIG. 1 .  FIG. 4  is a schematic view of a nozzle face of an inkjet head of the inkjet printing machine shown in  FIG. 1 . In the following explanation, top and bottom, right and left, front and back shown with arrows in  FIG. 1  define a vertical direction, a right-left direction, and a front-back direction, respectively. 
     (Configuration of Inkjet Printing Machine) 
     As shown in  FIGS. 1 to 3 , an inkjet printing machine  1  includes a shuttle base unit  2 , a flatbed unit  3 , a shuttle unit  4 , a displacement detecting unit  7 , a storing unit  5 , and a controller  6 . 
     The shuttle base unit  2  supports the shuttle unit  4  and moves the shuttle unit  4  in the front-back direction (sub-scanning direction). The shuttle base unit  2  includes a stand  11  and a sub-scanning driving motor  12 . 
     The stand  11  supports the shuttle unit  4 . The stand  11  has a shape of a rectangular frame. Sub-scanning drive guides  13 A and  13 B that extend in the front-back direction are arranged on a left and a right frames of the stand  11 , respectively. The sub-scanning drive guides  13 A and  13 B guide the shuttle unit  4  that moves in the front-back direction. 
     The sub-scanning driving motor  12  is a sub-scanning driving unit for relatively moving inkjet heads  41  and a printing medium  15  in the sub-scanning direction. In particular, the sub-scanning driving motor  12  moves the shuttle unit  4  in the front-back direction. 
     The flatbed unit  3  supports the printing medium  15  constituted by a building material and the like. The flatbed unit  3  is arranged inside the stand  11  (a space surrounded by the frame thereof) of the shuttle base unit  2 . The flatbed unit  3  has a medium placement surface  3   a  that is a horizontal surface for placing the printing medium  15 . The height of the medium placement surface  3   a  of the flatbed unit  3  can be adjusted by using an elevator mechanism (not shown) constituted by a hydraulic driving mechanism and the like. 
     The shuttle unit  4  prints an image on the printing medium  15 . As shown in  FIGS. 1 to 3 , the shuttle unit  4  includes a housing  21 , a main-scanning driving unit  22 , a main-scanning movable table  23 , and a head unit  24 . 
     The housing  21  houses all the parts such as the head unit  24 . The housing  21  is formed in the form of a gate that arches over the flatbed unit  3  in the right-left direction. The housing  21  has a left leg member  26 A and a right leg member  26 B that are supported by the stand  11  of the shuttle base unit  2 , and the housing  21  is movable along the sub-scanning drive guides  13 A and  13 B. The housing  21  includes a horizontal member  27  between the leg members  26 A and  26 B. A bottom side of the horizontal member  27  has an opening for discharging ink from the head unit  24  to the printing medium  15 . The leg members  26 A and  26 B of the housing  21  include openings  28 A and  28 B formed on respective surfaces facing the displacement detecting unit  7 . The openings  28 A and  28 B communicate with the inner space of the housing  21 . 
     The main-scanning driving unit  22  moves the main-scanning movable table  23  in the right-left direction to move the head unit  24  in the right-left direction (main-scanning direction). The main-scanning driving unit  22  includes a driving belt  31 , a pair of pulleys  32 A and  32 B, a main-scanning driving motor  33 , and a main-scanning drive guide  34 . 
     The driving belt  31  moves the main-scanning movable table  23  by making a circular movement. The driving belt  31  is stretched over the pulleys  32 A and  32 B. 
     The pulleys  32 A and  32 B support the driving belt  31  and cause the driving belt  31  to make the circular movement. The pulleys  32 A and  32 B are rotatably supported by a back-side wall of the housing  21 . The pulleys  32 A and  32 B are separated in the right-left direction and are arranged at the same height. The pulley  32 B is connected to an output shaft of the main-scanning driving motor  33  and transmits a rotary driving force of the main-scanning driving motor  33  to the driving belt  31 . 
