Patent Publication Number: US-8529012-B2

Title: Image forming apparatus and method for correcting landing positions of liquid droplets

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates generally to an image forming apparatus, and more particularly, to an image forming apparatus using a recording head including a liquid ejection head that ejects liquid droplets, and a method for correcting landing positions of liquid droplets. 
     2. Description of the Background 
     One example of related-art image forming apparatuses such as printers, copiers, plotters, facsimile machines, and multifunction devices having two or more of printing, copying, plotting, and facsimile functions is an inkjet recording device employing a liquid ejection recording method. The inkjet recording device includes a recording head that ejects droplets of a recording liquid such as ink onto a sheet of a recording medium while the sheet is conveyed to form an image on the sheet. 
     Examples of the inkjet recording device include a serial-type image forming apparatus, in which the recording head ejects liquid droplets while moving in a main scanning direction to form an image on the sheet as the sheet is moved in a sub-scanning direction perpendicular to the main scanning direction, and a line-type image forming apparatus equipped with a line-type recording head that ejects liquid droplets and does so without moving to form an image on the sheet as the sheet is moved in the sub-scanning direction. 
     A maintenance mechanism that maintains performance of the recording head is essential for the image forming apparatus employing the liquid ejection recording method. One of the functions of the maintenance mechanism is to discharge bubbles, foreign substances, coagulated ink, and so forth present in the recording head through nozzles in the recording head in order to prevent irregular ejection of the ink from the nozzles in the recording head. 
     In addition, a full-color image forming apparatus that forms full-color images using the liquid ejection recording method generally includes two separate recording heads, that is, a recording head that ejects black ink droplets (hereinafter referred to as the first recording head) and a recording head that ejects color ink droplets (hereinafter referred to as the second recording head). In such a full-color image forming apparatus, not only black ink but also color ink is ejected for maintenance of the recording heads even when monochrome printing is performed using only the first recording head, causing a waste of color ink and a concomitant cost increase. 
     In order to solve those problems, some image forming apparatuses deploy separate carriages for the black and color inks. That is, they include a first carriage mounting a first recording head that ejects black ink droplets and a second carriage mounting a second recording head that ejects color ink droplets. The first and second carriages are separatably dockable with each other. 
     Meanwhile, there are also image forming apparatuses that correct a timing of ejection of ink droplets from recording heads (hereinafter referred to as an ejection timing) in order to prevent a shift in positions on a sheet to which black and color ink droplets are ejected (hereinafter referred to as landing positions of ink droplets). Specifically, the image forming apparatus forms an adjustment pattern and reads the adjustment pattern using an optical sensor to adjust the ejection timing of the black and color ink droplets, thereby correcting the landing positions of the black and color ink droplets on the sheet and reducing color shift during full-color image formation. 
     However, because the carriage mounting the recording head scans reciprocally to form images on the sheet for each of outward and homeward scanning movement, a shift in the landing positions of the ink droplets between outward and homeward scanning movement tends to occur especially upon formation of ruled lines in the above-described image forming apparatuses. In addition, in a case in which the first and second carriages respectively mounting the first and second recording heads are docked together to form full-color images, a color shift tends to occur when ink droplets of different colors are superimposed one atop the other to form the full-color images on the sheet. 
     To solve the above-described problems, an arrangement is often employed in which a test pattern formed on a recording medium or a conveyance belt is read by a sensor installed on a carriage to adjust landing positions of ink droplets ejected from recording heads by, for example, controlling the timing of the ejection of the ink droplets. 
     It is to be noted that any variation or instability in scanning speed of the carriage adversely affects accuracy in reading of the test pattern using the sensor installed on the carriage. Therefore, it is preferable that the carriage be as heavy as possible to accurately read the test pattern. 
     In the above-described case in which the two separate carriages dockable with each other are used for full-color image formation, the carriages are docked together to make the carriage having the sensor thereon heavier so that the scanning speed of the carriages is stabilized during reading of the test pattern, thereby accurately reading the test pattern. 
     However, when the landing positions of the ink droplets are corrected during monochrome image formation, the first carriage needs to be separated from the second carriage, to form monochrome images by scanning only the first carriage. As a result, docking and separation of the first and second carriages must be performed between formation and reading of the test pattern. 
     In addition, when multiple first recording heads are offset laterally from each other on the first carriage in order to increase productivity during monochrome image formation, the number of times the first and second carriages are separated from and docked with each other for forming and reading the test pattern, respectively, is further increased. 
     Consequently, a period of time required for docking and separating the carriages with and from each other and moving the carriages to a position where docking and separation of the carriages are performed is increased, thereby extending downtime for adjustment of the landing positions. 
     SUMMARY 
     In this disclosure, a novel image forming apparatus including first and second carriages separatably dockable with each other is provided that reduces a number of times the carriages are docked with and separated from each other upon automatic adjustment of landing positions of ink droplets, thereby reducing downtime. 
     In one illustrative embodiment, an image forming apparatus includes a first carriage movable in a main scanning direction and having at least two first recording heads offset laterally from each other to eject black liquid droplets, a second carriage separatably dockable with the first carriage and having a second recording head to eject color liquid droplets, a pattern forming unit to control the at least two first recording heads to form on a recording medium adjustment patterns for correcting a shift in landing positions of the liquid droplets ejected from the at least two first recording heads, a pattern detector provided to the first carriage to read the adjustment patterns, and a landing position corrector to correct the shift in the landing positions of the liquid droplets. Each of the adjustment patterns includes at least two reference patterns and a measured pattern sandwiched by the two reference patterns aligned in the main scanning direction, and the at least two first recording heads form multiple rows of the adjustment patterns in a sub-scanning direction perpendicular to the main scanning direction. The pattern detector successively reads at least two rows of the multiple rows of the adjustment patterns formed in the sub-scanning direction without docking and separation of the first and second carriages. The landing position corrector determines one of a distance between the measured pattern and at least one of the two reference patterns and a scanning time of the first carriage based on a result obtained by the pattern detector and corrects the shift in the landing positions of the liquid droplets. 
     Another illustrative embodiment provides a method for correcting a shift in landing positions of liquid droplets ejected from at least two first recording heads mounted on a first carriage movable in a main scanning direction and separatably dockable with a second carriage having a second recording head. The method includes the steps of: forming on a recording medium multiple rows of adjustment patterns in a sub-scanning direction perpendicular to the main scanning direction for correcting the shift in the landing positions of the liquid droplets, each of the adjustment patterns including at least two reference patterns and a measured pattern sandwiched by the two reference patterns aligned in the main scanning direction; reading successively at least two rows of the multiple rows of the adjustment patterns formed in the sub-scanning direction using a pattern detector, without docking and separation of the first and second carriages; determining one of a distance between the measured pattern and at least one of the two reference patterns and a scanning time of the first carriage based on a result obtained by the reading; and correcting the shift in the landing positions of the liquid droplets based on the determined distance or determined scanning time. 
