Patent Publication Number: US-2011050774-A1

Title: Liquid Ejecting Apparatus and Liquid Ejecting Method

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     Japanese Patent application No. 2009-195941 is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a liquid ejecting apparatus and a liquid ejecting method. 
     2. Description of Related Art 
     Ink jet printers that form images by ejecting ink while moving heads are used. Such printers include a printer that forms an image by ejecting ink in both forward and backward scanning directions of a head. 
     When the printer ejects ink in both directions, an ink-landing position on a medium in the forward scanning direction of the head should be aligned with an ink-landing position on the medium in the backward scanning direction, to increase image quality of an image that is formed on the medium. Owing to this, patterns for inspecting the ink-landing positions in the forward and backward scanning directions are printed. Ejection timings of the ink are corrected on the basis of the patterns. The ink-landing positions in the forward and backward scanning directions are adjusted to be aligned with one another. (For example, see JP-A-2002-205385 and JP-A-2005-138323.) 
     Even if the ejection timings are corrected during the forward scanning and the backward scanning, it is still difficult to correct the landing positions under an environment with a severe temperature condition, resulting in the image quality being degraded. It is necessary to reduce a shift between the liquid-landing positions under such an environment. 
     SUMMARY OF INVENTION 
     An advantage of some aspects of the invention is to reduce a shift between liquid-landing positions. 
     According to an aspect of the invention, a liquid ejecting apparatus includes a head that ejects liquid onto a medium; a head-moving unit that moves the head in a moving direction; a temperature-acquiring unit that acquires a temperature relating to the head; and a control unit that controls the head and the head-moving unit. The control unit corrects an ejection timing of the liquid and causes the head to eject the liquid during forward scanning and backward scanning in the moving direction if the temperature acquired by the temperature-acquiring unit is within a predetermined range. The control unit causes the head to eject the liquid during one of the forward scanning and the backward scanning in the moving direction if the temperature acquired by the temperature-acquiring unit is outside the predetermined range. 
     Other features of the invention will be described in the specification with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is an explanatory view showing an external configuration of a print system according to an exemplary embodiment. 
         FIG. 2  is a block diagram showing a general configuration of a printer according to the embodiment. 
         FIG. 3A  briefly illustrates a general configuration of the printer according to the embodiment. 
         FIG. 3B  is a cross-sectional view showing the general configuration of the printer according to the embodiment. 
         FIG. 4A  briefly illustrates a configuration of a linear encoder. 
         FIG. 4B  schematically illustrates a configuration of a detector. 
         FIG. 5A  is a timing chart showing waveforms of two output signals of the detector during forward rotation of a carriage motor. 
         FIG. 5B  is a timing chart showing waveforms of two output signals of the detector during reverse rotation of the carriage motor. 
         FIG. 6A  is an explanatory view showing a structure of a head. 
         FIG. 6B  is an explanatory view showing arrangement of nozzles in a lower surface of the head. 
         FIG. 7A  is an explanatory view showing a drive circuit of a head unit. 
         FIG. 7B  is an explanatory view showing the drive circuit. 
         FIG. 8  is a timing chart for explaining respective signals. 
         FIG. 9  is an explanatory view showing ink-landing positions during bidirectional printing. 
         FIG. 10A  is an explanatory view showing a pattern for inspecting a shift between ink-landing positions. 
         FIG. 10B  is an explanatory view showing a pattern after an ejection timing of the ink is adjusted. 
         FIG. 11A  is an explanatory view showing a relationship between an original signal and control signals before the adjustment of the ejection timing of the ink. 
         FIG. 11B  is an explanatory view showing a relationship between the original signal and the control signals after the adjustment of the ejection timing of the ink. 
         FIG. 12  is a flowchart showing a printing process according to a first embodiment. 
         FIG. 13  is a flowchart showing a printing process according to a second embodiment. 
         FIG. 14  is a graph plotting a relationship between a temperature and a viscosity of ink. 
         FIG. 15  is a table explaining a correction value with respect to a thermistor-detected temperature. 
         FIG. 16  is an explanatory view showing acquisition of a correction value for a thermistor-detected temperature. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the specification and the attached drawings, at least the following matters will be defined. 
     A liquid ejecting apparatus includes a head that ejects liquid onto a medium; a head-moving unit that moves the head in a moving direction; a temperature-acquiring unit that acquires a temperature relating to the head; and a control unit that controls the head and the head-moving unit. The control unit corrects an ejection timing of the liquid and causes the head to eject the liquid during forward scanning and backward scanning in the moving direction if the temperature acquired by the temperature-acquiring unit is within a predetermined range. The control unit causes the head to eject the liquid during one of the forward scanning and the backward scanning in the moving direction if the temperature acquired by the temperature-acquiring unit is outside the predetermined range. 
     Accordingly, a shift between liquid-landing positions can be reduced. 
     In the liquid ejecting apparatus, it may be determined whether the temperature is within the predetermined range, for each page of the medium. Accordingly, the ejection timing is not changed from the middle of a page of the medium, and hence image quality can be prevented from being changed. 
