Patent Publication Number: US-8974021-B2

Title: Printing apparatus and processing method thereof

Description:
The present application is a continuation of U.S. application Ser. No. 13/604,253, filed on Sep. 5, 2012, now U.S. Pat. No. 8,814,296, which claims priority to JP 2011-224307, filed Oct. 11, 2011, the entire disclosure of each of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus and processing method thereof. 
     2. Description of the Related Art 
     There is known a printing apparatus which employs an inkjet method of printing an image on a printing medium using a printhead including orifice arrays each configured by arraying a plurality of printing elements (orifices) (integrating and arraying many printing elements). As printing apparatuses of this type require higher printing operation speeds and higher resolutions, the number of orifices arrayed on a printhead is increasing. 
     When all printing elements are simultaneously driven in a printing operation, discharge becomes unstable owing to pressure interference (crosstalk) between neighboring orifices, and the like. Since a large current is supplied, a voltage drop arising from power loss on a common power line becomes large near the printhead. As the number of simultaneously driven orifices increases, driving voltage applied to orifices (printing elements) drops more steeply, impairing the printing stability. Further, a power supply instantaneously resistant to a large current is necessary, inhibiting the design of a compact, low-cost apparatus. 
     To solve these problems, all orifices are generally divided into a plurality of driving blocks in a printhead, and orifices in the respective driving blocks are time-divisionally driven sequentially. This driving method is called time divisional driving (or block divisional driving). 
     When a printhead in which printing elements are arranged on a single straight line is time-divisionally driven for respective driving blocks, the printing position shifts between the driving blocks because the printhead moves in the scanning direction during the time divisional driving. For example, when expressing tonality using a unit matrix (an image processing control unit formed from M×N pixels), a dot pattern in the matrix may shift in every printing scan of the printhead in accordance with the relationship between the matrix size and the pattern size of the driving block. To solve this problem, Japanese Patent Laid-Open No. 2006-159698 proposes a method of shifting the arrangement of binary image data in every printing scan of the printhead in accordance with the relationship between the matrix size and the pattern size of the driving block. 
     Conventional printing by time divisional driving suffers the following problem regardless of whether to express tonality using a unit matrix. 
       FIG. 13  is a view showing the relationship between the orifice array of a printhead, the driving signal of each orifice, and a dot which is discharged from each orifice and attached to a printing medium.  FIG. 13  shows 2-pass printing (that is, in which an image is printed by two printing scans) in the same printing region on a printing medium. 
     In this case, every time a printing scan is performed, the printing medium is conveyed by a distance corresponding to eight orifices. Reference numeral  401  denotes a first printing scan;  402  and  412 , a second printing scan; and  403  and  413 , a third printing scan. 
     An orifice array denoted by reference numeral  402  is illustrated at a position shifted from an orifice array denoted by reference numeral  401  by eight orifices in the orifice array direction (printing medium conveyance direction). This is because in the second printing scan, the printing medium is conveyed in the conveyance direction by a distance corresponding to eight orifices from a position in the first printing scan. Similarly, an orifice array denoted by reference numeral  403  is illustrated at a position shifted along with conveyance of the printing medium. 
     An orifice array  500  of the printhead is divided into two, groups 1 and 2 each including eight adjacent orifices, as denoted by reference numerals  401  to  403 . Each of eight orifices in each group belongs to one of eight driving blocks. In a printing operation, the eight orifices are time-divisionally driven for the respective driving blocks (orifices of the same driving block are driven simultaneously). Note that numerals on the left side of respective orifices indicate orifice numbers 1-1 to 2-8, and numerals on the right side of respective orifices indicate block numbers 1 to 8. 
     In the orifice array  500 , the first and ninth orifices 1-1 and 2-1 from the top in  FIG. 13  are assigned to the first driving block. The second and 10th orifices 1-2 and 2-2 from the top in  FIG. 13  are assigned to the second driving block. All orifices are assigned to driving blocks. The first to eighth driving blocks are sequentially driven in the ascending order based on a pulse-like block selection signal  300  as denoted by reference numerals  411  to  413 , and a printing signal complying with image data. Then, ink is discharged from the respective orifices, forming dots on a printing medium, as denoted by reference numeral  414 . 