     Additional aspects, features, and advantages of the present disclosure will be more fully apparent from the following detailed description of illustrative embodiments, the accompanying drawings, and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views and wherein: 
         FIG. 1  is a perspective view illustrating an example of a configuration of an image forming apparatus according to illustrative embodiments; 
         FIG. 2  is a vertical cross-sectional view illustrating the example of the configuration of the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a front view illustrating an example of a configuration of an image forming unit of the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 4  is a perspective view illustrating an example of a configuration of first and second carriages separated from each other according to illustrative embodiments; 
         FIG. 5  is a top view illustrating an example of a configuration of the first and second carriages docked with each other according to illustrative embodiments; 
         FIG. 6  is a top view illustrating the example of the configuration of the first and second carriages separated from each other; 
         FIG. 7  is a block diagram illustrating an example of a configuration and operation of a control unit of the image forming apparatus according to illustrative embodiments; 
         FIG. 8  is a block diagram illustrating an example of a configuration and operation of a shift corrector; 
         FIGS. 9(   a ) and  9 ( b ) are schematic views illustrating operation of correcting a shift in landing positions; 
         FIG. 10  is a schematic view illustrating an example of a configuration of a pattern detector; 
         FIGS. 11(   a ) and  11 ( b ) are schematic views illustrating a first example of detection of an adjustment pattern; 
         FIG. 12A  is a graph illustrating an output voltage obtained by scanning the pattern detector on the adjustment pattern in a second example of detection of the adjustment pattern; 
         FIG. 12B  is an enlarged graph illustrating a portion at a falling edge of the output voltage illustrated in  FIG. 12A ; 
         FIGS. 13(   a ) and  13 ( b ) are schematic views illustrating a third example of detection of the adjustment pattern; 
         FIG. 14  is a schematic view illustrating an example of a basic configuration of the adjustment pattern; 
         FIGS. 15A to 15D  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern according to a first illustrative embodiment; 
         FIGS. 16A to 16E  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern according to a second illustrative embodiment; 
         FIGS. 17A to 17C  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern according to a third illustrative embodiment; 
         FIGS. 18A to 18F  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern according to a first comparative example; 
         FIGS. 19A to 19E  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern according to a second comparative example; and 
         FIGS. 20A to 20D  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern according to a third comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     In describing illustrative embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Image forming apparatuses hereinafter described form an image on a recording medium, such as paper, string, fiber, cloth, lather, metal, plastics, glass, wood, and ceramics by ejecting liquid droplets onto the recording medium. In this specification, an image refers to both signifying images such as characters and figures, as well as a non-signifying image such as patterns. In addition, ink includes any material which is a liquid when ejected from a recording head, such as a DNA sample, a resist material, and a pattern material. Further, an image formed on the recording medium is not limited to a flat image, but also includes an image formed on a three-dimensional object, a three-dimensional image, and so forth. 
     A description is now given of a configuration and operation of an inkjet recording device serving as an image forming apparatus  1  according to illustrative embodiments with reference to  FIGS. 1 to 3 .  FIG. 1  is a perspective view illustrating an example of a configuration of the image forming apparatus  1 .  FIG. 2  is a vertical cross-sectional view illustrating the configuration of the image forming apparatus  1 .  FIG. 3  is a front view illustrating an example of a configuration of an image forming unit  2  of the image forming apparatus  1 . 
     The image forming apparatus  1  is a serial-type inkjet recording device, and includes the image forming unit  2 , a sheet conveyance unit  3 , a sheet roll storage  4 , an electrical substrate storage  6 , an image reading unit  7  provided at the top thereof, and so forth. It is to be noted that the image reading unit  7  is omitted in  FIG. 1  for ease of illustration. 
     In the image forming unit  2 , a guide rod  13  and a guide rail  14  are extended between lateral plates  51  and  52 , and a first carriage  15  that ejects black ink droplets is slidably held by the guide rod  13  and the guide rail  14  in a direction indicated by a double-headed arrow A in  FIG. 1  (hereinafter referred to as the main scanning direction). A second carriage  16  that ejects color ink droplets can be docked with and separated from the first carriage  15 . It is to be noted that  FIG. 1  illustrates a state in which the first and second carriages  15  and  16  are docked together, and  FIG. 3  illustrates a state in which the first and second carriages  15  and  16  are separated from each other. 
     A main scanning mechanism that moves the first carriage  15  reciprocally back and forth in the main scanning direction includes a drive motor  21  positioned at one end of the image forming apparatus  1  in the main scanning direction, a drive pulley  22  rotatively driven by the drive motor  21 , a driven pulley  23  provided at the other end of the image forming apparatus  1  in the main scanning direction, and a belt member  24  wound around the drive pulley  22  and the driven pulley  23 . A tension spring, not shown, applies tension to the driven pulley  23  to separate the driven pulley  23  from the drive pulley  22 . A part of the belt member  24  is fixed to a mount provided to a back surface of the first carriage  15  to guide the first carriage  15  in the main scanning direction. 
     An encoder sheet, not shown, is provided along the main scanning direction in order to detect a main scanning position of the first carriage  15 . The encoder sheet is read by an encoder sensor, not shown, provided to the first carriage  15 . 
     The first carriage  15  has a main scanning range through which it scans, and within this range is a recording range. A sheet S fed from a sheet roll  30  is intermittently conveyed to the recording range by the sheet conveyance unit  3  in a direction perpendicular to the main scanning direction as indicated by an arrow B in  FIG. 1  (hereinafter referred to as the sub-scanning direction). 
     An ink cartridge  19  that stores ink of a specific color, that is, magenta (M), cyan (C), yellow (Y), or black (K), to be supplied to sub-tanks included in recording heads provided to the first and second carriages  15  and  16 , is detachably attached to the image forming apparatus  1  at the one end of the image forming apparatus  1  in the main scanning direction, that is, a portion outside the main scanning range of the first carriage  15 . A maintenance mechanism  18  that services and moisturizes the recording heads using caps  71  and  72  is provided at the other end of the image forming apparatus  1  in the main scanning direction within the main scanning range of the first carriage  15 . 
     The sheet roll  30  is set in the sheet roll storage  4  serving as a sheet feed unit. The sheet roll  30  having different widths can be set in the sheet roll storage  4 . Flanges  31  are attached to both ends of a paper core of the sheet roll  30  and are placed on flange bearings  32 , respectively. Support rollers, not shown, are provided to the flange bearings  32  to contact outer circumferential surfaces of the flanges  31 , respectively, thereby rotating the flanges  31  to feed the sheet S from the sheet roll  30 . 
     The sheet S fed from the sheet roll  30  set in the sheet roll storage  4  is conveyed by conveyance members such as a pair of rollers  33 , a drive roller  34 , and a driven roller  35  from the back to the front of the image forming apparatus  1  to reach the recording range. In monochrome printing, the first carriage  15  is moved reciprocally in the main scanning direction, and the recording heads of the first carriage  15  are driven to eject black ink droplets onto the sheet S based on image data while the sheet S is intermittently conveyed in the sub-scanning direction. By contrast, in full-color printing, the first and second carriages  15  and  16  are docked together, and the recording heads of the first and second carriages  15  and  16  are together driven to eject ink droplets of the specified color onto the sheet S based on image data. Accordingly, a desired image is formed on the sheet S. The sheet S having the image thereon is then cut to a predetermined length and is discharged to a discharge tray, not shown, provided to the front of the image forming apparatus  1 . 
     A description is now given of a configuration of each of the first and second carriages  15  and  16  according to illustrative embodiments with reference to  FIGS. 4 to 6 .  FIG. 4  is a perspective view illustrating an example of a configuration of the first and second carriages  15  and  16  separated from each other according to illustrative embodiments.  FIG. 5  is a top view illustrating an example of a configuration of the first and second carriages  15  and  16  docked together.  FIG. 6  is a top view illustrating the example of the configuration of the first and second carriages  15  and  16  separated from each other. 