     The main-scanning driving motor  33  causes the driving belt  31  to make the circular movement by rotating the pulley  32 B. The main-scanning drive guide  34  guides the main-scanning movable table  23  to move along the right-left direction. The main-scanning drive guide  34  is an elongated member and extends in the right-left direction. The main-scanning drive guide  34  is mounted on the back-side wall of the housing  21 . 
     The main-scanning movable table  23  is a table on which the head unit  24  is placed. The main-scanning movable table  23  is fixed to the driving belt  31 . The main-scanning movable table  23  moves along the main-scanning drive guide  34  in the right-left direction by the circular movement of the driving belt  31 . 
     The head unit  24  includes four inkjet heads  41 . The head unit  24  discharges ink while moving in the right-left direction, thereby printing the image on the printing medium  15 . The head unit  24  is mounted on the main-scanning movable table  23 . The head unit  24  moves in the right-left direction along with the main-scanning movable table  23 . The head unit  24  includes openings  25 A and  25 B formed on both sides thereof at respective positions facing the openings  28 A and  28 B. Since the housing  21  is formed with the openings  28 A and  28 B and the head unit  24  is formed with the openings  25 A and  25 B, an inkjet head  41  inside the housing  21  is visible from each side of the housing  21  as shown in  FIG. 6 . 
     The four inkjet heads  41  are arranged side-by-side in the right-left direction. As shown in  FIG. 4 , each of the inkjet heads  41  includes a plurality of nozzles  43  arranged along the sub-scanning direction. These nozzles  43  open on a nozzle surface  41   a , which is a lower surface of the inkjet head  41 . Ink drops are discharged on the printing medium  15  from the nozzles  43 . Each of the four inkjet heads  41  discharges ink of a different color (for example, cyan, black, magenta, and yellow). 
     As shown in  FIGS. 5A and 5B , the inkjet heads  41  print images on the printing medium  15  by alternatively repeating: an operation in which the inkjet heads  41  perform printing for one pass by discharging ink drops from the nozzles  43  on the printing medium  15  while being moved in the main-scanning direction by the main-scanning driving motor  33 ; and an operation in which the inkjet heads  41  are relatively moved in the sub-scanning direction by the sub-scanning driving motor  12 . 
       FIG. 5A  is a diagram illustrating operations of the inkjet heads  41  inside the shuttle unit  4  along the sub-scanning direction in respective steps, which are viewed from a side of the housing  21 .  FIG. 5B  is a diagram illustrating operations of the inkjet heads  41  inside the shuttle unit  4  along the main-scanning direction in the respective steps, which are viewed from the front of the housing  21 .  FIGS. 5A and 5B  illustrate operations of the inkjet heads  41  by dividing the operations into steps S 1  to S 11 . It is noted that the inkjet heads  41  are omitted in steps S 2  to S 11  in  FIGS. 5A and 5B  because the inkjet heads  41  vibrate the same as the head unit  24 . 
     Step S 1  shows a state in which the inkjet heads  41  are positioned on the left side in the main-scanning direction. Step S 2  shows a state in which the inkjet heads  41  are moving toward the right side in the main-scanning direction. Step S 3  shows a state in which the inkjet heads  41  reach the right end in the main-scanning direction. 
     Step S 4  shows a state in which the inkjet heads  41  move in the sub-scanning direction by one pass to reach a position for the next one pass. Step S 5  shows a state in which the inkjet heads  41  start moving toward the left side in the main-scanning direction. Step S 6  shows a state in which the inkjet heads  41  are moving toward the left side in the main-scanning direction. Step S 7  shows a state in which the inkjet heads  41  reach the left end in the main-scanning direction. 
     Step S 8  shows a state in which the inkjet heads  41  move in the sub-scanning direction by one pass to reach a position for the next one pass. Step S 9  shows a state in which the inkjet heads  41  start moving toward the right side in the main-scanning direction. Step S 10  shows a state in which the inkjet heads  41  are moving toward the right side in the main-scanning direction. Step S 11  shows a state in which the inkjet heads  41  reach the right end in the main-scanning direction. 