     As the layout positions of dots formed on a printing medium, dots are formed in a staggered pattern in the first scan (first scanning), and dots are formed in an inverse staggered pattern in the second printing scan (second scanning) in the same printing region, as denoted by reference numeral  414 . Printing of an image is completed by 2-pass printing. 
     To the contrary,  FIG. 14  shows printing using only a predetermined number (eight in this case) of orifices positioned at the center, unlike printing using all orifices in  FIG. 13 . For example, an orifice array denoted by reference numeral  601  prints using orifices 1-5 to 1-8 and 2-1 to 2-4. Note that the arrangement of the printhead and original image data are the same as those in  FIG. 13 . 
     A comparison between dot layout positions denoted by reference numeral  414  in  FIG. 13  and those denoted by reference numeral  616  in  FIG. 14  reveals that they are different from each other, though original image data is the same. 
     More specifically, dots are laid out with almost no gap on a printing medium at dot layout positions denoted by reference numeral  414  in  FIG. 13 . In contrast, gaps are generated between dots at dot layout positions denoted by reference numeral  616  in  FIG. 14 . This dot layout position difference is generated because all dots are formed by the same driving blocks in  FIG. 13 , whereas dots formed by different driving blocks coexist in  FIG. 14 . 
     When a region printed using all orifices and a region printed using only some orifices exist, the relationship between the printing medium conveyance amount and the driving block cycle changes, and fill of dots differs between the respective regions. This appears as density nonuniformity, degrading image uniformity. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the conventional problems, and provides a technique advantageous for suppressing a density change arising from the relationship between the driving block and the printing medium conveyance amount in time divisional driving, and suppressing a decrease in image uniformity. 
     One of the aspects of the present invention provides a printing apparatus comprising, a printhead including an element array in which a plurality of printing elements are arrayed, scanning unit configured to reciprocally scan the printhead in a direction perpendicular to an array direction of the printing elements, driving unit configured to divide the element array into a plurality of groups each including consecutive printing elements, and time-divisionally driving the printing elements in each group, conveyance unit configured to convey a printing medium in the array direction of the printing elements, and setting unit configured to set a driving order in the time divisional driving, wherein the conveyance unit performs a first conveyance operation of conveying the printing medium by a conveyance amount which is an integer multiple of a width of the group, and a second conveyance operation of conveying the printing medium by a conveyance amount which is not an integer multiple of the width of the group, and the setting unit sets the driving order in the time divisional driving for each scan based on the conveyance amount by the conveyance unit. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are views exemplifying the arrangement of a printing apparatus  10  according to an embodiment of the present invention; 
         FIG. 2  is a diagram exemplifying the arrangement of the driving circuit of a printhead  20 ; 
         FIG. 3  is a block diagram exemplifying the arrangement (electrical circuit) of a control system in the printing apparatus  10 ; 
         FIG. 4  is a block diagram exemplifying the internal arrangement of a main board  40  shown in  FIG. 3 ; 
         FIGS. 5A to 5C  are views for explaining an outline of conveyance control in the printing apparatus  10 ; 
         FIG. 6  is a view for explaining an outline of conveyance control in the printing apparatus  10 ; 
         FIG. 7  is a view for explaining the relationship between orifices used in a printing operation and a conveyance amount in each printing scan; 
         FIG. 8  is a view for explaining the relationship between orifices used in a printing operation and a conveyance amount in each printing scan; 
         FIG. 9  is a view for explaining the relationship between orifices used in a printing operation and a conveyance amount in each printing scan; 
         FIG. 10  is a view for explaining an outline of time divisional driving of an orifice array when printing in a region indicated by dotted lines shown in  FIG. 7 ; 
         FIG. 11  is a flowchart exemplifying a processing sequence in the printing apparatus  10 ; 
         FIG. 12  is a view for explaining an outline of time divisional driving of an orifice array when printing in a region indicated by dotted lines shown in  FIG. 8 ; 
         FIG. 13  is a view for explaining a conventional technique; and 
         FIG. 14  is a view for explaining a conventional technique. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the following description, a printing apparatus using an inkjet printing method will be exemplified. The printing apparatus may be, for example, a single-function printer having only a printing function, or a multifunction printer having a plurality of functions including a printing function, FAX function, and scanner function. Also, the printing apparatus may be, for example, a manufacturing apparatus used to manufacture a color filter, electronic device, optical device, micro-structure, and the like using a predetermined printing system. 