     Also, the control unit may determine whether an image to be formed on the medium contains a line if the temperature is outside the predetermined range, and the control unit may cause the head to eject the liquid during one of the forward scanning and the backward scanning in the moving direction if the image contains the line. Further, the line may extend in a direction intersecting with the moving direction. Accordingly, even if the temperature relating to the head is outside the predetermined range, the head can eject the liquid during the forward scanning and the backward scanning as long as the image does not contain the line. Thus, a printing speed can be increased. 
     Further, the head may include a plurality of nozzle arrays that eject liquid of a plurality of colors, and the ejection timing of the liquid when the head ejects the liquid during the forward scanning and the backward scanning may be corrected, for each of the nozzle arrays. Accordingly, even if liquid with different viscosities depending on nozzle arrays is ejected, the liquid can be ejected at ejection timings suitable for each of the nozzle arrays. 
     Further, the ejection timing of the liquid may be corrected when the head ejects the liquid during one of the forward scanning and the backward scanning. Accordingly, even if the liquid is ejected during one of the forward scanning and the backward scanning, the liquid-landing position can be adjusted in the moving direction by correcting the ejection timing. 
     Further, a correction value that corrects the ejection timing of the liquid may be determined in accordance with the temperature relating to the head when the head ejects the liquid during one of the forward scanning and the backward scanning. Accordingly, the ejection timing can be corrected in accordance with the viscosity of the liquid, the viscosity which changes with temperature. 
     A liquid ejecting method includes acquiring a temperature relating to a head that ejects liquid onto a medium; correcting an ejection timing of the liquid and causing the head to eject the liquid during forward scanning and backward scanning in a moving direction of the head if the temperature relating to the head is within a predetermined range; and causing the head to eject the liquid during one of the forward scanning and the backward scanning in the moving direction of the head if the temperature relating to the head is outside the predetermined range. 
     Accordingly, a shift between liquid-landing positions can be reduced. 
     Exemplary Embodiment 
     Configuration of Print System 
     An exemplary embodiment of a print system (computer system) will be described below with reference to the attached drawings. It is to be noted that the following embodiments include embodiments of a computer program and a storage medium storing a computer program. 
       FIG. 1  is an explanatory view showing an external configuration of a print system  100 . The print system  100  includes a printer  1 , a computer  10 , a display device  120 , an input device  130 , and a recording and reproducing device  140 . The printer  1  is a printing device that prints an image on a medium, such as a sheet of paper, a piece of cloth, or a film. A computer  110  is electrically connected to the printer  1 . To cause the printer  1  to print an image, the computer  110  outputs print data to the printer  1 . The print data corresponds to the image to be printed. The display device  120  includes a display, and displays user interfaces of, for example, an application program and a printer driver. The input device  130  includes, for example, a keyboard  130 A and a mouse  130 B. The input device  130  is used for operating the application program and for setting the printer driver with the user interfaces displayed on the display device  120 . The recording and reproducing device  140  includes, for example, a flexible disk drive  140 A and a CD-ROM drive  140 B. 
     A printer driver is installed in the computer  110 . The printer driver is a program that provides a function for causing the display device  120  to display the user interfaces, and a function for converting image data output from the application program into print data. The printer driver is stored in a storage medium (a computer-readable storage medium), such as a flexible disk (FD) or a CD-ROM. Alternatively, the printer driver may be downloaded to the computer  110  through the Internet. The program includes codes for providing the functions. 
     “The printing device” is the printer  1  in a narrow sense, but is a system including the printer  1  and the computer  110  in a broad sense. 
     Configuration of Ink Jet Printer 
       FIG. 2  is a block diagram showing a general configuration of the printer  1  according to this embodiment.  FIG. 3A  briefly illustrates the general configuration of the printer  1  according to the embodiment.  FIG. 3B  is a cross-sectional view showing the general configuration of the printer  1  according to the embodiment. A basic configuration of the printer  1  according to this embodiment will be described below. 
     The printer  1  according to this embodiment includes a transport unit  20 , a carriage unit  30 , a head unit  40 , a detectors group  50 , and a controller  60 . The printer  1  that has received the print data from the computer  110 , which serves as an external device, uses the controller  60  to control the respective units (the transport unit  20 , the carriage unit  30 , and the head unit  40 ). The controller  60  controls the respective units in accordance with the print data received from the computer  110 , to form an image on a sheet. The detectors group  50  monitors the state in the printer  1 . The detectors group  50  outputs the detection result to the controller  60 . When the controller  60  receives the detection result from the detectors group  50 , the controller  60  controls the respective units on the basis of the detection result. 
     The transport unit  20  feeds a medium (for example, sheet S) to a printable position, and transports the medium in a predetermined direction (hereinafter, referred to as transport direction) at a predetermined transport rate during printing. That is, the transport unit  20  functions as a transport mechanism that transports a sheet. The transport unit  20  includes a sheet-feed roller  21 , a transport motor  22  (also referred to as PF motor), a transport roller  23 , a platen  24 , and a sheet-output roller  25 . However, when the transport unit  20  functions as the transport mechanism, not all the components are required. The sheet-feed roller  21  automatically feeds a sheet, which has been inserted to a sheet insertion port, into the printer  1 . The sheet-feed roller  21  has a D-shaped cross section. The sheet-feed roller  21  has a larger length of a circumferential portion than a transport distance from the sheet-feed roller  21  to the transport roller  23 . Hence, the sheet-feed roller  21  can transport a sheet S to the transport roller  23  by using the circumferential portion. The transport motor  22  is a DC motor, and transports the sheet S in the transport direction. The transport roller  23  transports the sheet S, which has been fed by the sheet-feed roller  21 , to a printable region. The transport roller  23  is driven by the transport motor  22 . The platen  24  supports the sheet S during the printing. The sheet-output roller  25  outputs the sheet S outside the printer  1  after the printing. The sheet-output roller  25  rotates synchronously with the transport roller  23 . 