     In this manner, the inkjet heads  41  inside the head unit  24  print images on the printing medium  15  by alternatively repeating: an operation in which the inkjet heads  41  perform printing for one pass by discharging ink drops from the nozzles on the printing medium  15  while being moved in the main-scanning direction by the main-scanning driving motor  33 ; and an operation in which the inkjet heads  41  are relatively moved in the sub-scanning direction by the sub-scanning driving motor  12 . 
     When the weight of the head unit  24  is heavy due to an increase in the number of mounted inkjet heads, the head unit  24  vibrates in the sub-scanning direction immediately after the inkjet heads  41  move in the sub-scanning direction by one pass to reach the position for the next one pass as in steps S 4  and S 8 . Further, as in steps S 5  and S 9 , even after a predetermined time passes, the inkjet heads  41  have residual vibration in the sub-scanning direction due to the movement in the sub-scanning direction. 
     Back to  FIGS. 1 to 4 , the displacement detecting unit  7  is a displacement detector for detecting displacement of the inkjet heads  41  in the sub-scanning direction, and arranged on the right and left of the shuttle base unit  2 . The displacement detecting unit  7  includes sub-scanning drive guides  72 A and  72 B extending in the front-back direction, displacement detecting sensors  71 A and  71 B movable along the sub-scanning drive guides  72 A and  72 B, and a sensor driving motor  73  for moving the displacement detecting sensors  71 A and  71 B. The sub-scanning drive guides  72 A and  72 B are provided independently from the shuttle base unit  2  so as not to receive the vibration of the shuttle base unit  2 . The sub-scanning drive guides  72 A and  72 B have the same length as that of the shuttle base unit  2  in the front-back direction. 
     The sensor driving motor  73  is synchronized with the sub-scanning driving motor  12  to move the displacement detecting sensors  71 A and  71 B in the front-back direction in accordance with operations of moving the shuttle unit  4  in the front-back direction by the sub-scanning driving motor  12 . 
     The displacement detecting sensors  71 A and  71 B are constituted by a displacement sensor, which is a transmission-type laser, for detecting displacement of the inkjet heads  41  in the sub-scanning direction. According to the present embodiment, the displacement detecting sensor  71 A is a light-emitting device to emit laser light and the displacement detecting sensor  71 B is a light receiving device to receive the laser light emitted from the displacement detecting sensor  71 A. 
     The displacement detecting sensor  71 A is arranged at a position facing the opening  28 A of the housing  21  and the opening  25 A of the head unit  24 . The displacement detecting sensor  71 B is arranged at a position facing the opening  28 B of the housing  21  and the opening  25 B of the head unit  24 . In this manner, the displacement detecting sensors  71 A and  71 B are arranged to be opposite each other across the head unit  24 . 
     This configuration enables the displacement detecting sensor  71 B to receive laser light emitted from the displacement detecting sensor  71 A through the openings  28 A and  28 B of the housing  21  and the openings  25 A and  25 B of the head unit  24 . It is noted that the openings  28 A and  28 B and the openings  25 A and  25 B have a size large enough to let the laser light emitted from the displacement detecting sensor  71 A pass therethrough such that the housing  21  and the head unit  24  does not interfere with the laser light even if the housing  21 , the head unit  24  and the inkjet heads  41  vibrate integrally. 
     The displacement detecting sensors  71 A and  71 B detect positions of the inkjet heads  41  arranged inside the housing  21 .  FIGS. 7A to 7C  show each state of the displacement detecting sensor  71  for detecting positions of the inkjet heads  41 , which is viewed from the top of the inkjet printing machine  1 .  FIGS. 7A to 7C  show a current position of the inkjet heads  41  by a solid line and positions of the inkjet heads  41  moving in the main-scanning direction and vibrating in the sub-scanning direction from a printing start position to the solid line by broken lines. It is noted that the housing  21  and the head unit  24  are omitted in  FIGS. 7A to 7C . 
       FIG. 7A  shows a state in which the head unit  24  does not vibrate at a position for each one pass in the sub-scanning direction, as in steps S 1  to S 3 , S 6  and S 7 , or S 10  and S 11  of  FIGS. 5A and 5B . In this state, the amount of received light W 1  received by the displacement detecting sensor  71 B is stored as a reference amount in the storing unit  5 . 