     In the following description, “print” not only includes the formation of significant information such as characters and graphics, but also broadly includes the formation of images, designs, patterns, structures, and the like on a printing medium, or processing of the medium, regardless of whether they are significant or insignificant and whether they are so visualized as to be visually perceived by humans. 
     Also, a “printing medium” not only includes paper used in general printing apparatuses, but also broadly includes materials capable of accepting ink, such as cloth, plastic film, metal plate, glass, ceramics, resin, wood, and leather. 
     Also, “ink” should be broadly interpreted, similar to the definition of “print” described above. “Ink” includes a liquid which, when applied onto a printing medium, can form images, designs, patterns, and the like, can process the printing medium, or can be used for ink processing (for example, solidification or insolubilization of a coloring material contained in ink applied to a printing medium). 
     Further, a “printing element” (to be also referred to as a “nozzle”) generically unit an ink orifice or a liquid channel communicating with it, and an element for generating energy used to discharge ink, unless otherwise specified. 
       FIG. 1A  is a view exemplifying the overall arrangement of a printing apparatus  10  according to an embodiment of the present invention. 
     In the printing apparatus  10 , an inkjet printhead (to be referred to as a printhead hereinafter)  20  which prints by discharging ink according to the inkjet method is mounted on a carriage  1 . The carriage  1  reciprocates in a predetermined direction (the main scanning direction) to print. The printing apparatus  10  conveys a printing medium P such as a printing sheet in a direction (the sub-scanning direction) perpendicular to the main scanning direction. The printing apparatus  10  prints by discharging ink from the printhead  20  to the printing medium P. 
     The carriage  1  receives the rotational force of a carriage motor (driving source)  2  via a belt  4 . The carriage  1  can therefore reciprocate on a chassis  9 . The printing apparatus  10  drives the carriage motor  2  while an encoder light-receiving unit  11  detects the displacement amount of a linear encoder  3 , thereby controlling the position of the carriage  1 . 
     The printing apparatus  10  rotates a conveyance roller  5  to convey the printing medium P in the sub-scanning direction. The conveyance roller  5  rotates upon receiving the rotational force of a conveyance motor  6  via a belt  8 . The printing apparatus  10  drives the conveyance motor  6  while the encoder light-receiving unit  11  detects the angular displacement of a rotary encoder  7  attached to the conveyance roller  5 . By this operation, the rotational amount of the conveyance roller  5  is controlled, and the conveyance amount of the printing medium P is controlled. 
     As shown in  FIG. 1B ,  12  orifice arrays  101  to  112  are arrayed in the printhead  20  according to the embodiment. The respective orifice arrays discharge inks of respective colors. The orifice arrays  101  to  112  can discharge, for example, inks of gray, photo black, light gray, dark gray, light cyan, magenta, yellow, light magenta, matte black, cyan, red, and clear. Each of the orifice arrays  101  to  112  of the respective colors is formed from, for example, two orifice arrays each including 512 orifices at a 600-dpi pitch. The two orifice arrays are shifted at half the pitch (1200-dpi interval) in the orifice array direction (sub-scanning direction). As a result, an orifice array including 1,024 orifices at the 1200-dpi interval is pseudo-formed for each color. 
     At each orifice, for example, an electrothermal transducer (heater) is arranged as a printing element. Each orifice discharges ink using thermal energy. In the embodiment, discharge of ink using the electrothermal transducer is described as an ink discharge method, but the ink discharge method is not limited to this. Various inkjet methods are available, including a method using a piezoelectric element, a method using an electrostatic element, and a method using a MEMS element. 
     A plurality of orifices (printing elements) arranged in the printhead  20  are divided into groups each including a predetermined number of printing elements. The printing elements in each group are time-divisionally driven. An outline of time divisional driving in the driving circuit of the printhead  20  shown in  FIG. 1B  will be explained briefly with reference to  FIG. 2 . 