     The carriage unit  30  moves a head (scans with a head) in a predetermined direction (hereinafter, referred to as moving direction). The carriage unit  30  includes a carriage  31  and a carriage motor  32  (also referred to as CR motor). The carriage  31  can reciprocate in the moving direction (accordingly, the head moves in the moving direction). Also, the carriage  31  detachably holds an ink cartridge containing ink. The carriage motor  32  is a DC motor, and moves the carriage  31  in the moving direction. 
     The head unit  40  ejects ink on a sheet. The head unit  40  includes a head  41 . The head  41  has a plurality of nozzles serving as ink ejection portions. The nozzles intermittently eject ink. The head  41  is provided on the carriage  31 . Hence, when the carriage  31  moves in the moving direction, the head  41  also moves in the moving direction. If the head  41  intermittently ejects the ink while the head  41  moves in the moving direction, a dot line (raster line) is formed on a sheet in the moving direction. The head unit  40  acquires data for driving the head  41  from the controller  60  in a printer body through a cable  45 . The cable  45  is a flexible and flat cable, and is electrically connected to the printer body and the carriage  31 . 
     The detectors group  50  includes a linear encoder  51 , a rotary encoder  52 , a sheet-detecting sensor  53 , an optical sensor  54 , etc. The linear encoder  51  detects the position of the carriage  31  in the moving direction. The rotary encoder  52  detects a rotating amount of the transport roller  23 . The sheet-detecting sensor  53  detects the position of the leading edge of the sheet to be printed. The sheet-detecting sensor  53  is provided at a position at which the sheet-detecting sensor  53  can detect the position of the leading edge of the sheet while the sheet-feed roller  21  feeds the sheet toward the transport roller  23 . The sheet-detecting sensor  53  is a mechanical sensor that detects the leading edge of the sheet by using a mechanical mechanism. To be more specific, the sheet-detecting sensor  53  includes a lever that is rotatable in the transport direction. The lever is arranged to protrude into a transport path. Thus, the leading edge of the sheet contacts the lever, and rotates the lever. The sheet-detecting sensor  53  detects the motion of the lever, and detects the position of the leading edge of the sheet. The optical sensor  54  is attached to the carriage  31 . The optical sensor  54  detects the presence of the sheet. In particular, the optical sensor  54  includes a light-emitting portion and a light-receiving portion, and detects the presence of the sheet Such that the light emitting portion irradiates the sheet with light and the light receiving portion detects the reflected light. The optical sensor  54  detects the position of the edge of the sheet while the optical sensor  54  is moved by the carriage  31 . The optical sensor  54  optically detects the edge of the sheet. Hence, the optical sensor  54  has a higher detection accuracy than the mechanical sheet-detecting sensor  53 . 
     The controller  60  is a control unit that controls the printer  1 . The controller  60  includes an interface (I/F) unit  61 , a CPU  62 , a memory  63 , and a units-controlling circuit  64 . The interface unit  61  enables data transmission between the computer  110 , which serves as the external device, and the printer  1 . The CPU  62  is a processing unit that controls the entire printer  1 . The memory  63  provides a storage area for a program of the CPU  62  and a work area for the CPU  62 . The memory  63  includes a storage unit, such as a RAM or an electrically erasable programmable read-only memory (EEPROM). The CPU  62  controls the respective units through the units-controlling circuit  64  in accordance with the program stored in the memory  63 . 
       FIG. 4A  briefly illustrates a configuration of the linear encoder  51 . The linear encoder  51  includes a linear-encoder code disc  564  and a detector  566 . Referring to  FIG. 3A , the linear-encoder code disc  564  is attached to a frame in the ink jet printer  1 . The detector  566  is attached to the carriage  31 . If the carriage  31  moves along a guide rail  36 , the detector  566  moves along the linear-encoder code disc  564  relative to the linear-encoder code disc  564 . Thus, the detector  566  detects a moving amount of the carriage  31 . 
     Configuration of Detector 
       FIG. 4B  schematically illustrates a configuration of the detector  566 . The detector  566  includes a light-emitting diode  552 , a collimator lens  554 , and a detection processing unit  556 . The detection processing unit  556  includes a plurality of (for example, four) photodiodes  558 , a signal-processing circuit  560 , and, for example, two comparators  562 A and  562 B. 
     If a voltage Vcc is applied to both ends of the light-emitting diode  552  through resistances, the light-emitting diode  552  emits light. The light is collimated by the collimator lens  554 , and passes through the linear-encoder code disc  564 . The linear-encoder code disc  564  has slits at a predetermined interval (for example, 1/180 inch, where 1 inch equals to 2.54 cm). 