     The first carriage  15  includes first recording heads  101   k   1  and  101   k   2  (hereinafter collectively referred to as first recording heads  101 ) each including a liquid ejection head that ejects black ink droplets. The first recording heads  101  are offset laterally from each other in the main scanning direction and the sub-scanning direction on the first carriage  15 , and the first carriage  15  is moved reciprocally in the main scanning direction along the guide rod  13  by the main scanning mechanism. Black ink is supplied from the ink cartridge  19  provided to the image forming apparatus  1  to the sub-tanks integrally formed with the first recording heads  101  through a tube  53 . Alternatively, replaceable ink cartridges may be attached to the first recording heads  101 . 
     The second carriage  16  includes second recording heads  102   m ,  102   c , and  102   y  (hereinafter collectively referred to as second recording heads  102 ), each including a liquid ejection head that ejects ink droplets of a specific color, that is, magenta (M), cyan (C), or yellow (Y). The second recording heads  102  are positioned at the same position as the first recording head  101   k   2  in the main scanning direction. The second carriage  16  is docked with the first carriage  15  to be moved reciprocally in the main scanning direction together with the first carriage  15  by reciprocating movement of the first carriage  15 . Ink of the specified color is supplied from the ink cartridge  19  provided to the image forming apparatus  1  to the sub-tanks integrally formed with the second recording heads  102  through a tube  54 . Alternatively, replaceable ink cartridges may be attached to the second recording heads  102 . 
     The first carriage  15  has mounts  55   i  and  55   ii  (hereinafter collectively referred to as mounts  55 ) to place the second carriage  16  thereon, and a cutout  56  is formed between the mounts  55 . When the second carriage  16  is placed on the mounts  55  to be docked with the first carriage  15 , the color ink droplets are ejected from the second recording heads  102  of the second carriage  16  onto the sheet S through the cutout  56 , and the caps  72  of the maintenance mechanism  18  are moved up and down within the cutout  56 . The mounts  55  respectively have engaging members  57   i  and  57   ii  (hereinafter collectively referred to as engaging members  57 ) each separatably engageable with engaging members  61   i  and  61   ii  (hereinafter collectively referred to as engaging members  61 ) provided to the second carriage  16 . 
     The first carriage  15  further includes a protrusion  58  that protrudes toward the lateral plate  52  beyond the second carriage  16  when the first carriage  15  is docked with the second carriage  16 . The protrusion  58  is used for detecting a reference position of the first carriage  15 . Specifically, a position where the protrusion  58  contacts the lateral plate  52  is detected by, for example, detecting a change in a drive current of the drive motor  21 , and the first carriage  15  is moved from that position to a direction opposite the lateral plate  52  by a predetermined amount and the resultant position of the first carriage  15  is set as the reference position. A home position of the first carriage  15  can be detected in a manner similar to detection of the reference position of the first carriage  15  as described above, and may be the same as or different from the reference position. 
     Alternatively, a detection member may be provided to the first carriage  15  in place of the protrusion  58  so that relative positions of the detection member and a reference position provided to the main body of the image forming apparatus  1  are detected to determine the reference position of the first carriage  15 . In such a case, the reference position of the first carriage  15  may be determined by, for example, a reference position detector such as a sensor provided to the main body of the image forming apparatus  1 , or by matching of a result detected by the encoder sensor that detects the position of the first carriage  15  and a preset reference position. 
     A pattern detector  401  serving as a pattern reader that reads an adjustment pattern  400  formed on the sheet S for automatically correcting a shift in positions to where the ink droplets are ejected from the first and second recording heads  101  and  102  onto the sheet S (hereinafter referred to as landing positions) is provided on a lateral surface of the first carriage  15 . The pattern detector  401  is an optical sensor including a reflective-type photosensor. Specifically, the pattern detector  401  includes a light emitter  402  that emits light onto the adjustment pattern  400  and a light receiver  403  that receives light reflected from the adjustment pattern  400 . 
     A description is now given of an example of a configuration and operation of a control unit  200  of the image forming apparatus  1  according to illustrative embodiments with reference to  FIG. 7 .  FIG. 7  is a block diagram illustrating an example of a configuration and operation of the control unit  200 . 
     The control unit  200  controls the image forming apparatus  1 , and includes a CPU  201  serving also as a landing position corrector, a ROM  202  that stores fixed data and various programs including a program for performing processing relating to correction of the landing positions performed by the CPU  201 , a RAM  203  that temporarily stores image data and so forth, a nonvolatile rewritable memory (NVRAM)  204  that holds data while power supply to the image forming apparatus  1  is blocked, and an ASIC  205  that performs signal processing for image data and image processing such as sorting of the image data and handles input/output signals for controlling the image forming apparatus  1 . 
     The control unit  200  further includes a host I/F  206  that sends and receives data and signals to and from a host; a print controller  207  including a data transfer unit for controlling driving of the liquid ejection heads, that is, the first and second recording heads  101  and  102 , and a drive waveform generator that generates a drive waveform; a motor driver  210  for driving the drive motor  21  and a sub-scanning motor  36  that rotates the drive roller  34 ; and an I/O  213  that inputs various detection signals output from encoder sensors  221  and  236  and the pattern detector  401  as well as various detection signals output from a detector group  212  including a temperature detector for detecting a surrounding temperature that may cause a shift in the landing positions. An operation panel  214  through which data necessary for the image forming apparatus  1  is input and on which such data is displayed is connected to the control unit  200 . 
     The control unit  200  receives image data and so forth sent from the host including an information processing device such as a personal computer and an image reading device such as an image scanner using the host I/F  206  through a cable or a network, which may be either wired or wireless. 
     The CPU  201  of the control unit  200  reads image data from a reception buffer included in the host I/F  206  and analyzes the image data so that image processing and sorting of the image data are performed by the ASIC  205  as needed. The resultant image data is transferred from the print controller  207  to a head driver  215  for the first recording heads  101  of the first carriage  15  and a head driver  216  for the second recording heads  102  of the second carriage  16 . It is to be noted that dot pattern data for outputting an image on the sheet S is generated by a printer driver provided to the host. 
     Specifically, the print controller  207  transfers the above-described image data as serial data to the head drivers  215  and  216  and outputs a transfer clock, a clutch signal, a mask signal, and so forth each necessary for transferring the image data and confirming transfer of the image data to the head drivers  215  and  216 . As described above, the print controller  207  includes the drive waveform generator having a voltage amplifier, a current amplifier, a D/A converter that performs digital/analog conversion of pattern data of a drive signal stored in the ROM  202 , and so forth. The print controller  207  further includes a drive waveform selector that outputs a drive waveform having a single drive pulse or multiple drive pulses generated by the drive waveform generator to the head drivers  215  and  216 . 
     The head drivers  215  and  216  selectively apply the drive signal forming the drive waveform output from the print controller  207  to a drive element such as a piezoelectric element that generates energy to drive the first and second recording heads  101  and  102  to eject the ink droplets based on a single line of the image data serially input to the first and second recording heads  101  and  102 . At this time, a size of a dot of the ink droplet ejected from the first and second recording heads  101  and  102  can be changed to small, medium, or large by selecting the drive pulse that forms the drive waveform as appropriate. 
