Patent Publication Number: US-2023146482-A1

Title: Flushing timing adjustment method and inkjet recording device

Description:
TECHNICAL FIELD 
     The present invention relates to a flushing timing adjustment method of an inkjet recording device and an inkjet recording device therefor. 
     BACKGROUND ART 
     Use of quick-drying ink in inkjet printers and other inkjet recording devices tends to clog nozzles of recording heads easily due to thickening of ink by drying. Thickened ink needs to be discharged before printing on the paper. An ink discharging operation that does not contribute to image formation on the paper is also referred to herein as flushing (idle discharge). 
     Conventional flushing techniques are disclosed in, for example, Patent Literature 1 to 3. In Patent Literature 1, the flushing is performed by discharging ink from the recording heads between sheets of paper (so-called “between the papers”) supplied to a conveyance belt and causing the ink to pass through openings of the conveyance belt. An opening detection sensor detects the positions of the openings in the conveyance belt, and the discharge of ink during flushing is controlled in accordance with the detection result. 
     Patent Literature 2 discloses a technique to correct the ink discharge timing relative to the openings by taking into account the effect of variations in the pitch of the openings in the paper conveyance direction due to, for example, a perimeter tolerance of the conveyance belt. 
     For example, when the paper is placed on the conveyance belt overlapping the openings, paper deflection occurs and a distance (gap) between the recording head and the paper becomes uneven. This may degrade the quality of the image to be recorded on the paper. In Patent Literature 3, the paper is supplied to the conveyance belt so as not to overlap the openings, thus suppressing the deterioration in quality of the recorded image due to the paper deflection. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Publication No. 2001-113690 
         PTL 2: Japanese Unexamined Patent Publication No. 2012-45867 
         PTL 3: Japanese Unexamined Patent Publication No. 2006-21399 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     An opening detection sensor has a detection tolerance (variations in detection accuracy) and an installation position tolerance (variations in installation position). Even when a device is designed using a center value of each tolerance, accumulating the tolerances should increase the variations in ink discharge timing during flushing (flushing timing) between devices. To facilitate handling of such variations in the flushing timing, it may be possible, for example, to enlarge the openings of the conveyance belt. However, the enlarged openings may lead to a decrease of the strength of the conveyance belt and eventually break the conveyance belt. In addition, the enlarged openings may also lead to a decrease in print quality due to the change of the gap caused by the deflection of the paper that overlaps the openings, as described in Patent Literature 3. For this reason, it is preferable to form the openings as small as possible. 
     Therefore, without forming the opening larger than necessary to reduce the effect of the tolerance of the opening detection sensor, it is necessary to properly adjust the flushing timing for each device such that the ink discharged from the recording heads during flushing can accurately pass through the openings. Such flushing timing adjustment, however, has not been proposed in the past. 
     In view of the above problems, it is an object of the present invention to provide a flushing timing adjustment method capable of properly adjusting the flushing timing and executing the flushing accurately, without unnecessarily enlarging the openings to decrease the effect of the tolerance of the opening detection sensor, and also provide an ink jet recording device capable of executing such flushing timing adjustment. 
     Means for Solving the Problem 
     A flushing timing adjustment method according to an aspect of the present invention is a flushing timing adjustment method of an inkjet recording device for adjusting a flushing timing which is an ink discharge timing for flushing to discharge ink from a recording head toward openings of an endless conveyance belt. The adjustment method includes detecting a recording medium supplied from a recording medium supply portion to the conveyance belt by a recording medium detection sensor, detecting the recording medium conveyed by the conveyance belt by an opening detection sensor that detects the openings that move by running of the conveyance belt, recording a first chart on the recording medium by discharging ink from a predetermined nozzle of the recording head at a plurality of discharge timings set in advance using a detection time point at which the recording medium is detected by the recording medium detection sensor as a reference time point, and recording a second chart on the recording medium alongside the first chart by discharging ink from another nozzle of the recording head at a plurality of discharge timings set in advance using a detection time point at which the recording medium is detected by the opening detection sensor as a reference time point, and adjusting the flushing timing in accordance with the an input of an instruction following the recording of the first chart and the second chart are recorded on the recording medium, the flushing timing being set using the detection time point at which the opening is detected by the opening detection sensor as a reference time point. 