     The parallel light (collimated light), which has passed through the linear-encoder code disc  564 , passes through a fixed slit (not shown), enters the photodiodes  558 , and is converted into an electric signal. Electric signals output from the four photodiodes  558  are processed in the signal-processing circuit  560 . The signals output from the signal-processing circuit  560  are compared in the comparators  562 A and  562 B. The comparison results are output in the form of pulses. The comparator  562 A outputs a pulse ENC-A, and the comparator  562 B outputs a pulse ENC-B. The pulses ENC-A and ENC-B serve as the outputs from the linear encoder  51 . 
     Output Signal 
       FIGS. 5A and 5B  are timing charts showing waveforms of the two output signals from the detector  566  during forward rotation and reverse rotation of the carriage motor  32 . Referring to  FIGS. 5A and 5B , the phase of the pulse ENC-A differs from the phase of the pulse ENC-B by 90 degrees during the forward rotation and the reverse rotation of the carriage motor  32 . When the carriage motor  32  rotates forward, that is, when the carriage  31  moves along the guide rail  36 , the phase of the pulse ENC-A is advanced by 90 degrees as compared with the phase of the pulse ENC-B as shown in  FIG. 5A . When the carriage motor  32  reversely rotates, the phase of the pulse ENC-A is delayed by 90 degrees as compared with the phase of the pulse ENC-B as shown in  FIG. 5B . A single period T of each of the pulse ENC-A and the pulse ENC-B is equivalent to a time in which the carriage  31  is moved by a distance corresponding to the interval of the slits. 
     Rising edges of each of the output pulses ENC-A and ENC-B of the linear encoder  51  are detected, the number of the detected edges is counted, and the rotational position of the carriage motor  32  is calculated on the basis of the count value. A value “+1” is added to the count value if one edge is detected while the carriage motor  32  rotates forward. A value “−1” is added to the count value if one edge is detected while the carriage motor  32  reversely rotates. The period of each of the pulses ENC-A and ENC-B is equivalent to a time from when a slit of the linear-encoder code disc  564  passes the detector  566  to when the next slit passes the detector  566 . Also, the phase of the pulse ENC-A differs from the phase of the pulse ENC-B by 90 degrees. Thus, the count value “1” corresponds to ¼ of the interval of the slits of the linear-encoder code disc  564 . By multiplying the count value by ¼, which is the interval of the slits, a moving amount of the carriage motor  32  from a rotational position, at which the count value is “0,” can be obtained on the basis of the multiplication value. At this time, the resolution of the linear encoder  51  is ¼, which is the interval of the slits of the linear-encoder code disc  564 . 
       FIG. 6A  is an explanatory view showing a structure of the head  41 .  FIG. 6A  illustrates a nozzle Nz, a piezoelectric element PZT, an ink supply channel  402 , a nozzle communication channel  404 , and an elastic plate  406 . 
     The ink supply channel  402  is supplied with ink from an ink tank (not shown). The ink is supplied to the nozzle communication channel  404 . A pulse of a drive signal (described later) is applied to the piezoelectric element PZT. When the pulse is applied, the piezoelectric element PZT expands and contracts in accordance with the signal of the pulse, and vibrates the elastic plate  406 . Accordingly, the nozzle Nz ejects ink droplets by a quantity corresponding to the amplitude of the pulse. 
     Also, a thermistor  502  is attached to the head  41 . The temperature of the thermistor  502  is output to the controller  60 . Since the thermistor  502  is attached to the head  41 , the temperature of the head  41  can be acquired. 
     Nozzles 
       FIG. 6B  is an explanatory view showing arrangement of nozzles in a lower surface of the head  41 . A black-ink nozzle array K, a cyan-ink nozzle array C, a magenta-ink nozzle array M, and a yellow-ink nozzle array Y are formed in the lower surface of the head  41 . Each nozzle array has a plurality of nozzles (180 nozzles in this embodiment). Each nozzle serves as an ejection port that ejects ink of each color. 
     The nozzles in each nozzle array are arranged in the transport direction at a regular interval (nozzle pitch of k·D). D is a minimum dot pitch in the transport direction (that is, an interval of dots formed on a sheet S with a highest resolution), and k is an integer equal to and greater than 1. For example, if the nozzle pitch is 180 dpi ( 1/180 inch), and the dot pitch in the transport direction is 720 bpi ( 1/270 inch), k=4. 
     Different numbers are assigned to the nozzles in each nozzle array (# 1  to # 180 ). A smaller number is assigned to a nozzle located at the downstream side. That is, the nozzle # 1  is located downstream of the nozzle # 180  in the transport direction. Each nozzle is provided with a piezoelectric element (not shown) serving as a drive element that drives the nozzle to eject ink droplets. 
     Driving Head 
       FIG. 7A  is an explanatory view showing a drive circuit of the head unit  40 . The drive circuit is provided in the units-controlling circuit  64 . Referring to  FIG. 7A , the drive circuit includes an original-drive-signal generating section  644 A and a drive-signal shaping section  644 B. The drive circuit for the nozzles # 1  to # 180  is provided for each nozzle group, that is, for each nozzle array of black (K), cyan (C), magenta (M), and yellow (Y). In addition, each nozzle is driven by the individual piezoelectric element. Referring to  FIG. 7A , a number in parentheses at the end of the name of each signal indicates the number of nozzle to which the signal is supplied. 