     The CPU  201  calculates a drive output value (or a control value) for the drive motor  21  based on a speed detection value and a position detection value each obtained by sampling a detection pulse output from the encoder sensor  221 , and a target speed value and a target position value obtained from prestored speed and position profiles to drive the drive motor  21  through the motor driver  210 . Similarly, the CPU  201  calculates a drive output value (or a control value) for the sub-scanning motor  36  based on a speed detection value and a position detection value each obtained by sampling a detection pulse output from the encoder sensor  236 , and a target speed value and a target position value obtained from prestored speed and position profiles to drive the sub-scanning motor  36  through the motor driver  210 . 
     As described previously, the CPU  201  also serves as a landing position corrector. Specifically, the CPU  201  causes the first and second recording heads  101  and  102  to form the adjustment pattern  400  for correcting a shift in the landing positions on the sheet S. The adjustment pattern  400  thus formed is read by the pattern detector  401 . The CPU  201  calculates a correction amount to adjust a timing at which the first and second recording heads  101  and  102  eject the ink droplets onto the sheet S (hereinafter referred to as an ejection timing) for image formation based on the result obtained by the pattern detector  401 . Thereafter, the CPU  201  sends the correction amount thus calculated to the print controller  207  to correct a shift in the landing positions. 
     A description is now given of correction of a shift in the landing positions in the image forming apparatus  1  with reference to  FIGS. 8 to 10 .  FIG. 8  is a block diagram illustrating an example of a configuration and operation of a shift corrector  505 .  FIGS. 9(   a ) and  9 ( b ) are schematic views illustrating operation of correcting a shift in the landing positions.  FIG. 10  is a schematic view illustrating an example of a configuration of the pattern detector  401 . 
     As described above, the first carriage  15  includes the pattern detector  401  that reads the adjustment pattern  400  formed on the sheet S for correcting a shift in the landing positions. It is to be noted that the adjustment pattern  400  is composed of at least a reference pattern  400   a  and a measured pattern  400   b.    
     The pattern detector  401  includes the light emitter  402  that emits light onto the adjustment pattern  400  formed on the sheet S and the light receiver  403  that receives light regularly or diffusively reflected from the adjustment pattern  400 . The light emitter  402  and the light receiver  403  are disposed side by side in a direction perpendicular to the main scanning direction, that is, the sub-scanning direction, and are held in a holder  404 . The holder  404  has a lens  405  at a portion through which the light is emitted or entered. 
     As described above, the light emitter  402  and the light receiver  403  are disposed side by side in the sub-scanning direction within the pattern detector  401 . Accordingly, a change in scanning speed of the first carriage  15  hardly affects the result detected by the pattern detector  401 . A relatively simple and inexpensive light source such as an optical LED may be used as the light emitter  402 . Further, an inexpensive lens is used for a spot size of the light source, thereby achieving mm-order detection accuracy. 
     The image forming apparatus  1  further includes a pattern controller  501  serving as a pattern forming unit that causes the first carriage  15  to move in the main scanning direction and the first and second recording heads  101  and  102  to eject the ink droplets through an ejection controller  502 . Accordingly, the adjustment pattern  400  including the reference pattern  400   a  and the measured pattern  400   b  each having a linear shape is formed on the sheet S. 
     In addition, the pattern controller  501  controls the pattern detector  401  to read the adjustment pattern  400  formed on the sheet S. Specifically, the pattern controller  501  drives the light emitter  402  of the pattern detector  401  to emit light while causing the first carriage  15  to move in the main scanning direction so that the light is emitted from the light emitter  402  to the adjustment pattern  400  formed on the sheet S. 
     The light emitted from the light emitter  402  to the adjustment pattern  400  is reflected from the adjustment pattern  400  and strikes the light receiver  403 . Accordingly, a detection signal is output from the light receiver  403  corresponding to an amount of light reflected from the adjustment pattern  400 . The detection signal thus output is then input into a shift amount calculator  503  included in the shift corrector  505 . 
     The shift amount calculator  503  obtains a time interval between each of the reference patterns  400   a  and a time interval between the reference pattern  400   a  and the measured patterns  400   b  based on the result output from the light receiver  403 . In addition, the shift amount calculator  503  obtains a distance between each of the reference patterns  400   a  based on the scanning speed of the first carriage  15 . Then, the shift amount calculator  503  calculates a distance between the reference pattern  400   a  and the measured patterns  400   b  and corrects the distance thus calculated based on the distance between each of the reference patterns  400   a  and a theoretical distance between each of the reference patterns  400   a  (or a scanning time of the first carriage  15 ). As a result, an amount of shift of the measured pattern  400   b  from the reference position, that is, an amount of shift in the landing positions, is calculated. 
     The amount of shift in the landing positions calculated by the shift amount calculator  503  is then sent to a correction amount calculator  504 . The correction amount calculator  504  calculates a correction amount that adjusts a timing at which the ejection controller  502  drives at least one of the first and second recording heads  101  and  102  to eject the ink droplets onto the sheet S, such that the amount of shift in the landing positions is eliminated. The correction amount thus calculated is set to the ejection controller  502 . Accordingly, the ejection controller  502  adjusts the ejection timing based on the correction amount and appropriately drives at least one of the first and second recording heads  101  and  102 , thereby preventing a shift in the landing positions. 
     A description is now given of examples of detection of the adjustment pattern  400  formed on the sheet S and calculation of a distance between the reference pattern  400   a  and the measured pattern  400   b  with reference to  FIGS. 11 to 13 .  FIGS. 11(   a ) and  11 ( b ) are schematic views illustrating a first example of detection of the adjustment pattern  400 .  FIG. 12A  is a graph illustrating an output voltage So obtained by scanning the pattern detector  401  on the adjustment pattern  400  in a second example of detection of the adjustment pattern  400 .  FIG. 12B  is an enlarged graph illustrating a portion at a falling edge of the output voltage So illustrated in  FIG. 12A .  FIGS. 13(   a ) and  13 ( b ) are schematic views illustrating a third example of detection of the adjustment pattern  400 . 
     In the example illustrated in  FIG. 11(   a ), the pattern detector  401  scans in the main scanning direction to read the reference pattern  400   a  and the measured pattern  400   b  of the adjustment pattern  400  formed on the sheet S. Accordingly, an output voltage So that falls upon detection of the reference pattern  400   a  and the measured pattern  400   b  as illustrated in  FIG. 11(   b ) is obtained from a result output from the light receiver  403  of the pattern detector  401 . 
     The output voltage So is compared to a predetermined threshold Vr to detect an edge of the reference pattern  400   a  and the measured pattern  400   b , that is, a position in which the output voltage So falls below the threshold Vr. At this time, a center of gravity of a range defined by the threshold Vr and the output voltage So, that is, a shaded parts in the graph shown in  FIG. 11(   b ), is calculated to use the center of gravity of the range thus calculated as the center of the reference pattern  400   a  and the measured pattern  400   b . Accordingly, an error caused by minute fluctuation in the output voltage So can be reduced. 
     In the second example, the pattern detector  401  scans on the adjustment pattern  400  formed on the sheet S so that an output voltage So illustrated in  FIG. 12A  is obtained. 
     The portion at the falling edge of the output voltage So is searched in a direction indicated by an arrow Q 1  in  FIG. 12B , and a point where the output voltage So falls below a minimum threshold Vrd is stored as a point P 2 . Next, the output voltage So is searched from the point P 2  in a direction indicated by an arrow Q 2  in  FIG. 12B , and a point where the output voltage So exceeds a maximum threshold Vru is stored as a point P 1 . Then, a regression line L 1  is calculated from the output voltage So between the points P 1  and P 2 , and an intersection point C 1  of the regression line L 1  and an intermediate threshold Vrc between the maximum and minimum thresholds Vru and Vrd is calculated using the regression line L 1  thus obtained. Similarly, a regression line L 2  at a portion at a rising edge of the output voltage So is calculated to calculate an intersection point C 2  of the regression line L 2  and the intermediate threshold Vrc. Thereafter, a midpoint between the intersection points C 1  and C 2  (C 1 +C 2 /2) is calculated to obtain a line center C 12 . 