     On the other hand,  FIGS. 7B and 7C  show a state in which the vibration due to the movement in the sub-scanning direction occurs at a position for each one pass in the sub-scanning direction and the inkjet head  41  vibrates in the sub-scanning direction (front-back direction), as in steps S 4  and S 5 , or S 8  and S 9  of  FIGS. 5A and 5B . In these cases, a difference between the amount of received light W 2  or the amount of received light W 3  received by the displacement detecting sensor  71 B and the amount of received light W 1 , which is the reference amount, is used to measure the displacement of the inkjet heads  41 . 
     The storing unit  5  stores data related to various image processing. The data includes positional information for the inkjet heads  41  that corresponds to the amount of received light to be detected by the displacement detecting sensors  71 A and  71 B. The positional information for the inkjet heads  41  corresponding to the amount of received light is measured in advance. The storing unit  5  is constituted by an HDD (Hard Disk Drive) and the like. 
     The controller  6  is a processing unit for controlling operations of respective parts of the inkjet printing machine  1 . The inkjet printing machine  1  has the controller  6  built-in, or the controller  6  can be realized with a dedicated control device such as an externally connected personal computer or a programmable logic controller (PLC). In particular, the controller  6  is constituted by a CPU, an RAM, a ROM and the like, and further includes a communication function as necessary. 
     The controller  6  controls the main-scanning driving motor  33 , the sub-scanning driving motor  12  and the inkjet heads  41  to print images on the printing medium  15  by alternatively repeating: an operation in which the inkjet heads  41  perform printing for one pass by discharging ink drops from the nozzles on the printing medium  15  while being moved in the main-scanning direction by the main-scanning driving motor  33 ; and an operation in which the inkjet heads  41  and the printing medium  15  are relatively moved in the sub-scanning direction by the sub-scanning driving motor  12 . 
     When the controller  6  controls the sub-scanning driving motor  12  to relatively move the inkjet heads  41  in the sub-scanning direction, the inkjet heads  41  vibrate as in steps S 4 , S 5 , or S 8 , S 9  of  FIGS. 5A and 5B , and thus printing is considered to be started after the vibration converges. This method however has a problem of a drop in productivity. On the other hand, when the inkjet heads  41  with the vibration or the residual vibration discharge ink drops from the nozzles  43 , landing misalignment of ink drops occurs, which may cause deterioration in picture quality. 
     To solve these problems, the controller  6  performs print processing by selecting nozzles  43  to be used. The following will describe operations of performing print processing by selecting nozzles  43  to be used in detail. 
       FIG. 8  is a diagram illustrating nozzles  43  of the inkjet head  41 , which are viewed from the top of the inkjet head  41 .  FIG. 9  is a diagram illustrating respective nozzle positions while the inkjet head  41  moves in the main-scanning direction, which are viewed from the top of the inkjet head  41 .  FIG. 10A  is a selection diagram illustrating a positional relationship among use nozzles, which is viewed from the top of the inkjet head  41 .  FIG. 10B  is a selection diagram illustrating a positional relationship among changed use nozzles, which is viewed from the top of the inkjet head  41 .  FIGS. 11A to 11C  are diagrams, each illustrating a dot pattern viewed from the top of the inkjet head  41  when print processing is performed by selecting nozzles. 
     In  FIGS. 8 to 11C , four inkjet heads  41  are indicated as one inkjet head  41  to simplify explanations. Moreover, in  FIGS. 8 to 11C , the number of nozzles  43  of the inkjet head  41  is set as twelve and the number of rows of nozzles  43  included in the inkjet head  41  is indicated as one row. The actual number of nozzles  43  for each inkjet head  41  is not limited to twelve and may be about several hundreds to several thousands as necessary. The actual number of rows of nozzles  43  for each inkjet head  41  is not limited to one row and may be plural rows as necessary. In  FIGS. 8 and 9 , for twelve nozzles  43 , the number  1  (or reference number # 1 ), the number  2  (or reference number # 2 ) . . . the number  12  (or reference number # 12 ) are assigned from the front toward the back along the sub-scanning direction. In  FIGS. 11A to 11C , nozzles  43  that discharge ink drops are shown as black dots and nozzles  43  that do not discharge ink drops are shown as white dots. 