     An inkjet recording device according to another aspect of the present invention includes a recording head having a plurality of nozzles for discharging ink, an endless conveyance belt that conveys a recording medium and has openings at a plurality of locations in the conveyance direction of the recording medium, the plurality of openings allowing ink to pass through when the recording head executes flushing for discharging ink at timing different from timing that contributes to image formation, a recording medium supply portion that supplies the recording medium to the conveyance belt, a recording medium detection sensor that detects the recording medium supplied from the recording medium supply portion to the conveyance belt, an opening detection sensor that detects the openings that move by running of the conveyance belt, and also detects the recording medium on the conveyance belt, and a discharge control portion that controls discharging of the ink in the recording head. The discharge control portion causes a first chart to be recorded on the recording medium by discharging ink from a predetermined nozzle of the recording head at a plurality of discharge timings set in advance using a detection time point at which the recording medium is detected by the recording medium detection sensor as a reference time point, and causes a second chart to be recorded on the recording medium alongside the first chart by discharging ink from another nozzle of the recording head at a plurality of discharge timings set in advance using a detection time point at which the recording medium is detected by the opening detection sensor as a reference time point. 
     Effect of the Invention 
     The flushing timing, which is set using the detection time point of the openings by the opening detection sensor as the reference time point, is adjusted in accordance with the input of the instruction following the recording of the first chart and the second chart on the recording medium. When the flushing is performed during the use of the device on the user side, the ink can be discharged from the recording heads at the pre-adjusted flushing timing and passes through the openings of the conveyance belt, even though the opening detection sensor has the tolerance (detection tolerance, installation position tolerance). Therefore, it is possible to perform the flushing accurately without enlarging the openings unnecessarily to reduce the effect of the tolerance. Furthermore, the inkjet recording device capable of such flushing timing adjustment can be implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an explanatory view schematically illustrating a structure of a printer as an inkjet recording device according to an embodiment of the present invention. 
         FIG.  2    is a plan view of a recording portion provided in the printer. 
         FIG.  3    is en explanatory view schematically illustrating a structure around a paper conveyance path from a paper-feed cassette to a second conveyance unit via a first conveyance unit in the printer. 
         FIG.  4    is a block diagram illustrating a hardware structure of a major part of the printer. 
         FIG.  5    is a plan view illustrating an example of a structure of a first conveyance belt included in the first conveyance unit. 
         FIG.  6    is an explanatory view schematically illustrating an example of a pattern of opening groups for flushing and the paper placed on the first conveyance belt according to the pattern when the first conveyance belt of  FIG.  5    is used. 
         FIG.  7    is an explanatory view schematically illustrating another example of the pattern and the paper placed on the first conveyance belt according to the pattern. 
         FIG.  8    is an explanatory view schematically illustrating still another example of the pattern and the paper placed on the first conveyance belt according to the pattern. 
         FIG.  9    is an explanatory view schematically illustrating still another example of the pattern and the paper placed on the first conveyance belt according to the pattern. 
         FIG.  10    is an explanatory view schematically illustrating a light amount distribution of reflected light when an object is irradiated with light. 
         FIG.  11    is a cross-sectional view illustrating a schematic structure of an opening detection sensor provided in the printer. 
         FIG.  12    is an explanatory view schematically illustrating variations in signals output from the opening detection sensor. 
         FIG.  13    is a flowchart illustrating a flow of operation according to a flushing timing adjustment method. 
         FIG.  14    is a timing chart illustrating examples of output signals from various sensors and various timings when paper is supplied to the first conveyance belt. 
         FIG.  15    is an explanatory view schematically illustrating an example of a first chart and a second chart recorded on paper when the adjustment method is implemented. 
         FIG.  16    is an explanatory view schematically illustrating a state in which a positional discrepancy has occurred between the first chart and the second chart in the structure illustrate in  FIG.  15   . 
         FIG.  17    is an explanatory view schematically illustrating another example of the first chart and the second chart. 
         FIG.  18    is an explanatory view schematically illustrating a state in which a positional discrepancy has occurred between the first chart and the second chart in the structure illustrated in  FIG.  17   . 
         FIG.  19    is a timing chart at least illustrating a flushing executable timing for each opening of two opening rows that belong to each opening group of the first conveyance belt. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     1. Structure of Inkjet Recording Device 
     An embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG.  1    is an explanatory view schematically illustrating the structure of a printer  100  as an inkjet recording device. The printer  100  includes a paper-feed cassette  2  as a paper storage portion. The paper-feed cassette  2  is disposed in a lower portion in a printer body  1 . Paper P is stored in the paper-feed cassette  2  as an example of a recording medium. 