     When a voltage with a predetermined time width is applied to electrodes at both ends of the piezoelectric element, the piezoelectric element expands in accordance with the voltage-applied time, and deforms a side wall of an ink flow channel. Accordingly, the volume of the ink flow channel contracts as the piezoelectric element expands. Each of the nozzles # 1  to # 180  of each color ejects ink droplets by an ink quantity corresponding to the contraction volume of the ink flow channel. 
     The original-drive-signal generating section  644 A generates an original signal ODRV that is commonly used for the nozzles # 1  to # 180 . The original signal ODRV includes a plurality of pulses within a scanning period for a single pixel (i.e., within a time in which the carriage  31  moves across a distance of a single pixel). 
     The drive-signal shaping section  644 B receives the original signal ODRV from the original-drive-signal generating section  644 A, and a print signal PRT as serial data. 
       FIG. 7B  is an explanatory view showing the drive circuit. The circuit shown in  FIG. 7B  performs serial/parallel conversion for the print signal PRT by using 360 shift resistors, so that the print signal PRT is converted into PRT(i) that indicates ON/OFF of each nozzle. The drive-signal shaping section  644 B shapes the original signal ODRV in accordance with the level of the print signal PRT(i), and outputs the signal as a drive signal DRV(i) to the piezoelectric element of each of the nozzles # 1  to # 180 . The piezoelectric element of each of the nozzles # 1  to # 180  is driven in accordance with the drive signal DRV from the drive-signal shaping section  644 B. 
     Drive Signal of Head 
       FIG. 8  is a timing chart for explaining respective signals. In particular,  FIG. 8  is a timing chart for the respective signals including the original signal ODRV, the print signal PRT(i), and the drive signal DRV(i). The print signal PRT(i) is generated from the print signal PRT. 
     The original signal ODRV is commonly supplied to the nozzles # 1  to # 180  from the original-drive-signal generating section  644 A. In this embodiment, the original signal ODRV includes two pulses of a first pulse W 1  and a second pulse W 2  within a main-scanning period for a single pixel (i.e., within a time in which the carriage  31  moves across a distance of a single pixel). The original signal ODRV is output from the original-drive-signal generating section  644 A to the drive-signal shaping section  644 B. 
     The print signal PRT(i) corresponds to pixel data that is allocated to a single pixel. That is, the print signal PRT(i) corresponds to pixel data contained in print data. In this embodiment, the print signal PRT(i) includes two-bit information for each pixel. The drive-signal shaping section  644 B shapes the original signal ODRV in accordance with the level of the print signal PRT(i), and outputs the drive signal DRV. 
     The drive signal DRV is obtained when the original signal ODRV is blocked in accordance with the level of the print signal PRT(i). In particular, when the print signal PRT(i) is at a level  1 , the drive-signal shaping section  644 B allows the pulse corresponding to the original signal ODRV to pass, so that the pulse directly becomes the drive signal DRV. In contrast, when the print signal PRT(i) is at a level  0 , the drive-signal shaping section  644 B blocks the pulse of the original signal ODRV. The drive-signal shaping section  644 B outputs the drive signal DRV to the piezoelectric element provided for each nozzle. Then, the piezoelectric element is driven in accordance with the drive signal DRV. 
     Referring to  FIG. 7B , the control signal S 1  is input to a latch circuit and a data selector. The control signal S 2  is input to the data selector. Referring to  FIG. 8 , the control signals S 1  and S 2  indicate timings at which the print signal PRT(i) is changed. 
     The serially transmitted print signal PRT is converted into 180 pieces of two-bit data (parallel data) as follows. First, the print signal PRT is input into 360 shift resistors. When the pulse of the control signal S 1  is input to the latch circuit, the 360 pieces of data in the respective shift resistors are latched. The data selector selects the data latched in the latch circuit and outputs the selected data. When the pulse of the control signal S 1  is input to the latch circuit, the pulse of the control signal S 1  is also input to the data selector. When the pulse of the control signal S 1  is input to the data selector, the data selector is brought into an initial state. The data selector in the initial state selects the data, which has been stored in a shift resistor W 2 - i  before the data is latched, and the data selector outputs the data as PRT(i). Next, when the pulse of the control signal S 2  is input to the data selector, the data selector selects the data, which has been stored in a shift resistor W 1 - i  before the data is latched, and the data selector outputs the data as PRT(i). In this way, the serially transmitted print signal PRT is converted into the 180 pieces of two-bit data. The control signal S 1  determines ejection or non-election in association with the second pulse W 2 . The second signal S 2  determines ejection or non-ejection in association with the first pulse W 1 . 
     When the print signal PRT(i) corresponds to two-bit data “01,” only the first pulse W 1  is output in the latter half of a single pixel period. Accordingly, the nozzle ejects a small-size ink droplet, and hence a small-size dot is formed on a sheet. When the print signal PRT(i) corresponds to two-bit data “10,” only the second pulse W 2  is output in the former half of a single pixel period. Accordingly, the nozzle ejects a middle-size ink droplet, and hence a middle-size dot is formed on the sheet. When the print signal PRT(i) corresponds to two-bit data “11,” the first pulse W 1  and the second pulse W 2  are output in a single pixel period. Accordingly, the nozzle ejects a large-size ink droplet, and hence a large-size dot is formed on the sheet. When the print signal PRT(i) corresponds to two-bit data “00,” the first pulse W 1  or the second pulse W 2  is not output. Accordingly, the ink is not ejected in a single pixel period, and hence, no dot is formed. 