     M printing elements R01 to RM are commonly connected to a driving voltage VH at one end, and connected to an M-bit driver  160  at the other end. The M printing elements are divided into L groups each including N adjacent printing elements. 
     The M-bit driver  160  receives AND signals between an output signal from an M-bit latch  170 , and N-bit block selection signals BE1 to BEN. 
     The M-bit latch  170  holds an M-bit signal output from an M-bit shift register  180 . Upon receiving a latch signal LAT, the M-bit latch  170  latches (holds) the M-bit data held in the M-bit shift register  180 . 
     The M-bit shift register  180  is a circuit which holds image data in correspondence with a printing signal. The M-bit shift register  180  receives image data sent via a signal line S_IN in synchronism with an image data transfer clock SCLK. 
     In the driving circuit having this arrangement, temporally divided driving signals are sequentially input as the N-bit (N) block enable selection signals BE1 to BEN. In response to this, the M printing elements are time-divisionally driven for N respective driving blocks each including one printing element in each group. That is, a plurality of printing elements in the printhead are divided into a plurality of driving blocks, and time-divisionally driven at timings different from each other. 
     The arrangement (electrical circuit) of a control system in the printing apparatus  10  shown in  FIG. 1A  will be exemplified with reference to  FIG. 3 . 
     The printing apparatus  10  includes, as building components of the control system, a carriage board  31 , main board  40 , power supply unit  32 , and front panel  33 . 
     The power supply unit  32  is connected to the main board  40 , and supplies driving power to each building component. 
     The carriage board  31  is a board unit mounted on the carriage  1 , and exchanges various signals with the printhead  20  via a head connector  201 . In addition, the carriage board  31  supplies head driving power via the head connector  201 . The carriage board  31  is connected to the main board  40  via a flexible flat cable (CRFFC)  210 . 
     The carriage board  31  detects a change of the positional relationship between an encoder scale  205  and an encoder sensor  204 , based on a pulse signal output from the encoder sensor  204  along with movement of the carriage  1 . The carriage board  31  outputs the output signal to the main board  40  via the CRFFC  210 . 
     The main board  40  is a board unit which performs driving control of the respective units of the printing apparatus  10 . A host I/F (InterFace)  41  is arranged on the main board  40 . The main board  40  receives data from a host computer (not shown) via the I/F  41 , and controls various printing operations based on the data. 
     The main board  40  includes the carriage motor  2  serving as a driving source for moving the carriage  1 , and the conveyance motor  6  serving as a driving source for conveying a printing medium. The main board  40  also includes an AP motor  208  and EP motor  209 . The main board  40  also controls driving of these motors. 
     The main board  40  exchanges a sensor signal  206  (including a control signal and detection signal) with various sensors (for example, the encoder sensor  204 ) which detect the operation statuses of the respective units of the printing apparatus. The main board  40  is also connected to the CRFFC  210  and power supply unit  32 . 
     The front panel  33  is a user interface between the user and the printing apparatus  10 . The front panel  33  includes a power key  211 , resume key  212 , LED  213 , flat pass key  214 , and device I/F  215 . The operation of the front panel  33  is controlled based on a panel signal  207  from the main board  40 . 
     The internal arrangement of the main board  40  shown in  FIG. 3  will be exemplified with reference to  FIG. 4 . 
     In addition to the host I/F  41 , the main board  40  includes a driver reset circuit  42 , RAM  43  (Random Access Memory), ROM (Read Only Memory)  44 , ASIC (Application Specific Integrated Circuit)  45 , EEPROM (Electrically Erasable PROM)  46 , power control circuit  47 , and head temperature detection circuit  48 . 
     The ASIC  45  is a one-chip semiconductor integrated circuit, and outputs a motor control signal  306 , power control signal  310 , power supply unit control signal  313 , and the like. The ASIC  45  is connected to the RAM  43  and ROM  44 , and performs various control operations in accordance with a program stored in the ROM  44  by using the RAM  43  as a work area. The RAM  43  is implemented by, for example, a DRAM (Dynamic Random Access Memory), and is used as a printing data buffer, a reception buffer for data from a host computer, or a work area necessary for various control operations. 