     As shown in  FIG. 8 , the inkjet head  41  forms pixels on the printing medium  15  by discharging ink drops from each nozzle  43  while the inkjet head  41  moves along the main-scanning direction from left to right on  FIG. 8 .  FIG. 8  shows a state in which the inkjet head  41  does not vibrate. In this case, while the inkjet head  41  moves along the main-scanning direction, each nozzle  43  moves parallel to the main-scanning direction and thus a locus of each nozzle  43  is drawn so that nozzles  43  to which the same reference number is assigned are always arranged on a straight line along the main-scanning direction. 
     On the other hand, as shown in  FIG. 9 , when the inkjet head  41  vibrates, a position of each nozzle  43  in the main-scanning direction fluctuates while the inkjet head  41  moves along the main-scanning direction. When all of the nozzles  43  discharge ink drops, there are formed regions in which pixels are not formed on straight lines extending in the main-scanning direction on the printing medium  15  (regions A 2  and A 3  in  FIG. 9 ). To deal with this problem, before starting printing, the controller  6  moves the inkjet head  41  in the sub-scanning direction and controls the displacement detecting sensors  71 A and  71 B to detect the maximum amplitude of the vibration in the sub-scanning direction. In accordance with the maximum amplitude of the vibration, the controller  6  determines the amount of driving by the sub-scanning driving motor  12  and the number of nozzles  43  to be used. In  FIGS. 9 ,  10 A,  10 B, and  13 , reference numbers  411 ,  412 ,  413 ,  414 ,  415 ,  416 , and  417  indicate the inkjet head  41  at respective positions when the controller  6  moves the inkjet head  41  in the main-scanning direction. 
     In particular, the controller  6  determines a region A 1 , which is an overlapped region of: the inkjet head  411  with the maximum amplitude toward the front direction along the sub-scanning direction; and the inkjet head  412  with the maximum amplitude toward the back direction along the sub-scanning direction, as the amount of driving by the sub-scanning driving motor  12 . The controller  6  then uses nozzles  43  within the region A 1 , for example, in  FIG. 9 , the fourth nozzle  43  to the ninth nozzle  43  of the inkjet head  413  where the inkjet head  41  stops vibrating. This enables the inkjet head  41  to form all the pixels on each straight line extending in the main-scanning direction within the region A 1  on the printing medium  15  even when the inkjet head  41  vibrates. 
     In this way, the number of nozzles  43  to be used is changed in accordance with the maximum amplitude of the vibration in the front direction along the sub-scanning direction and the maximum amplitude of the vibration in the back direction along the sub-scanning direction. However, as shown in  FIG. 10A , when the inkjet head  41  vibrates, positions of dots formed by ink drops discharged from respective nozzles  43  are displaced as the inkjet head  41  moves in the main-scanning direction. In particular, when the fourth nozzle  43  to the ninth nozzle  43  of the inkjet head  41  are fixed as use nozzles  1  to  6  in an example shown in  FIG. 10A , the inkjet head  411  is displaced by three nozzles in the front direction and the inkjet head  411  performs printing over the region A 1 , which is to be printed, by three dots in the front direction. In the same way, the inkjet head  412  is displaced by three nozzles in the back direction and the inkjet head  412  performs printing over the region A 1 , which is to be printed, by three dots in the back direction. 
     To deal with this problem, the controller  6  performs printing by selecting nozzles  43  to be used in accordance with displacement of the inkjet head  41  in the sub-scanning direction, which is detected by the displacement detecting sensors  71 A and  71 B, while moving the inkjet head  41  in the main-scanning direction. 