     In the third example, the pattern detector  401  scans in the main scanning direction to read the reference pattern  400   a  and the measured pattern  400   b  respectively formed on the sheet S as illustrated in  FIG. 13(   a ). Accordingly, an output voltage So shown in  FIG. 13(   b ) is obtained. 
     At this time, for example, harmonic noises are removed using an IIR filter, and then quality of detection signals is evaluated. Next, a sloped portion near the threshold Vr is detected to calculate a regression line. Thereafter, intersection points a 1 , a 2 , b 1 , and b 2  of the regression line and the threshold Vr are calculated to calculate a midpoint A between the intersection points a 1  and a 2  and a midpoint B between the intersection points b 1  and b 2 , respectively. 
     A description is now given of adjustment of the ejection timing based on scanning speed of the first and second carriages  15  and  16  between the reference pattern  400   a  and the measured pattern  400   b  of the adjustment pattern  400  with reference to  FIG. 14 .  FIG. 14  is a schematic view illustrating an example of a basic configuration of the adjustment pattern  400 . 
     Here, the minimum unit of the adjustment pattern  400  for detecting a shift in the landing positions is composed of the two reference patterns  400   a  and the measured pattern  400   b  arranged side by side in the main scanning direction without overlapping with each other. The measured pattern  400   b  is sandwiched by the two reference patterns  400   a . In  FIG. 14 , one of the two reference patterns  400   a  formed in the left of the measured pattern  400   b  is hereinafter referred to as a reference pattern  400   a   1 , and the other one of the reference patterns  400   a  formed in the right of the measured pattern  400   b  is hereinafter referred to as a reference pattern  400   a   2 . 
     A distance between the reference patterns  400   a   1  and  400   a   2  and a distance between one of the reference patterns  400   a  and the measured pattern  400   b  are calculated by multiplying a difference between timings when the pattern detector  401  provided to the first carriage  15  detects each of the reference patterns  400   a   1  and  400   a   2  and the measured pattern  400   b  by a predetermined scanning speed of the first carriage  15 . Next, a predetermined correction ratio of fluctuation in the scanning speed of the first carriage  15  calculated from the distance between the reference patterns  400   a   1  and  400   a   2  is added to the calculated distances to correct an amount of a positional shift of the measured pattern  400   b  from the reference patterns  400   a . Then, the ejection timing is adjusted based on the corrected amount of the positional shift. 
     Specifically, when the first carriage  15  is moved in the main scanning direction so that the pattern detector  401  reads the adjustment pattern  400 , a period of time from when the reference pattern  400   a   1  is detected to when the measured pattern  400   b  is detected is referred to as a time t 2 , and a period of time from when the reference pattern  400   a   1  is detected to when the reference pattern  400   a   2  is detected is referred to as a time t 1 . Referring the scanning speed of the first carriage  15  to as Vc, a distance L 1  between the reference patterns  400   a   1  and  400   a   2  is calculated by a formula of L 1 =t 1 ×Vc, and a distance L 2  between the reference pattern  400   a   1  and the measured pattern  400   b  is calculated by a formula of L 2 =t 2 ×Vc. 
     Here, a theoretical distance La 2  from the reference pattern  400   a   1  to the measured pattern  400   b  is determined in advance. Therefore, the distance L 2  is subtracted from the theoretical distance La 2  to obtain the amount of positional shift of the measured pattern  400   b  from the reference pattern  400   a   1 . 
     Meanwhile, a theoretical distance between the reference patterns  400   a   1  and  400   a   2  is referred to as a theoretical distance La 1  when the first carriage  15  is moved at the predetermined scanning speed Vc. The distance L 1  and the theoretical distance La 1  are the same when the scanning speed Vc of the first carriage  15  is constant during reading of the adjustment pattern  400 . However, when the scanning speed Vc of the first carriage  15  is changed during reading of the adjustment pattern  400 , the distance L 1  and the theoretical distance La 1  are different from each other. 
     Therefore, the theoretical distance La 1  is divided by the distance L 1  to calculate the correction ratio of fluctuation in the scanning speed of the first carriage  15 . The correction ratio thus calculated is multiplied by the amount of positional shift of the measured pattern  400   b  from the reference pattern  400   a   1  to obtain the accurate amount of positional shift when the first carriage  15  is moved at the predetermined scanning speed Vc. 
     A description is now given of formation and reading of the adjustment pattern  400  performed by the CPU  200  of the image forming apparatus  1  according to a first illustrative embodiment with reference to  FIGS. 15A to 15D .  FIGS. 15A to 15D  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern  400  according to the first illustrative embodiment. It is to be noted that, in  FIGS. 15A  to  15 D and successive drawings, the reference pattern  400   a  and the measured pattern  400   b  formed by the first recording head  101   k   2  during outward scanning movement of the first carriage  15  are denoted by “a reference pattern  400   a  ( 101   k   2 -O)” and “a measured pattern  400   b  ( 101   k   2 -O)”, respectively. Similarly, the reference pattern  400   a  and the measured pattern  400   b  formed by the first recording head  101   k   1  during outward scanning movement of the first carriage  15  is denoted by “a reference pattern  400   a  ( 101   k   1 -O)” and “a measured pattern  400   b  ( 101   k   1 -O)”, respectively. The measured pattern  400   b  formed by the first recording head  101   k   2  during homeward scanning movement of the first carriage  15  is denoted by “a measured pattern  400   b  ( 101   k   2 -H)”, and the measured pattern  400   b  formed by the first recording head  101   k   1  during homeward scanning movement of the first carriage  15  is denoted by “a measured pattern  400   b  ( 101   k   1 -H)”. Each row of the adjustment pattern  400  (hereinafter also referred to as an adjustment pattern row) is composed of a set of multiple reference patterns  400   a  and a measured pattern(s)  400   b  formed on the sheet S in the main scanning direction, and multiple rows of the adjustment patterns  400  are formed on the sheet S in a direction of feeding of the sheet S, that is, the sub-scanning direction. Specifically, a first row of the adjustment pattern  400  (hereinafter referred to as a first pattern row), a second row of the adjustment pattern  400  (hereinafter referred to as a second pattern row), and so on are formed on the sheet S from downstream to upstream in the sub-scanning direction, in that order. 
     At step S 1 , the first and second carriages  15  and  16  are separated from each other (first separation of the first and second carriages  15  and  16 ) at a docking/separation position where the first and second carriages  15  and  16  are docked with and separated from each other. 
     At step S 2 , the first carriage  15  is moved outward from the docking/separation position so that the first recording head  101   k   2  forms two reference patterns  400   a  of a first pattern row at a first pattern formation position on the sheet S. At step S 3 , the first carriage  15  is moved homeward to the docking/separation position, and the sheet S is fed in the sub-scanning direction such that the first pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . 
     At step S 4 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -O) between the two reference patterns  400   a  of the first pattern row formed at step S 2 . Accordingly, formation of the first pattern row is completed. In addition, during the same outward scanning movement of the first carriage  15 , the first recording head  101   k   2  forms two reference patterns  400   a  of a second pattern row at a second pattern formation position on the sheet S. 