     The ASIC  45  exchanges the sensor signal  206  regarding various sensors, and detects, for example, the state of an encoder signal (ENC)  310 . The ASIC  45  executes various logical operations, condition determination, and the like in accordance with the connection of the host I/F  41  and the data input state, controls the respective units, and controls the printing apparatus  10 . 
     The ASIC  45  detects the state of the encoder signal (ENC)  310  to generate a timing signal, and controls the printing operation of the printhead  20  using a head control signal  312 . The encoder signal (ENC)  310  is an output signal which is input from the encoder sensor  204  via the CRFFC  210 . 
     The EEPROM  46  stores various types of information such as the printing history. For example, the ASIC  45  counts the number of dots from the respective orifices of the printhead  20  based on monitoring of the head control signal  312 , and stores, as a printing history in the EEPROM  46 , a numerical value obtained by calculating the accumulation. The value of the printing history is called, as needed. 
     The power control circuit  47  controls power supply to each sensor including a light-emitting element, and the like in accordance with the power supply control signal  310  from the ASIC  45 . The head temperature detection circuit  48  detects the temperature of the printhead  20  based on the head control signal  312 . 
     The host I/F  41  outputs a host I/F signal  307  from the ASIC  45  to a host I/F cable  308  (connected to the outside), and inputs a signal from the cable  308  to the ASIC  45 . 
     The power supply unit  32  supplies power to the respective units based on the power supply unit control signal  313  from the ASIC  45 . If necessary, the supplied power is converted into a voltage, and then supplied to the respective units inside and outside the main board  40 . The power supply unit  32  shifts the printing apparatus  10  to a low power consumption mode or the like based on the power supply unit control signal  313 . 
     The ASIC  45  includes, as functional components, a conveyance amount determination unit  81 , cumulative conveyance amount calculation unit  82 , conveyance control unit  83 , pattern setting unit  84 , and driving control unit  85 . 
     The conveyance amount determination unit  81  determines the conveyance amount of a printing medium in each printing scan by the printhead  20 . The conveyance amount is determined based on, for example, a plurality of conveyance amounts held in advance in the ROM  44  or the like. The ROM  44  or the like stores a conveyance pattern (conveyance amount) corresponding to printing using all orifices, and a conveyance pattern corresponding to printing using some orifices. 
     The cumulative conveyance amount calculation unit  82  calculates the cumulative conveyance amount (within a page) of a printing medium. The conveyance control unit  83  controls a conveyance unit (for example, conveyance roller and discharge roller) to convey a printing medium based on the conveyance amount determined by the conveyance amount determination unit  81 . The conveyance control unit  83  controls conveyance of the printing medium using the distance (600 dpi in this case) between orifices (between printing elements) as a unit. The conveyance unit (for example, conveyance roller and discharge roller) can change the printing medium conveyance amount using 600 dpi as a unit. 
     The pattern setting unit  84  sets driving block patterns (driving order patterns) for respective orifices (for respective printing elements) in each group based on a conveyance amount calculated by the cumulative conveyance amount calculation unit  82 . The driving block patterns represent information which defines the driving order of printing elements. 
     The driving control unit  85  time-divisionally drives a plurality of printing elements in accordance with driving block patterns set by the pattern setting unit  84 . The functional components implemented on the ASIC  45  have been exemplified. 
     An outline of conveyance control of the printing medium P in the printing apparatus  10  shown in  FIG. 1A  will be described with reference to  FIGS. 5A to 5C  and  6 . 
     When printing in a downstream region (leading end) on the printing medium P in the conveyance direction, the conveyance roller  5  and a pinch roller  51  support an upstream region (trailing end) on the printing medium in the conveyance direction, as shown in  FIG. 5A . However, the downstream region (leading end) is not supported by the rollers, and the conveyance state becomes unstable. 
     When printing in a center region on the printing medium P, the conveyance roller  5  and pinch roller  51  support the leading end region on the printing medium P, as shown in  FIG. 5B . Further, a discharge roller  53  and spur roller  52  support the trailing end. That is, when printing in the center region, the printing medium is conveyed to the position of a platen  54  while its leading and trailing ends are supported by the rollers. At a position where the printing medium faces the platen  54 , the carriage  1  scans to print. Hence, printing is performed on the printing medium in a stable conveyance state. 