     In particular, the controller  6  changes nozzles  43  to be used by the number of nozzles that are displaced in the main-scanning direction due to the vibration of the inkjet head  41  so that nozzles  43  to be used fit within the region A 1  to be printed. For example, as the inkjet head  411  shown in  FIG. 10B  is displaced by 3 nozzles in the front direction, the controller  6  shifts the nozzles  43  to be used by 3 nozzles in the back direction (the seventh nozzle  43  to the twelfth nozzle  43  of the inkjet head  43  in  FIG. 9 , use nozzles  1  to  6  of the inkjet head  411  in  FIG. 10B ). Further, as the inkjet head  412  shown in  FIG. 10B  is displaced by 3 nozzles in the back direction, the controller  6  shifts the nozzles  43  to be used by 3 nozzles in the front direction (the first nozzle  43  to the sixth nozzle  43  of the inkjet head  412  in  FIG. 9 , use nozzles  1  to  6  of the inkjet head  412  in  FIG. 10B ). 
     In this way, regarding the inkjet head  41  that has nozzles  43  displaced in the sub-scanning direction, the controller  6  changes the nozzles  43  to be used. As shown in  FIG. 10B , this achieves printing by the use nozzles  1  to  6  within the region A 1  to be printed. 
     As shown in  FIGS. 11A and 11B , the print processing with the control by the controller  6  forms images without ink misalignment both for the first pass and the second pass. As shown in  FIG. 11C , by moving the inkjet head  41  to connect the images, an image that absorbs the ink misalignment due to the vibration is formed on the printing medium  15 . 
     (Operations of Inkjet Printing Machine) 
     Next, operations of the inkjet printing machine  1  having the above-described configuration will be described.  FIG. 12  is a flowchart illustrating operations of the inkjet printing machine  1 . The processing shown in the flowchart of  FIG. 12  is started when a print job is input into the inkjet printing machine  1  from an outside personal computer. Before starting the processing shown in the flowchart of  FIG. 12 , the printing medium  15  is set on the medium placement surface  3   a  of the flatbed unit  3 . 
     In step S 1 , before starting printing, the controller  6  controls the sub-scanning driving motor  12  to move the inkjet head  41  in the sub-scanning direction to a printing start position. In step S 2 , the controller  6  detects the maximum amplitude of the vibration in the front direction and the maximum amplitude of the vibration in the back direction along the sub-scanning direction using the displacement detecting sensors  71 A and  71 B. In step S 3 , the controller  6  determines the amount of driving by the sub-scanning driving motor  12  and the number of nozzles  43  to be used in accordance with the detected maximum amplitudes. 
     In step S 4 , the controller  6  starts print processing for the first pass. In step S 5 , the controller  6  obtains displacement of the inkjet head  41  in the sub-scanning direction at each position of the inkjet head  41  along the main-scanning direction, which is detected by the displacement detecting sensors  71 A and  71 B, while the inkjet head  41  moves in the main-scanning direction at a current pass. The controller  6  then selects nozzles  43  to be used in accordance with the detected displacement and performs print processing. 
     In step S 6 , the controller  6  determines whether or not to perform printing for the next pass. When printing is not performed (step S 6 : NO), the controller  6  performs processing in step S 7 . In step S 7 , the controller  6  controls the sub-scanning driving motor  12  to move the inkjet head  41  in the sub-scanning direction to a waiting position, and printing is ended. On the other hand, when printing is performed (step S 6 : YES), the controller  6  performs processing in step S 8 . In step S 8 , the controller  6  controls the sub-scanning driving motor  12  to relatively move the inkjet head  41  and the printing medium  15  in the sub-scanning direction and starts print processing for the next pass. 
     (Operation and Effect) 
     As described above, in the inkjet head printing machine  1 , before starting printing, the controller  6  moves the inkjet head  41  in the sub-scanning direction and controls the displacement detecting sensors  71 A and  71 B to detect the maximum amplitude of the vibration in the sub-scanning direction. In accordance with the detected maximum amplitude, the controller  6  determines the amount of driving by the sub-scanning driving motor  12  and the number of nozzles  43  to be used, and determines a printing pitch for one pass in the sub-scanning direction. The controller  6  then performs printing by alternatively repeating: an operation A of discharging ink drops from nozzles  43  on the printing medium  15  while controlling the main-scanning driving unit  22  to move the inkjet head  41  in the main-scanning direction; and an operation B of moving the inkjet head  41  in the sub-scanning direction by the printing pitch. 