     As described above, the drive signal DRV(i) in the single pixel period is shaped so as to have four different waveforms in accordance with the four different values of the print signal PRT(i). 
       FIG. 9  is an explanatory view showing ink-landing positions during bidirectional printing.  FIG. 9  illustrates speeds, at which ink is ejected during the forward scanning and the backward scanning, in the form of vectors. Herein, the head  41  moves at a moving speed Vt during the forward scanning and the backward scanning. In this case, it is desirable to eject the ink onto the sheet S at an ejection speed V 1  and to direct the vector of DV 1  to a landing position A, so that the ink is ejected onto the landing position A during both the forward scanning and the backward scanning. 
     However, the ejection speed of the ink may be higher than V 1  for some reason.  FIG. 9  illustrates an ejection speed V 2  of the ink when the ink ejection speed is higher than V 1 . When the ejection speed is increased, although the ink is ejected at the same timing as the former case, the vector of DV 2  is not directed to the position A, resulting in that the ink is landed at a position short of the target landing position A. Then, an ink-landing position during the forward scanning may be shifted from an ink-landing position during the backward scanning in the moving direction of the head  41 . 
     Therefore, the ejection timing has to be adjusted so that the ink-landing position during the forward scanning is aligned with the ink-landing position during the backward scanning. 
       FIG. 10A  is an explanatory view showing a pattern for inspecting a shift between ink-landing positions.  FIG. 10A  illustrates a pattern P 1  including a pattern that is formed during the forward scanning and a pattern that is formed during the backward scanning. In both the pattern formed during the forward scanning and the pattern formed during the backward scanning, dots are arranged in a nozzle-array direction, in which the nozzles are arrayed. 
     The patterns are formed by ejecting the ink at predetermined ejection timings during the forward scanning and the backward scanning of the head  41 . However, a line of the pattern during the forward scanning is shifted from a line of the pattern during the backward scanning by Δx in the moving direction of the head  41  because, for example, the ejection speed of the ink is increased as described above. If the shift between the ink-landing positions is obtained (here, Δx), a time, by which the ejection timing should be shifted, can be obtained as long as the moving speed of the head  41  is previously determined. 
       FIG. 10B  is an explanatory view showing a pattern after the ejection timing of the ink is adjusted. In this case, the ejection timing of the ink during the backward scanning is adjusted such that the ink-landing position during the backward scanning is shifted by Δx leftward in  FIG. 10B  as compared with the case in  FIG. 10A . As a result, the landing position of the ink ejected during the forward scanning is aligned with the landing position of the ink ejected during the backward scanning in the moving direction of the head  41 . 
       FIG. 11A  is an explanatory view showing a relationship between the original signal ODRV and the control signals S 1  and S 2  before the adjustment of the ejection timing of the ink.  FIG. 11A  extracts the original signal ODRV and the controls signals S 1  and S 2  corresponding to the main-scanning period for a single pixel, from the timing chart shown in  FIG. 8 . 
     The control signals S 1  and S 2  are generated on the basis of pulse timing signals (PTS signals). The PTS signals regulate timings at which pulses are generated for the control signals S 1  and S 2 . Pulses of the PTS signals are generated on the basis of the output pulses ENC-A and ENC-B from the linear encoder  51  (the detector  566 ). That is, a pulse of a PTS signal is generated in accordance with a moving amount of the carriage  31 . 
     Hence, if the generation timing of the original signal ODRV with respect to the control signals S 1  and S 2  can be shifted, the ejection timing can be changed with respect to the control signals S 1  and S 2 . Also, the ejection timing can be changed with respect to the position of the head  41  on the sheet S in the moving direction. 
       FIG. 11B  is an explanatory view showing a relationship between the original signal ODRV and the control signals S 1  and S 2  after the adjustment of the ejection timing of the ink. Comparing the shapes of the signals in  FIG. 11B  to those in  FIG. 11A , the shape of the original signal ODRV in  FIG. 11B  is as the same as that in  FIG. 11A . However, the generation timing of the original signal ODRV is delayed by Δt with respect to the control signals S 1  and S 2 , as compared with that in  FIG. 11A . 
     When the generation timing of the original signal ODRV is shifted by Δt, the generation timing of the drive signal DRV is delayed by Δt accordingly. Since the ink is ejected because the drive signal DRV is applied to the piezoelectric element PZT in the head  41 , if the generation timing of the drive signal DRV is delayed by Δt, the ejection timing of the ink is delayed by Δt accordingly. In this embodiment, the memory  63  of the printer  1  previously stores Δt as a correction amount of the ejection timing corresponding to Δx shown in  FIG. 10A . To delay the ejection timing of the ink by Δt in the backward scanning direction during bidirectional printing, the generation timing of the original signal ODRV is delayed by Δt, so that the landing position during the forward scanning is aligned with the landing position during the backward scanning as shown in  FIG. 10B . 
     To delay the generation timing of the original signal ODRV, the original-drive-signal generating section  644 A delays the generation timing of the original signal ODRV. 