     A paper feeder  3  is disposed on a downstream side of the paper-feed cassette  2  in a paper conveyance direction, that is, on an upper right side of the paper-feed cassette  2  in  FIG.  1   . The paper feeder  3  separates and feeds paper P one sheet at a time toward the upper right side of the paper-feed cassette  2 . 
     The printer  100  includes a first paper conveyance path  4   a  therein. The first paper conveyance path  4   a  is located on the upper right side of the paper-feed cassette  2  in the paper-feed direction. The paper P, which is fed out of the paper-feed cassette  2 , is conveyed perpendicularly upward along the first paper conveyance path  4   a  on a side surface of the printer body  1 . 
     A registration roller pair  13  is provided at a downstream end of the first paper conveyance path  4   a  in the paper conveyance direction. In addition, a first conveyance unit  5  and a recording portion  9  are disposed in the immediate vicinity on a downstream side of the registration roller pair  13  in the paper conveyance direction. The paper P is fed out of the paper-feed cassette  2  and reaches the registration roller pair  13  through the first paper conveyance path  4   a . The registration roller pair  13  measures timing for an ink discharge operation executed by the recording portion  9  while correcting diagonal feed of the paper P, and feeds the paper P toward the first conveyance unit  5  (particularly to a first conveyance belt  8  which will be described later). That is, the registration roller pair  13  is a recording medium supply portion that supplies the paper P onto the first conveyance belt  8 . 
     The paper P fed to the first conveyance unit  5  by the registration roller pair  13  is conveyed by the first conveyance belt  8  to a position facing the recording portion  9  (in particular, recording heads  17   a  to  17   c , which will be described below). Ink is discharged onto the paper P from the recording portion  9  to record an image on the paper P. The ink discharge in the recording portion  9  is controlled by a controller  110  in the printer  100 . 
     In the paper conveyance direction, a second conveyance unit  12  is disposed on a downstream side (left side in  FIG.  1   ) of the first conveyance unit  5 . The paper P, on which the image is recorded by the recording portion  9 , is conveyed to the second conveyance unit  12 . The ink discharged onto the surface of the paper P is dried while the paper P passes through the second conveyance unit  12 . 
     In the paper conveyance direction, a de-curler portion  14  is provided on the downstream side of the second conveyance unit  12  and near the left side of the printer body  1 . The paper P, on which the ink has been dried by the second conveying portion  12 , is conveyed to the de-curler portion  14  to uncurl the curling of the paper P. 
     In the paper conveyance direction, a second paper conveyance path  4   b  is provided on the downstream side (upper side in  FIG.  1   ) of the de-curler portion  14 . In a case where double-sided recording is not performed, the paper P, which has passed through the de-curler portion  14 , passes through the second paper conveyance path  4   b  and is discharged to a paper discharge tray  15  that is provided on the outside of a left surface of the printer  100 . 
     A reverse conveyance path  16  for the double-sided recording is provided at an upper portion of the printer body  1  and above the recording portion  9  and the second conveyance unit  12 . In a case where the double-sided recording is performed, the paper P, on which recording is completed on one surface (first surface) thereof and which has passed through the second conveyance unit  12  and the de-curler portion  14 , is conveyed to the reverse conveyance path  16  through the second paper conveyance path  4   b.    
     The conveyance direction of the paper P, which has been conveyed to the reverse conveyance path  16 , is switched for subsequent recording on the other surface (second surface) of the paper P. Then, the paper P passes through the upper portion of the printer body  1  to proceed rightward, and is conveyed to the first conveyance unit  5  again via the registration roller pair  13  with the second surface thereof faces upward. In the first conveyance unit  5 , the paper P is conveyed to the position facing the recording portion  9 , and the ink is discharged from the recording portion  9  to the second surface to record the image thereon. After the double-sided recording is performed, the paper P is discharged to the paper discharge tray  15  through the second conveyance unit  12 , the de-curler portion  14 , and the second paper conveyance path  4   b  in this order. 
     A maintenance unit  19  and a cap unit  20  are provided below the second conveyance unit  12 . The maintenance unit  19  moves horizontally below the recording portion  9  in execution of purging, wipes the ink pushed out of ink discharge port of the recording head, and collects the wiped ink. Purging refers to an operation to forcibly push out the ink from the ink discharge port of the recording head in order to discharge thickened ink, a foreign substance, or air bubbles in the ink discharge ports. The cap unit  20  moves horizontally toward the position below the recording portion  9  when capping the ink discharge surface of the recording head, then farther moves upward and is mounted on a lower surface of the recording head. 