     When printing on the trailing end of the printing medium P, the discharge roller  53  and spur roller  52  support the leading end region on the printing medium P, as shown in  FIG. 5C . However, the trailing end region on the printing medium P is not supported by the rollers, and the conveyance state becomes unstable. 
     In the states shown in  FIGS. 5A and 5C , printing is performed on the printing medium P in an unstable conveyance state. As shown in  FIG. 6 , a region on a printing medium is roughly divided into three regions  61 ,  62 , and  63 . Printing is executed in a stable state only in the center region  62  among these regions. 
     To ensure the conveyance accuracy in printing in a region in an unstable conveyance state, the embodiment performs the printing operation using not all but only some orifices of the orifice array. 
     The relationship between orifices used in the printing operation and a conveyance amount in each printing scan will be explained with reference to  FIGS. 7 to 9 . 
       FIG. 7  shows a state in which an image is printed using all orifices when printing in a center region on a printing medium. In  FIG. 7 , an orifice array formed from 512 orifices at a 600-dpi pitch on one side is divided into 32 groups S1 to S32. An orifice array on the leftmost side in  FIG. 7  represents the first printing scan. An orifice array immediately adjacent to the right is positioned downstream by 48 orifices in the conveyance direction. A conveyance amount in the second printing scan will be referred to as 48 (48 orifices at the 600-dpi pitch). 
     When printing an image using all orifices, the printing medium P is conveyed in conveyance amounts of 48, 48, and 32 as repetitive units. Printing of an image in a predetermined region is completed by a total of 12 printing scans. 
       FIG. 8  shows a state in which an image is printed using only some orifices when printing in leading and trailing end regions on a printing medium. Printing uses groups S13 to S20 corresponding to 128 orifices out of 512 orifices at the 600-dpi pitch on one side (shaded region of the orifice array in  FIG. 8 ). 
     When printing an image using some orifices, the printing medium P is conveyed in conveyance amounts of 16, 8, and 8 as repetitive units. Printing of an image in a predetermined region is completed by a total of 12 printing scans. 
       FIG. 9  shows an intermediate state in which printing in a leading end region using only some orifices shifts to printing in a center region using all orifices. In this case, the printing medium conveyance amount and the number of orifices used in the printing operation are switched during the printing operation. More specifically, the number of orifices used in the printing operation gradually increases to print. 
       FIG. 10  is a view showing an outline of time divisional driving of an orifice array when printing in a region indicated by dotted lines shown in  FIG. 7  (printing using all orifices). 
     Reference numeral  71  denotes an outline of time divisional driving in group S32 in the first printing scan. All orifices to be described here belong to 16 orifice groups. The discharge timing is shifted to drive orifices so that orifices in a group discharge ink at timings different from each other. As for numerals described on the left and right sides of each orifice, a numeral on the left side indicates an orifice number (for example, S32-1), and a numeral on the right side indicates a driving timing. For example, among orifices of group S32, S32-1 is the first orifice, and S32-16 is the 16th orifice. In time divisional driving denoted by reference numeral  71 , the orifice S32-1 is driven at the first driving timing. The orifice S32-9 is driven at the second driving timing. That is, a plurality of orifices belonging to each group are driven in the order of patterns (driving block patterns) which define the driving order of orifices. 
     The embodiment executes multi-pass printing (2-pass printing in this case) in two directions in the same printing region on a printing medium. More specifically, time divisional driving operations denoted by reference numerals  71  and  73  represent forward printing scans, and a time divisional driving operation denoted by reference numeral  72  represents a reverse printing scan. In the forward printing scan, a block selection signal is generated to drive orifices in the order of the first driving timing, second driving timing, . . . , 16 driving timing in accordance with the driving block patterns shown in  FIG. 10 , thereby discharging ink onto a printing medium. In the reverse printing scan, a block selection signal is generated to drive orifices in the order of the 16th driving timing, 15th driving timing, . . . , first driving timing in accordance with the driving block patterns shown in  FIG. 10 , thereby discharging ink onto the printing medium. 
     A time divisional driving processing sequence in the printing operation shown in  FIG. 10  will be described with reference to  FIG. 11 . More specifically, a processing sequence when setting patterns (driving block patterns) which define the driving order of orifices will be explained. As described above, when printing using all orifices, the printing medium P is conveyed in conveyance amounts of 48, 48, and 32 as repetitive units. 