     In the operation A, the controller  6  performs printing by selecting nozzles  43  to be used in accordance with the displacement of the inkjet head  41  in the sub-scanning direction, which is detected by the displacement detecting sensors  71 A and  71 B while the inkjet head  41  is moved in the main-scanning direction. This enables a region of use nozzles, which is to be formed by selected nozzles  43  for the printing pitch, to be moved following the displacement due to the vibration occurring when the inkjet head  41  relatively moves in the sub-scanning direction. As shown in  FIGS. 11A to 11C , it is therefore possible to absorb landing misalignment of ink drops due to the vibration of the inkjet head  41 . As a result, the productivity is improved without causing degradation in the printing quality. 
     Second Embodiment 
     Next, a second embodiment according to the present invention will be described. The present embodiment relates to a control in a case in which a nozzle pitch is displaced due to amplitude.  FIG. 13A  is a diagram illustrating dots/printing pattern formed by ink drops discharged from use nozzles according to the first embodiment, which are viewed from the top of the inkjet head  41 .  FIG. 13B  is a diagram illustrating dots/printing pattern formed by ink drops discharged from use nozzles according to a second embodiment, which are viewed from the top of the inkjet head  41 . 
     When the inkjet head  41  vibrates in the sub-scanning direction, a nozzle pitch in the main-scanning direction is not always formed steady as shown in  FIGS. 7A to 10B . There is a case in which dots are not aligned on the nozzle pitch. In an example shown in  FIG. 13A , a dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  414  is displaced by one-half dot with respect to a dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  417  that is the inkjet head  41  at a point of time when the inkjet head  41  stops vibration. A dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  415  is displaced by one-fourth of a dot with respect to a dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  417 . A dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  416  is displaced by one-half dot from a dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  417 . 
     In such cases, nozzles  43  to be used are selected in accordance with the displacement of the inkjet head  41  in the sub-scanning direction, which is detected by the displacement detecting sensors  71 A and  71 B, and the printing is performed. As shown in  FIG. 13A , if the use nozzle  1  and the use nozzle  6 , which are positioned on both ends of the region of use nozzles formed by selected use nozzles  1  to  6  (refer to  FIG. 10B ), discharge a normal droplet quantity, misalignment in the image occurs in the part of the use nozzles  1  and  6 , which causes degradation in the printing quality. 
     To deal with this problem, in the present embodiment, when the inkjet head  41  is displaced in the back direction in the sub-scanning direction, in accordance with the amount of displacement in the back direction in the sub-scanning direction, the controller  6  determines whether or not dots formed by ink drops discharged from nozzles  43  of the inkjet head  41  are on the nozzle pitch of nozzles  43  of the inkjet head  41  that does not vibrate. When the dots are not on the nozzle pitch, the controller  6  adds a nozzle  43  that is adjacent to the use nozzle  1  of the inkjet head  41  in the front direction in the sub-scanning direction as a use nozzle  0 . The controller  6  then changes the quantity of an ink drop discharged from the use nozzle  0 . The controller  6  also changes the quantity of an ink drop discharged from the use nozzle  6 . 
     On the other hand, when the inkjet head  41  is displaced in the front direction in the sub-scanning direction, in accordance with the amount of displacement in the front direction in the sub-scanning direction of the inkjet head  41 , the controller  6  determines whether or not dots formed by ink drops discharged from nozzles  43  of the inkjet head  41  are on the nozzle pitch of nozzles  43  of the inkjet head  41  that does not vibrate. When the dots are not on the nozzle pitch, the controller  6  adds a nozzle  43  that is adjacent to the use nozzle  6  in the back direction in the sub-scanning direction as a use nozzle  7 . The controller  6  then changes the quantity of an ink drop discharged from the use nozzle  7 . The controller  6  also changes the quantity of an ink drop discharged from the use nozzle  1  of the inkjet head  41 . 