       FIG.  2    is a plan view of the recording portion  9 . The recording portion  9  includes a head housing  10  and line heads  11 Y,  11 M,  11 C, and  11 K. The line heads  11 Y to  11 K are held by the head housing  10  at a height capable of forming a predetermined distance (for example, 1 mm) from the conveyance surface of the endless first conveyance belt  8  stretching between a plurality of rollers including a drive roller  6   a , a driven roller  6   b , and tension rollers  7   a ,  7   b . The drive roller  6   a  causes the first conveyance belt  8  to move in the conveyance direction of the paper P (arrow A direction). This drive roller  6   a  is driven by the main control portion  110   c  of the controller  110  (see  FIG.  4   ). The plurality of rollers are arranged in an order of the tension roller  7   a , the tension roller  7   b , the driven roller  6   b , and the drive roller  6   a  along a moving direction of the first conveyance belt  8 . 
     The line heads  11 Y to  11 K each have a plurality of (three in this case) recording heads  17   a  to  17   c . The recording heads  17   a  to  17   c  are arranged in a staggered manner along a paper width direction (arrow BB′ direction) perpendicular to the paper conveyance direction (the arrow A direction). Each of the recording heads  17   a  to  17   c  has a plurality of ink discharge ports  18  (nozzles). The ink discharge ports  18  are arranged at equally-spaced intervals in a width direction of the recording head, that is, the paper width direction (arrow BB′ direction). The inks of yellow (Y), magenta (M), cyan (C), and black (K) colors are discharged respectively onto the paper P that is conveyed on the first conveyance belt  8  from each of the line heads  11 Y to  11 K via the ink discharge ports  18  of the recording heads  17   a  to  17   c.    
       FIG.  3    schematically illustrates the structure around the conveyance path of the paper P from the paper-feed cassette  2  to the second conveyance unit  12  via the first conveyance unit  5 .  FIG.  4    is a block diagram illustrating a hardware structure of the major part of the printer  100 . In addition to the above structure, the printer  100  further includes a registration sensor  21 , a paper detection sensor  22 , an opening detection sensor  23 , and a belt sensor  24 . 
     The registration sensor  21  detects the paper P that is conveyed from the paper-feed cassette  2  by the paper feeder  3  and is fed to the registration roller pair  13 . The registration sensor  21  is located on the upstream side of the registration roller pair  13  in a paper supply direction of the paper P. A paper supply control portion  110   b , which will be described later, of the controller  110 , can control a rotation start timing of the registration roller pair  13  in accordance with the detection result of the registration sensor  21 . For example, the paper supply control portion  110   b  can control the paper supply timing of the paper P to the first conveyance belt  8  after skew correction by the registration roller pair  13  in accordance with the detection result of the registration sensor  21 . 
     The paper detection sensor  22  is a recording medium detection sensor that detects the passage (timing) of a leading edge of the paper P fed from the registration roller pair  13  to the first conveyance belt  8 . The paper detection sensor  22  is located on the upstream side of the opening detection sensor  23  in the paper conveyance direction. In accordance with the detection result of the paper P by the paper detection sensor  22 , the discharge control portion  110   a  can control the ink discharge timing for the paper P that reaches the position facing line heads  11 Y to  11 K (recording heads  17   a  to  17   c ) by the first conveyance belt  8 . 
     The opening detection sensor  23  detects openings  80  (see  FIG.  5   ), which will be described later, formed in the first conveyance belt  8 . The opening detection sensor  23  is located on the upstream side of the recording portion  9  and on the downstream side of the paper detection sensor  22  in the paper conveyance direction (running direction of the first conveyance belt  8 ). The discharge control portion  110   a  can control the recording heads  17   a  to  17   c  in accordance with the detection result of the openings  80  by the opening detection sensor  23 , and discharge the ink at a predetermined timing during flushing which will be described later. 
     The belt sensor  24  detects the position of an opening group  82  (openings  80 ) which is used as a reference during conveyance of the paper among a plurality of opening groups  82  of the first conveyance belt  8 . For example, a marker may be attached on the first conveyance belt  8  at a position corresponding to the reference opening group  82 , so that the belt sensor  24  can detect the reference opening group  82  by detecting the marker. 