     At step S 5 , the first carriage  15  is moved homeward so that the first recording head  101   k   2  forms a measured pattern  400   b  ( 101   k   2 -H) between the two reference patterns  400   a  of the second pattern row formed at step S 4 . Accordingly, formation of the second pattern row is completed. Then, the first carriage  15  is returned to the docking/separation position. 
     At step S 6 , the first and second carriages  15  and  16  are docked with each other (first docking of the first and second carriages  15  and  16 ) at the docking/separation position. 
     At step S 7 , the first and second carriages  15  and  16  docked with each other are together moved outward from the docking/separation position so that the pattern detector  401  provided to the first carriage  15  reads the measured pattern  400   b  ( 101   k   1 -O) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -O) of the first pattern row. After the sheet S is fed in the sub-scanning direction such that the second pattern row is positioned corresponding to the pattern detector  401 , at step S 8  the first and second carriages  15  and  16  are moved homeward. 
     At step S 9 , the first carriage  15  with which the second carriage  16  is docked is moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  of the second pattern row. 
     At step S 10 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     At step S 11 , the first and second carriages  15  and  16  are separated from each other (second separation of the first and second carriages  15  and  16 ). 
     At step S 12 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  of a third pattern row at a third pattern formation position on the sheet S, and then the sheet S is fed in the sub-scanning direction such that the third pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . At step S 13 , the first carriage  15  is moved homeward to the docking/separation position so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -H) between the two reference patterns  400   a  of the third pattern row formed at step S 12  to complete formation of the third pattern row. 
     At step S 14 , the first and second carriages  15  and  16  are docked with each other (second docking of the first and second carriages  15  and  16 ). 
     At step S 15 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -H) and the two reference patterns  400   a  on either side of the measured pattern  400   b  of the third pattern row, and then at step S 16  the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     As described above, the two reference patterns  400   a  are formed by the first recording head  101   k   2  during outward scanning movement of the first carriage  15 , and then the measured pattern  400   b  is formed between the reference patterns  400   a  by the first recording head  101   k   2  during homeward scanning movement of the first carriage  15  and by the first recording head  101   k   1  during outward and homeward scanning movement of the first carriage  15 . 
     Specifically, according to the first illustrative embodiment, the first recording head  101   k   2  positioned upstream from the first recording head  101   k   1  in the direction of sheet feed forms the reference patterns  400   a  during outward scanning movement of the first carriage  15 . In addition, the measured pattern  400   b  is formed between the reference patterns  400   a  by the first recording head  101   k   2  during homeward scanning movement of the first carriage  15  and the first recording head  101   k   1  during outward and homeward scanning movement of the first carriage  15  so as to correct the landing positions of the ink droplets. 
     At this time, the pattern detector  401  successively reads at least two rows of the adjustment patterns  400  formed on the sheet S in the sub-scanning direction without docking and separation of the first and second carriages  15  and  16 . 
     In the first illustrative embodiment, at least the two reference patterns  400   a  and the measured pattern  400   b  sandwiched by the two reference patterns  400   a  are formed in a row in the main scanning direction to form an adjustment pattern row on the sheet S. In addition, multiple adjustment pattern rows are formed on the sheet S in the sub-scanning direction. A distance between at least one of the two reference patterns  400   a  and the measured pattern  400   b  or a scanning time of the first carriage  15  is calculated based on a result detected by the pattern detector  401  to correct a shift in the landing positions of the ink droplets. At this time, the pattern detector  401  successively reads at least two rows of the adjustment patterns  400  formed on the sheet S in the sub-scanning direction without docking and separation of the first and second carriages  15  and  16 . Accordingly, the number of times the first and second carriages  15  and  16  are docked with or separated from each other is reduced during automatic adjustment of the landing positions, thereby shortening downtime. 
     In addition, the first recording head  101   k   2  positioned upstream from the first recording head  101   k   1  in the direction of sheet feed, that is, the sub-scanning direction, is used for forming the reference patterns  400   a . As a result, the number of times the first and second carriages  15  and  16  are docked with and separated from each other can be reduced compared to a case, described in detail later, in which the reference patterns  400   a  are formed by the first recording head  101   k   1 . 
     The first recording head  101   k   1  positioned downstream from the first recording head  101   k   2  in the direction of sheet feed forms the measured pattern  400   b  while the first recording head  101   k   2  positioned upstream from the first recording head  101   k   1  in the direction of sheet feed is forming the reference patterns  400   a  of the next pattern row during single outward scanning movement of the first carriage  15 . Accordingly, the number of times the first and second carriages  15  and  16  are docked with and separated from each other can be reduced. 
     Further, the first and second carriages  15  and  16  docked with each other are together moved in a single direction from the docking/separation position when the adjustment pattern  400  is read by the pattern detector  401 . Accordingly, the adjustment pattern  400  can be read with substantially consistent accuracy. 
     The pattern detector  401  is disposed on the first carriage  15  closer to the first recording head  101   k   1  positioned downstream from the first recording head  101   k   2  in the direction of sheet feed than to the first recording head  101   k   2 . Accordingly, the number of times the first and second carriages  15  and  16  are docked with and separated from each other can be reduced. 
     A description is now given of formation and reading of the adjustment pattern  400  according to a second illustrative embodiment with reference to  FIGS. 16A to 16E .  FIGS. 16A to 16E  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern  400  according to the second illustrative embodiment. 
     At step S 101 , the first and second carriages  15  and  16  are separated from each other (first separation of the first and second carriages  15  and  16 ) at the docking/separation position. 
     At step S 102 , the first carriage  15  is moved outward from the docking/separation position so that the first recording head  101   k   2  forms two reference patterns  400   a  of a first pattern row at a first pattern formation position on the sheet S. Then, the sheet S is fed in the sub-scanning direction such that the first pattern formation position is positioned corresponding to the first recording heads  101   k   1 . At step S 103 , the first carriage  15  is moved homeward to the docking/separation position so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -H) between the two reference patterns  400   a  of the first pattern row formed at step S 102  to complete formation of the first pattern row while the first recording head  101   k   2  is forming a measured pattern  400   b  ( 101   k   2 -H) of a second pattern row at a second pattern formation position on the sheet S. 
     At step S 104 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  that sandwich the measured pattern  400   b  ( 101   k   2 -H) of the second pattern row formed at step S 103  to complete formation of the second pattern row. At step S 105 , the first carriage  15  is moved homeward to the docking/separation position. 
     At step S 106 , the first and second carriages  15  and  16  are docked with each other (first docking of the first and second carriages  15  and  16 ). 
     At step S 107 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -H) of the first pattern row. After the sheet S is fed in the sub-scanning direction such that the second pattern formation position on the sheet S is positioned corresponding to the pattern detector  401 , at step S 108  the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     At step S 109 , the first carriage  15  with which the second carriage  16  is docked is moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   2 -H) of the second pattern row. At step S 110 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     At step S 111 , the first and second carriages  15  and  16  are separated from each other (second separation of the first and second carriages  15  and  16 ). 
     At step S 112 , the first carriage  15  is moved outward so that the first recording heads  101   k   2  forms two reference patterns  400   a  of a third pattern row at a third pattern formation position on the sheet S, and then the sheet S is fed such that the third pattern formation position is positioned corresponding to the first recording head  101   k   1 . At step S 113 , the first carriage  15  is moved homeward. 