     In this way, the controller  6  determines whether or not the displacement in the sub-scanning direction, which is detected by the displacement detecting sensors  71 A and  71 B, is a displacement in which dots are not on the nozzle pitch. When there is the displacement, the controller  6  calculates a ratio of the amount of displacement. Among the use nozzles  1  to  6  (see  FIG. 10B ), the controller  6  reduces the quantity of an ink drop discharged from the use nozzle  1  (or  6 ) and the quantity of an ink drop discharged from a nozzle  43  (use nozzle  7  or  0 ) adjacent to the use nozzle  6  (or  1 ). The quantity of a drop is adjusted by a multi drop method. 
     In an example shown in  FIG. 13B , a dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  414  or that of the inkjet head  416  is displaced by one-half dot in the sub-scanning direction with respect to a dot pattern formed by ink drops discharged from respective use nozzles of the inkjet head  417 . The controller  6  suppresses the quantity of an ink drop to be discharged from the nozzle  43  (use nozzle  0 ) adjacent to the use nozzle  1  and that to be discharged from the use nozzle  6  to form a dot having the size of one-half dot. 
     A dot pattern formed by ink drops to be discharged from respective use nozzles of the inkjet head  415  is displaced by one-fourth dot in the back direction in the sub-scanning direction with respect to a dot pattern formed by ink drops to be discharged from respective use nozzles of the inkjet head  417 . The controller  6  suppresses the quantity of an ink drop discharged from the use nozzle  6  to form a dot having the size of three-fourth dot, and suppresses the quantity of an ink drop discharged from the nozzle  43  (use nozzle  0 ) adjacent to the use nozzle  1  to form a dot having the size of one-fourth dot. 
     According to the present embodiment, in accordance with displacement and a direction of the inkjet head  41  in the sub-scanning direction due to the vibration, among the use nozzles  1  to  6  (see  FIG. 10B ), the controller  6  changes the quantity of an ink drop to be discharged from the use nozzle  1  (or  6 ) and that to be discharged from the nozzle  43  (use nozzle  7  or  0 ) adjacent to the use nozzle  6  (or  1 ). At each inkjet head  41 , it is therefore possible to deal with the displacement smaller than the diameter of the nozzle  43 , which cannot be solved only by selecting nozzles to be used, by changing the size of an ink drop to be discharged from the use nozzle  1  (or  6 ) and that to be discharged from the nozzle  43  (use nozzle  7  or  0 ) adjacent to the use nozzle  6  (or  1 ) among the use nozzles  1  to  6  (refer to  FIG. 10B ), in accordance with the displacement and the direction of the inkjet head  41  in the sub-scanning direction due to the vibration. As a result, the degradation in the printing quality is further reduced. 
     The present invention is not limited to the above embodiments and the structural components can be realized by modifying them without departing from the gist at the implementation stage. Moreover, various inventions can be constituted by appropriately combining the various structural components disclosed in the above embodiment. For example, some of the structural components among all the structural components described in the embodiments can be omitted. 
     For example, the above-described embodiments exemplify a case with four inkjet heads, but the present invention is not limited to this. The number of inkjet heads may be less or more than four. Moreover, in the present embodiment, the controller  6  sets the region A 1 , which is an overlapped region of: the inkjet head  411  with the maximum amplitude toward the front direction in the sub-scanning direction; and the inkjet head  412  with the maximum amplitude toward the back direction in the sub-scanning direction, as the amount of driving by the sub-scanning driving motor  12 , and the present embodiment exemplifies a case in which nozzles  43  within the region A 1  are used. The present invention is however not limited to this. For example, a region smaller than the region A 1  in which the inkjet head  411  with the maximum amplitude in the front direction in the sub-scanning direction and the inkjet head  412  with the maximum amplitude in the back direction in the sub-scanning direction overlap each other may be set as the amount of driving by the sub-scanning driving motor  12 , and nozzles  43  within the region smaller than the region A 1  may be used.