     The belt sensor  24  is located on the upstream side of the driven roller  6   b  on which the first conveyance belt  8  stretches in the paper conveyance direction (running direction of the first conveyance belt  8 ). Therefore, the belt sensor  24  may be located between the driven roller  6   b  and the other roller  7   b , or between the rollers  7   a  and  7   b . The driven roller  6   b  is located on the upstream side of the recording portion  9  in the running direction of the first conveyance belt  8 . The paper supply control portion  110   b  can control the registration roller pair  13  such that the paper P is supplied to the first conveyance belt  8  at a predetermined timing in accordance with the detection result of the opening group  82  by the belt sensor  24 . 
     The paper detection sensor  22 , the opening detection sensor  23 , and the belt sensor  24 , which have been described above, can be provided as transmissive or reflective optical sensors, contact image (CIS) sensors, or the like. In particular, the opening detection sensor  23  is desirably provided as the optical sensor capable of detecting both regularly reflected light and the diffusely reflected light, the reasons for which will be described later. 
     Furthermore, the printer  100  may include a meandering detection sensor that detects meandering of the first conveyance belt  8  and correct the meandering of the first conveyance belt  8  in accordance with the detection result. 
     The printer  100  further includes an operation panel  27 , a storage  28 , and a communication portion  29 . 
     The operation panel  27  is a control panel for accepting various types of input for setting. For example, the user can operate the operation panel  27  to enter information on the size of the paper P to be set in the paper-feed cassette  2 , that is, the information on the size of the paper P to be conveyed by the first conveyance belt  8 . The user can also operate the operation panel  27  to enter the number of sheets of the paper P to be printed or instruct to start print jobs. 
     The storage  28  is a memory that stores an operating program of the controller  110  and various types of information, and is configured to include a read only memory (ROM), a random access memory (RAM), a non-volatile memory, or the like. The storage  28  stores the information set using the operation panel  27  (for example, the information on the size of the paper P). The storage  28  also stores information on the position of each opening  80  of the first conveyance belt  8  and a pattern of the openings  80  through which ink is passed during the flushing which will be described later. 
     The communication portion  29  is a communication interface used to exchange information with an external device (for example, a personal computer (PC)). For example, when the user operates the PC to send a print command and image data to the printer  100 , the image data and the print command are entered to the printer  100  via the communication portion  29 . In the printer  100 , the discharge control portion  110   a  controls the recording heads  17   a  to  17   c  in accordance with the image data to discharge ink to record the image on the paper P. 
     In view of the above, the operation panel  27  and communication portion  29  can also be regarded as functioning as an input accepting portion  30  that accepts input of various types of information. 
     The printer  100  of the present embodiment also includes the controller  110 . The controller  110  is composed of, for example, a central processing unit (CPU), and includes the discharge control portion  110   a , the paper supply control portion  110   b , and the main control portion  110   c . The discharge control portion  110   a  controls the discharge of ink from the recording heads  17   a  to  17   c . The paper supply control portion  110   b  is a recording medium supply control portion that controls the registration roller pair  13  as the recording medium supply portion in accordance with the detection result of the opening group  82  by the belt sensor  24 , thus controlling the supply of the paper P to the registration roller pair  13 . The main control portion  110   c  controls the operation of individual parts in the printer  100 . The discharge control portion  110   a , the paper supply control portion  110   b , and the main control portion  110   c  may further function as an arithmetic portion that performs necessary calculations and as a timekeeping portion that measures time. 
     Furthermore, as illustrated in  FIG.  3   , the printer  100  includes ink receiving portions  31 Y,  31 M,  31 C, and  31 K on an inner circumferential surface side of the first conveyance belt  8 . The ink receiving portions  31 Y to  31 K receive and collect the ink discharged from the recording heads  17   a  to  17   c  and passed through the openings  80  of the first conveyance belt  8 , when the recording heads  17   a  to  17   c  executes the flushing. Accordingly, the ink receiving portions  31 Y to  31 K are disposed at positions facing the recording heads  17   a  to  17   c  of the line heads  11 Y to  11 K via the first conveyance belt  8 . The ink collected by the ink receiving portions  31 Y to  31 K is sent to a waste ink tank, for example, or may be reused without discarding. 
     Here, the flushing means the discharge of ink from the ink discharge ports  18  at timing different from timing that contributes to the image formation (image recording) on the paper P for the purpose of reducing or preventing clogging of the ink discharge ports  18  caused by drying of the ink. The discharge control portion  110   a  controls the execution of flushing in the recording heads  17   a  to  17   c.    