     In the above description, the ejection timing during the backward scanning has been delayed by Δt, however, the ejection timing during the forward scanning may be delayed by Δt, so that the landing position during the forward scanning is aligned with the landing position during the backward scanning as shown in  FIG. 10B . Alternatively, the ejection timings during the forward scanning and the backward scanning may be delayed by Δt/2 each, so that the landing position during the forward scanning is aligned with the landing position during the backward scanning as shown in  FIG. 10B . 
     In the above description, the ejection timing of the ink has been delayed. However, the generation timing of the original signal ODRV may be advanced by Δt with respect to the control signals S 1  and S 2 , so that the ejection timing of the ink is advanced. 
     In the above description, only the single original signal ODRV has been generated. However, if ejection timings are adjusted for ink of a plurality of colors, original signals corresponding to the ink of the plurality of colors may be generated. Then, a generation timing of each original signal with respect to control signals S 1  and S 2  may be adjusted. 
     In the above description, the ejection timing of the ink has been adjusted by changing the generation timing of the original signal with respect to the control signals S 1  and S 2 . However, the ejection timing of the ink may be adjusted by changing positions of pixels to be printed in pixel data. 
     In the above description, only the single correction value has been provided to adjust the ejection timing during the bidirectional printing. However, a plurality of correction values may be provided in accordance with temperatures relating to the head  41 . 
     Although the ejection timing is adjusted by using the above-described correction value, if the temperature of the ink is too high, the viscosity of the ink may be too high, and hence the ejection speed may excessively increase, resulting in that the landing position during the forward scanning may not be aligned with the landing position during the backward scanning even after the ejection timing is delayed. In contrast, if the temperature of the ink is too low, the ejection speed of the ink may be too low, resulting in that the landing position during the forward scanning may not be aligned with the landing position during the backward scanning even after the ejection timing is advanced. 
     Therefore, in this embodiment, if a temperature relating to the head  41  is within a predetermined range, the ejection timing is corrected and the bidirectional printing is performed, to keep print quality and to increase a printing speed. In contrast, if the temperature relating to the head  41  is outside the predetermined range, since certain print quality is no longer kept during the bidirectional printing, the bidirectional printing is not performed, and the printing is performed by ejecting the ink only during the forward or backward scanning. By ejecting the ink only during the forward or backward scanning, the misalignment between the ink-landing positions during the forward scanning and the backward scanning does not occur. The print quality can be kept even if the temperature is outside the predetermined range. 
     First Embodiment 
       FIG. 12  is a flowchart showing a printing process according to a first embodiment. 
     When printing is started, a temperature of the head  41  is acquired via the thermistor  502  (S 102 ). The temperature of the head  41  is acquired every sheet S to be printed. In particular, the temperature of the head  41  is acquired immediately before a single sheet S is printed. 
     Then, it is judged whether the acquired temperature is within a predetermined range (S 104 ). In this embodiment, the predetermined range is from 10° C. to 40° C. If the acquired temperature is within the predetermined range (i.e., in the range from 10° C. to 40° C.), the ejection timing is corrected with the correction value stored in the memory  63  and the printing is performed by ejecting the ink during the forward scanning and the backward scanning (S 106 ). Accordingly, if the temperature is within the predetermined range, the printing speed can be increased by the bidirectional printing. 
     In contrast, if the acquired temperature is outside the predetermined range, the printing is performed by ejecting the ink only during the forward scanning (or the backward scanning) (S 108 ). Accordingly, in a situation in which it is difficult to align the ink-landing position during the forward scanning with the ink-landing position during the backward scanning although the ejection timing is corrected, the printing is performed by ejecting the ink only during the forward scanning (or the backward scanning), to avoid the misalignment between the ink-landing positions. Thus, the print quality can be kept. 
     In this way, when the printing for a single sheet S is completed in step S 106  or S 108 , the printing process is ended. If another sheet to be printed is present, the printing process is repeatedly performed. 
     The printer in this embodiment includes the plurality of nozzle arrays for ejecting the ink of the plurality of colors. Therefore, if the bidirectional printing is performed (S 106 ), the ejection timing is corrected for each of the nozzle arrays. 
     Second Embodiment 
       FIG. 13  is a flowchart showing a printing process according to a second embodiment. 
     In the second embodiment, the control for printing is changed depending on whether an image to be printed contains a line, in addition to the steps described in the first embodiment. Also, when the printing is performed during the forward scanning (or the backward scanning), the temperature relating to the head  41  is acquired, and the ejection timing is corrected even when the printing is performed only during the forward scanning (or the backward scanning) in accordance with the acquired temperature. 
     When the printing is started, a temperature of the head  41  is acquired via the thermistor  502  (S 202 ). The temperature of the head  41  is acquired every sheet S to be printed. 
     Then, it is judged whether the acquired temperature is within the predetermined range (S 204 ). If the acquired temperature is within the predetermined range, the ejection timing is corrected with the correction value and the printing is performed by ejecting the ink during the forward scanning and the backward scanning (S 206 ). 
     In contrast, if the acquired temperature is outside the predetermined range, it is judged whether the image to be printed contains a line (S 208 ). The judgment whether the image contains a line is made, for example, by analyzing pixel data indicative of whether a dot is formed on each pixel in the image to be printed. In particular, this step desirably judges whether the line extends in a direction intersecting with the moving direction of the head  41 . That is, this step desirably judges whether the image contains a line extending as shown in  FIGS. 10A and 10B . 