     Note that the driving block patterns are common to all groups in the same printing scan. In the second printing scan, orifices of a group (group S29 in  FIG. 10 ) different from that in the first printing scan print in the same printing region on the printing medium. Driving block patterns at this time are determined at the start of a printing scan. 
     The printing apparatus  10  controls the cumulative conveyance amount calculation unit  82  to calculate a cumulative conveyance amount (step S 101 ). The cumulative conveyance amount unit a total conveyance amount from the first printing scan of a target page, and is calculated for the number (16 in this case) of orifices of one group as a unit. 
     In the embodiment, the conveyance amount is calculated at every 600 dpi. Assume that a cumulative conveyance amount in a printing scan denoted by reference numeral  71  is 16N (N is an integer). Then, a cumulative conveyance amount at the time (second printing scan) denoted by reference numeral  72  is a value obtained by adding, to the cumulative conveyance amount up to the time denoted by reference numeral  71 , a conveyance amount of 48 from reference numeral  71  to reference numeral  72 , that is, 16N+48. 
     Subsequently, the printing apparatus  10  controls the pattern setting unit  84  to calculate a remainder by dividing the cumulative conveyance amount of 16N+48 at this time by the number of driving block patterns of one cycle. In the embodiment, one cycle of driving block patterns is set for one group, one group includes 16 orifices, and thus the number of driving block patterns of one cycle is 16. Hence, “(16N+48)/16”, and the remainder is 0. 
     Since the remainder is 0 (YES in step S 102 ), the printing apparatus  10  controls the pattern setting unit  84  to assign block numbers to respective orifices belonging to a target group (group S29) in accordance with original driving block patterns (step S 103 ). In the second printing scan, a printing scan is executed using the original driving block patterns, similar to the printing scan denoted by reference numeral  71  (step S 105 ). 
     In the third printing scan, printing is performed in the same printing region on the printing medium using orifices belonging to group S27. Driving block patterns at this time are set in the above-described way. In this case, the cumulative conveyance amount is a value obtained by adding, to the cumulative conveyance amount (16N+48) up to the time denoted by reference numeral  72 , a conveyance amount of 32 from reference numeral  72  to reference numeral  73 , that is, 16N+80. 
     The printing apparatus  10  controls the pattern setting unit  84  to calculate a remainder by dividing the cumulative conveyance amount of 16N+80 by the number (16 in this case) of block patterns of one cycle. More specifically, “(16N+80)/16”, and the remainder is 0. 
     Since the remainder is 0 (YES in step S 102 ), the printing apparatus  10  controls the pattern setting unit  84  to assign block numbers to respective orifices belonging to a target group (group S27) in accordance with original driving block patterns (step S 103 ). In the third printing scan (reference numeral  73 ), a printing scan is executed using the original driving block patterns, similar to the printing scan denoted by reference numeral  71  (step S 105 ). 
     As described above, when printing using all orifices, conveyance amounts of 48, 48, and 32 serve as repetitive units. For this reason, the original driving block patterns are used in all 12 printing scans which are repeated in a region to be printed on a printing medium. 
       FIG. 12  shows the state of time divisional driving of an orifice array when printing in a region indicated by dotted lines shown in  FIG. 8  (printing using some orifices). A time divisional driving processing sequence in the printing operation shown in  FIG. 12  will be described with reference to  FIG. 11 . As described above, when printing using some orifices, conveyance amounts of 16, 8, and 8 serve as repetitive units in conveyance of the printing medium P. As for numerals described on the left and right sides of each orifice, a numeral on the left side indicates an orifice number (for example, S20-1), and a numeral on the right side indicates a driving timing, similar to  FIG. 10 . 
     In the second printing scan denoted by reference numeral  75  in  FIG. 12 , printing is performed in the same printing region on a printing medium using orifices S19-9 to S19-16 of the lower half of group S19 and orifices S20-1 to S20-8 of the upper half of group S20. 