     At step S 114 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -O) between the reference patterns  400   a  of the third pattern row formed at step S 112  to complete formation of the third pattern row. At step S 115 , the first carriage  15  is moved homeward to the docking/separation position. 
     At step S 116 , the first and second carriages  15  and  16  are docked with each other (second docking of the first and second carriages  15  and  16 ). 
     At step S 117 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -O) and the two reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -O) of the third pattern row, and then at S 118  the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     Similar to the first illustrative embodiment, the pattern detector  401  successively reads at least two rows of the adjustment patterns  400  formed on the sheet S in the sub-scanning direction without docking and separation of the first and second carriages  15  and  16 . However, in the second illustrative embodiment, the measured pattern  400   b  ( 101   k   1 -O) is not formed during outward scanning movement of the first carriage  15  while the reference patterns  400   a  are formed by the first recording head  101   k   2 . As a result, the number of scanning movements of the first carriage  15  is increased by one reciprocating movement compared to the first illustrative embodiment. 
     A description is now given of formation and reading of the adjustment pattern  400  according to a third illustrative embodiment with reference to  FIGS. 17A to 17C .  FIGS. 17A to 17C  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern  400  according to the third illustrative embodiment. 
     At step S 201 , the first and second carriages  15  and  16  are separated from each other (first separation of the first and second carriages  15  and  16 ) at the docking/separation position. 
     At step S 202 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms three reference patterns  400   a  of a first pattern row at a first pattern formation position on the sheet S. Then, at S 203  the first carriage  15  is moved homeward and the sheet S is fed in the sub-scanning direction such that the first pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . 
     At step S 204 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -O) between the left and middle reference patterns  400   a  of the three reference patterns  400   a  of the first pattern row formed at step S 202  while the first recording head  101   k   2  forms three reference patterns  400   a  of a second pattern row at a second pattern formation position on the sheet S. 
     At step S 205 , the first carriage  15  is moved homeward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -H) between the right and middle reference patterns  400   a  of the three reference patterns  400   a  of the first pattern row to complete formation of the first pattern row while the first recording head  101   k   2  forms a measured pattern  400   b  ( 101   k   2 -H) between the left and middle reference patterns  400   a  of the three reference patterns  400   a  of the second pattern row formed at step S 204  on the sheet S. Then, the first carriage  15  is returned to the docking/separation position. 
     At step S 206 , the first and second carriages  15  and  16  are docked with each other (first docking of the first and second carriages  15  and  16 ). 
     At step S 207 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured patterns  400   b  ( 101   k   1 -H and  101   k   1 -O) and the three reference patterns  400   a  respectively on either side of the measured patterns  400   b  ( 101   k   1 -H and  101   k   1 -O) of the first pattern row. After the sheet S is fed in the sub-scanning direction such that the second pattern formation position on the sheet S is positioned corresponding to the pattern detector  401 , at step S 208  the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     At step S 209 , the first carriage  15  with which the second carriage  16  is docked is moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -H) and the two reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   2 -H), that is, the left and middle reference patterns  400   a  of the three reference patterns  400   a  of the second pattern row. At step S 210 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     Similar to the first and second illustrative embodiments, the pattern detector  401  successively reads at least two rows of the adjustment patterns  400  formed on the sheet S in the sub-scanning direction without docking and separation of the first and second carriages  15  and  16 . However, in the third illustrative embodiment, the three reference patterns  400   a  are formed all at once during the same outward scanning movement of the first carriage  15 . Accordingly, the measured patterns  400   b  ( 101   k   1 -O), ( 101   k   1 -H), and ( 101   k   2 -H), each necessary for correction of the landing positions, are formed by a single reciprocating movement of the first carriage  15 , thereby reducing the number of times the first and second carriages  15  and  16  are docked with and separated from each other and the number of scanning movements of the first carriage  15  compared to the first illustrative embodiment. 
     To better appreciate the advantages and unpredicted effect of the above-described embodiments of the present invention, a description is now given of formation and reading of the adjustment pattern  400  according to comparative examples. In the comparative examples described below, the pattern detector  401  does not successively reads multiple rows of the adjustment patterns  400  formed on the sheet S in the sub-scanning direction without docking and separation of the first and second carriages  15  and  16 . 
       FIGS. 18A to 18F  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern  400  according to a first comparative example. 
     Similar to the foregoing illustrative embodiments, the first recording head  101   k   2  forms the reference patterns  400   a  in the first comparative example. 
     At step S 301 , the first and second carriages  15  and  16  are separated from each other (first separation of the first and second carriages  15  and  16 ) at the docking/separation position. 
     At step S 302 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  of a first pattern row at a first pattern formation position on the sheet S. At step S 303 , the first carriage  15  is moved homeward so that the first recording head  101   k   2  forms a measured pattern  400   b  ( 101   k   2 -H) between the two reference patterns  400   a  of the first pattern row formed at step S 302  to complete formation of the first pattern row. Then, the sheet S is fed in the sub-scanning direction such that the first pattern formation position on the sheet S is positioned corresponding to the pattern detector  401 , and the first carriage  15  is returned to the docking/separation position. 
     At step S 304 , the first and second carriages  15  and  16  are docked with each other (first docking of the first and second carriages  15  and  16 ). 
     At step S 305 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   2 -H) of the first pattern row. At step S 306 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     At step S 307 , the first and second carriages  15  and  16  are separated from each other (second separation of the first and second carriages  15  and  16 ). 
     At step S 308 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  of a second pattern row at a second pattern formation position on the sheet S, and then the sheet S is fed in the sub-scanning position such that the second pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . At step S 309 , the first carriage  15  is moved homeward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -H) between the two reference patterns  400   a  of the second pattern row formed at step S 308  to complete formation of the second pattern row. Then, the first carriage  15  is returned to the docking/separation position. 
     At step S 310 , the first and second carriages  15  and  16  are docked with each other (second docking of the first and second carriages  15  and  16 ). 
     At step S 311 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -H) and the two reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -H) of the second pattern row, and then at S 312 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     At step S 313 , the first and second carriages  15  and  16  are separated from each other (third separation of the first and second carriages  15  and  16 ). 
     At step S 314 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  of a third pattern row at a third pattern formation position on the sheet S. At step S 315 , the first carriage  15  is moved homeward. At this time, the sheet S is fed in the sub-scanning direction such that the third pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . 
     At step S 316 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -O) between the two reference patterns  400   a  of the third pattern row formed at step S 314  to complete formation of the third pattern row. At step S 317 , the first carriage is moved homeward to the docking/separation position. 
     At step S 318 , the first and second carriages  15  and  16  are docked with each other (third docking of the first and second carriages  15  and  16 ). 
     At step S 319 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -O) and the two reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -O) of the third pattern row, and then at S 320 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     As described above, in the first comparative example, the pattern detector  401  does not successively read multiple rows of the adjustment patterns  400  formed on the sheet S in the sub-scanning direction without docking and separation of the first and second carriages  15  and  16 . Consequently, docking or separation of the first and second carriages  15  and  16  needs to be performed each time the adjustment pattern row is formed or read. As a result, the number of times the first and second carriages  15  and  16  are docked with and separated from each other is increased, thereby extending downtime for correcting the landing positions. 