     If the image does not contain a line, the process in step S 206  is performed. In particular, the printing is performed by ejecting the ink during the forward scanning and the backward scanning while the ejection timing is corrected with the correction value. 
     A line (in particular, a line that extends in the direction intersecting with the moving direction) is noticeable if the landing positions are shifted as shown in  FIG. 10A . In contrast, if no line is contained, the shift between the landing positions may not be noticeable. Thus, if no line is contained, the printing is desirably performed by ejecting the ink during both the forward scanning and the backward scanning to increase the printing speed. Hence, in the second embodiment, if the image contains no line, the printing is performed by ejecting the ink during both the forward scanning and the backward scanning even if the temperature is outside the predetermined range. 
     If the image contains the line, the printing is performed by ejecting the ink only during the forward scanning (or the backward scanning) (S 210 ). At this time, the printing is performed while the ejection timing is corrected during the forward scanning (or backward scanning) for ink of each color in accordance with the acquired temperature by the thermistor  502 . 
     When step S 206  or S 210  is completed, the printing process is ended. 
     In step S 210 , even when the printing is performed by ejecting the ink only during the forward scanning, the printing is performed while the ejection timing during the forward scanning is corrected for ink of each color in accordance with the acquired temperature via the thermistor  502  because the following reason. 
       FIG. 14  is a graph plotting a relationship between a temperature and a viscosity of ink. If the temperature of the ink is outside a predetermined range including an ordinary temperature, the rate of change in viscosity with respect to the temperature may be increased (referring to  FIG. 14 , the rate of change is high particularly when the temperature is low). Also, the rate of change in viscosity of ink with temperature varies depending on the color of ink. Then, when the printing is performed by ejecting the ink only during the forward scanning, the ink-landing positions may be shifted from one another due to the viscosity although the landing positions of the ink of the respective colors should be aligned with one another in the moving direction of the head  41 . In particular, if the temperature of the ink is outside the predetermined temperature including the ordinary temperature, the amount of the shift between the landing positions may become large. Then, a color, which is expected to be obtained by superposing the ink of respective colors, is not obtained. The print quality is degraded. 
     Therefore, even if the printing is performed by ejecting the ink during the forward scanning or the backward scanning, the correction value for the ejection timing with respect to the thermistor-detected temperature (the detected temperature by the thermistor  502 ) for ink of each color is previously obtained, and the printing is performed while the ejection timing is corrected by using the correction value for ink of each color (for each nozzle array). 
       FIG. 15  is a table explaining a correction value with respect to a thermistor-detected temperature. In the second embodiment, the correction value for the ejection timing with respect to the thermistor-detected temperature shown in  FIG. 15  is previously acquired and stored in the memory  63 . 
       FIG. 16  is an explanatory view showing acquisition of a correction value with respect to a thermistor-detected temperature. Described here is a method of acquiring a correction value for an ejection timing of cyan C with respect to an ejection timing of black K. As illustrated in  FIG. 16 , ink is ejected from the nozzle array of the black K during first forward scanning, to print a line extending in the direction intersecting with the moving direction of the head  41 . Then, the ink is ejected from the nozzle array of the cyan C during the next forward scanning, to print a line extending in the direction intersecting with the moving direction of the head  41 . 
     Then, a shift amount Δy is measured. In this case, since the ejection timing of the cyan C is delayed by Δy, a correction amount, with which the generation timing of the original signal ODRV of cyan C is advanced by Δy, is obtained. The obtained correction amount for each thermistor-detected temperature is stored in the memory  63 . 
     In the above description, the method of obtaining the correction amount for the ejection timing of the cyan C with respect to the ejection timing of the black K has been described. Similarly, correction amounts for magenta M and yellow Y can be obtained. 
     In this way, if the ink-landing positions are shifted from one another because the viscosity of the ink is changed with temperature, the certain print quality can be kept. 
     Modifications 
     In the above-described embodiments, the printer  1  has been described as the liquid ejecting apparatus, however, it is not limited thereto. The apparatus may be implemented by a liquid ejecting apparatus that ejects liquid other than ink (liquid, a liquid-like object in which particles of a functional material are dispersed, or a fluid-like object such as gel). For example, a technique similar to that according to any of the embodiments may be applied to various apparatuses, such as a color-filter manufacturing apparatus, a dyeing apparatus, a microprocessing apparatus, a semiconductor fabricating apparatus, a surface processing apparatus, a three-dimensional molding apparatus, a liquid vaporizing apparatus, an organic electroluminescence (EL) manufacturing apparatus (in particular, a polymer EL manufacturing apparatus), a display manufacturing apparatus, a film forming apparatus, and a DNA-chip manufacturing apparatus, which use the ink jet technique. Also, a method derived from such an apparatus and a manufacturing method of such an apparatus may be included in the range of application. 
     The embodiments are provided for easy understanding of the invention, but not for interpretation of the invention in a limited way. The invention may be modified and improved within the scope of the invention, and may include equivalents thereof. 
     Head 
     In any of the above-described embodiments, the ink has been ejected by using the piezoelectric element. However, the method of ejecting liquid is not limited thereto, and other methods may be used. For example, a method of generating bubbles in a nozzle using heat may be applied.