     The printing apparatus  10  controls the cumulative conveyance amount calculation unit  82  to calculate a cumulative conveyance amount (step S 101 ). Assume that a cumulative conveyance amount in a printing scan denoted by reference numeral  74  is 16N (N is an integer). Then, a cumulative conveyance amount at the time (second printing scan) denoted by reference numeral  75  is a value obtained by adding, to the cumulative conveyance amount up to the time denoted by reference numeral  74 , a conveyance amount of 8 from reference numeral  74  to reference numeral  75 , that is, 16N+8. 
     The printing apparatus  10  controls the pattern setting unit  84  to calculate a remainder by dividing the cumulative conveyance amount of 16N+8 by the number (16 in this case) of driving block patterns of one cycle. More specifically, “(16N+8)/16”, and the remainder is 8. 
     Since the remainder is 8 (NO in step S 102 ), the printing apparatus  10  controls the pattern setting unit  84  to shift the driving block patterns by the remainder of 8, and assign block numbers to respective orifices belonging to target groups (step S 104 ). 
     More specifically, block numbers different from those of the original driving block patterns are set for orifices belonging to groups S19 and S20, as denoted by reference numeral  75 . 
     When the original driving block patterns are set, the driving order is set in the order of “1, 11, 5, 15, 9, 3, 13, 7, 2, 12, 6, 16, 10, 4, 14, 8” from the top orifice in the group, as denoted by reference numeral  74 . 
     In contrast, the driving block patterns are shifted by eight orifices and assigned, as denoted by reference numeral  75 . The driving order is set in the order of “2, 12, 6, 16, 10, 4, 14, 8, 1, 11, 5, 15, 9, 3, 13, 7”. 
     In the third printing scan, printing is performed using orifices belonging to group S19 in the same printing region on the printing medium. Driving block patterns at this time are set by the above-described processing. In this case, the cumulative conveyance amount is a value obtained by adding, to the cumulative conveyance amount (16N+8) up to the time denoted by reference numeral  75 , a conveyance amount of 8 from reference numeral  75  to reference numeral  76 , that is, 16N+16. 
     The printing apparatus  10  controls the pattern setting unit  84  to calculate a remainder by dividing the cumulative conveyance amount of 16N+16 by the number (16 in this case) of driving block patterns of one cycle. More specifically, “(16N+16)/16”, and the remainder is 0. 
     Since the remainder is 0 (YES in step S 102 ), the printing apparatus  10  controls the pattern setting unit  84  to assign block numbers to respective orifices belonging to a target group (group S19) in accordance with original driving block patterns (step S 103 ). In the third printing scan (reference numeral  76 ), a printing scan is executed using the original driving block patterns, similar to the printing scan denoted by reference numeral  74  (step S 105 ). In this case, the original driving block patterns are set as driving block patterns. The driving order is set in the order of “1, 11, 5, 15, 9, 3, 13, 7, 2, 12, 6, 16, 10, 4, 14, 8” from the top orifice in the group. 
     As described above, according to the embodiment, when printing in the same printing region on a printing medium by multi-pass printing, driving block patterns are set to time-divisionally drive respective printing elements in the same driving order in respective printing scans. In other words, the driving orders of a plurality of printing elements used in printing in respective printing scans coincide with each other. 
     This can prevent degradation of image uniformity caused by disturbance of the layout position of a dot formed on a printing medium when the printing medium conveyance amount does not coincide with an integer multiple of the number of driving block patterns of one cycle. The printing quality can therefore be improved. 
     A typical embodiment of the present invention has been exemplified. However, the present invention is not limited to the above-described embodiment illustrated in the drawings, and can be properly modified without departing from the scope of the invention. 
     For example, in the above-described embodiment, the number of driving block patterns of one cycle is 16, and printing medium conveyance amounts are “48, 48, 32” or “16, 8, 8”. However, the present invention is not limited to them. That is, a combination of the number of driving block patterns and conveyance amounts is arbitrary. 
     For example, in the above-described embodiment, the driving block patterns are shifted based on the printing medium conveyance amount. However, the present invention is not limited to this. The embodiment can adopt any method as long as the driving orders of a plurality of printing elements used in printing in respective printing scans of the same printing region coincide with each other when performing multi-pass printing in the same printing region on a printing medium. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-224307, filed Oct. 11, 2011, which is hereby incorporated by reference herein in its entirety.