     A description is now given of formation and reading of the adjustment pattern  400  according to a second comparative example with reference to  FIGS. 19A to 19E .  FIGS. 19A to 19E  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern  400  according to the second comparative example. In the second comparative example, in place of the first recording head  101   k   2 , the first recording head  101   k   1  positioned downstream from the first recording head  101   k   2  in the direction of sheet feed forms the reference patterns  400   a.    
     At step S 401 , the first and second carriages  15  and  16  are separated from each other (first separation of the first and second carriages  15  and  16 ) at the docking/separation position. 
     At step S 402 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms two reference patterns  400   a  of a first pattern row at a first pattern formation position on the sheet S while the first recording head  101   k   2  forms a measured pattern  400   b  ( 101   k   2 -O) of a second pattern row at a second pattern formation position on the sheet S. 
     At step S 403 , the first carriage  15  is moved homeward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -H) between the two reference patterns  400   a  of the first pattern row formed at step S 402  to complete formation of the first pattern row. Then, the first carriage  15  is returned to the docking/separation position. 
     At S 404 , the first and second carriages  15  and  16  are docked with each other (first docking of the first and second carriages  15  and  16 ). 
     At step S 405 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -H) of the first pattern row. At step S 406 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. At this time, the sheet S is fed in the sub-scanning position such that the second pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . 
     At step S 407 , the first and second carriages  15  and  16  are separated from each other (second separation of the first and second carriages  15  and  16 ). 
     At step S 408 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms two reference patterns  400   a  of the second pattern row that sandwich the measured pattern  400   b  ( 101   k   2 -O) formed at step S 402  to complete formation of the second pattern row. At step S 409 , the first carriage  15  is moved homeward so that the first recording head  101   k   2  form a measured pattern  400   b  ( 101   k   2 -H) of a third pattern row at a third pattern formation position on the sheet S. Then, the first carriage  15  is returned to the docking/separation position. 
     At step S 410 , the first and second carriages  15  and  16  are docked with each other (second docking of the first and second carriages  15  and  16 ). 
     At step S 411 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -O) and the two reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   2 -O) of the second pattern row, and then at S 412 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. At this time, the sheet S is fed in the sub-scanning direction such that the third pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   1 . 
     At step S 413 , the first and second carriages  15  and  16  are separated from each other (third separation of the first and second carriages  15  and  16 ). 
     At step S 414 , the first carriage  15  is moved outward so that the first recording head  101   k   1  forms two reference patterns  400   a  that sandwich the measured pattern  400   b  ( 101   k   2 -H) of the third pattern row formed at step S 409  to complete formation of the third pattern row. At step S 415 , the first carriage  15  is moved homeward to the docking/separation position. 
     At step S 416 , the first and second carriages  15  and  16  are docked with each other (third docking of the first and second carriages  15  and  16 ). 
     At step S 417 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -H) and the two reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   2 -H) of the third pattern row, and then at S 418 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     As described above, in the second comparative example, the reference patterns  400   a  are formed by one of the two first recording heads offset laterally from each other in the main scanning direction, that is, the first recording head  101   k   1 , provided downstream from the other one of the first recording heads, that is, the first recording head  101   k   2 , in the direction of sheet feed. In such a case, docking of the first and second carriages  15  and  16 , reading of the adjustment pattern  400 , feeding of the sheet S, separation of the first and second carriages  15  and  16  from each other, and formation of the reference patterns  400   a  using the first recording head  101   k   1  must be performed, in that order. Consequently, the adjustment pattern  400  cannot be read by the pattern detector  401  without separation of the first and second carriages  15  and  16  from each other, thereby preventing reduction of the number of times the first and second carriages  15  and  16  are docked with and separated from each other. 
     A description is now given of formation and reading of the adjustment pattern  400  according to a third comparative example with reference to  FIGS. 20A to 20D .  FIGS. 20A to 20D  are explanatory drawings illustrating steps in a process of formation and reading of the adjustment pattern  400  according to the third comparative example. In the third comparative example, the sheet S is not only fed in the single direction, that is, the sub-scanning direction, but also rewound in the middle of sheet feed. 
     At step  501 , the first and second carriages  15  and  16  are separated from each other (first separation of the first and second carriages  15  and  16 ) at the docking/separation position. 
     At step S 502 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  of a second pattern row at a second pattern formation position on the sheet S while the first recording heads  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -O) of a first pattern row at a first pattern formation position on the sheet S. Then, the sheet S is fed in the sub-scanning direction such that the second pattern formation position on the sheet S is positioned corresponding to the first recording heads  101   k   1 . 
     At step S 503 , the first carriage  15  is moved homeward so that the first recording head  101   k   1  forms a measured pattern  400   b  ( 101   k   1 -H) between the two reference patterns  400   a  of the second pattern row formed at step S 502  to complete formation of the second pattern row while the first recording head  101   k   2  forms a measured pattern  400   b  ( 101   k   2 -H) of a third pattern row at a third pattern formation position on the sheet S. 
     At S 504 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  that sandwich the measured pattern  400   b  ( 101   k   2 -H) of the third pattern row formed at step S 503  to complete formation of the third pattern row. 
     At S 505 , the first carriage  15  is moved homeward. At this time, the sheet S is rewound in a direction opposite the direction of sheet feed, that is, the sub-scanning direction, such that the first pattern formation position on the sheet S is positioned corresponding to the first recording head  101   k   2 . 
     At S 506 , the first carriage  15  is moved outward so that the first recording head  101   k   2  forms two reference patterns  400   a  that sandwich the measured pattern  400   b  ( 101   k   1 -O) of the first pattern row formed at step S 502  to complete formation of the first pattern row. Then, at S 507 , the first carriage  15  is moved homeward to the docking/separation position. At this time, the sheet S is fed in the sub-scanning direction such that the first pattern formation position on the sheet S is positioned corresponding to the pattern detector  401 . 
     At step S 508 , the first and second carriages  15  and  16  are docked with each other (first docking of the first and second carriages  15  and  16 ). 
     At step S 509 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -O) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -O) of the first pattern row. At step S 510 , the first and second carriages  15  and  16  are moved homeward. At this time, the sheet S is fed in the sub-scanning direction such that the second pattern formation position on the sheet S is positioned corresponding to the pattern detector  401 . 
     At step S 511 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   1 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   1 -H) of the second pattern row. Then, at step S 512 , the first and second carriages  15  and  16  are moved homeward. At this time, the sheet S is fed in the sub-scanning direction such that the third pattern formation position on the sheet S is positioned corresponding to the pattern detector  401 . 
     At step S 513 , the first and second carriages  15  and  16  docked with each other are together moved outward so that the pattern detector  401  reads the measured pattern  400   b  ( 101   k   2 -H) and the reference patterns  400   a  on either side of the measured pattern  400   b  ( 101   k   2 -H) of the third pattern row. Then, at step S 514 , the first and second carriages  15  and  16  are moved homeward to the docking/separation position. 
     As described above, the sheet S is rewound in the direction opposite the direction of sheet feed according to the third comparative example. Accordingly, the pattern detector  401  reads the adjustment patterns  400  successively after formation of all of the adjustment patterns  400  is completed, and the first and second carriages  15  and  16  need to be separated from and docked with each other only once. However, positions where the adjustment patterns  400  are formed vary due to a skew caused by rewinding of the sheet S, thereby degrading accuracy in correction of the landing positions. 
     As can be appreciated by those skilled in the art, numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     This patent specification is based on Japanese Patent Application No. 2010-045337, filed on Mar. 2, 2010 in the Japan Patent Office, which is hereby incorporated herein by reference in its entirety.