     The above-described second conveyance unit  12  includes a second conveyance belt  12   a  and a dryer  12   b . The second conveyance belt  12   a  stretches over two drive rollers  12   c  and a driven roller  12   d . The paper P, which is conveyed by the first conveyance unit  5 , and on which the image is recorded by the ink discharge by the recording portion  9 , is conveyed by the second conveyance belt  12   a , dried by the dryer  12   b  during the conveyance, and is then conveyed to the de-curler portion  14 . 
     2. Details of First Conveyance Belt 
     2-1. Example Structure of First Conveyance Belt 
     Next, the first conveyance belt  8  in the first conveyance unit  5  is described in detail below.  FIG.  5    is a plan view illustrating an example of the structure of the first conveyance belt  8 . The present embodiment adopts a negative-pressure suction method for suctioning and conveying the paper P onto the first conveyance belt  8  by negative-pressure suction. Thus, the first conveyance belt  8  includes innumerable suction holes  8   a  to allow the suction air generated by the negative-pressure suction passes through. 
     The first conveyance belt  8  also includes the opening groups  82 . The opening groups  82  is a group of the openings  80  to allow the ink discharged from the nozzles (ink discharge ports  18 ) of the recording heads  17   a  to  17   c  to pass through during flushing. An opening area of one opening  80  is larger than an opening area of one suction hole  8   a . The first conveyance belt  8  includes a plurality of opening groups  82  in one cycle of conveying the paper P in the conveyance direction of the paper P (direction A), and six group are provided in the present embodiment. One cycle means a period in which the first conveyance belt  8  makes one round. To distinguish the opening groups  82  from each other, the six opening groups  82  are referred to as opening groups  82 A to  82 F from the downstream side in the direction A. The suction holes  8   a  are located between adjacent opening groups  82  in the direction A. That is, in the first conveyance belt  8 , no suction holes  8   a  are formed in an area overlapping the opening groups  82  (around the openings  80 ). 
     In one cycle of the first conveyance belt  8 , the opening groups  82  are irregularly located in the direction A. That is, in the direction A, a distance between adjacent opening groups  82  is not constant but varies. At this time, the maximum distance between the two adjacent opening groups  82  in the direction A (for example, a distance between the opening group  82 A and the opening group  82 B in  FIG.  5   ) is longer than a length of the paper P in the direction A at the time when the paper P in the minimum printable size (for example, the paper P of A4 size (landscape)) is placed on the first conveyance belt  8 . 
     Each opening group  82  includes opening rows  81 . Each opening row  81  is formed by arranging the plurality of openings  80  in the belt width direction (paper width direction, direction BB′) perpendicular to the direction A. One opening group  82  includes at least one opening row  81  in the direction A, and two opening rows  81  are provided in the present embodiment. To distinguish the two opening rows  81  from each other, it is assumed that one of them is an opening row  81   a , and the other is an opening row  81   b.    
     In one opening group  82 , the openings  80  in any one of the opening rows  81  (for example, the opening row  81   a ) are shifted in the direction BB′ from the openings  80  in the other opening row  81  (for example, the opening row  81   b ), and are arranged so as to partially overlap the openings  80  in the other opening row  81  (for example, the opening row  81   b ) when viewed in the direction A. In each opening row  81 , the plurality of openings  80  are located at equally-spaced intervals in the direction BB′. 
     Since the plurality of opening rows  81  are arranged in the direction A to form one opening group  82  as described above, a width of the opening group  82  in the direction BB′ is greater than a width of the recording heads  17   a  to  17   c  in the direction BB′. This means that each opening group  82  covers an entire ink discharge area in the direction BB′ of the recording heads  17   a  to  17   c , and the ink discharged from all the ink discharge ports  18  of the recording heads  17   a  to  17   c  during flushing passes through any one of the openings  80  in the opening group  82 . 
     In view of the above, the first conveyance belt  8  can be regarded as including the opening groups  82  each including the openings  80  through which the ink discharged from the recording heads  17   a  to  17   c  passes during flushing at the plurality of locations at different intervals in the direction A which is the conveyance direction. 
     2-2. Patterns of Opening Groups Used for Flushing 
     In the present embodiment, the discharge control portion  110   a  controls the recording heads  17   a  to  17   c  to record images on the paper P, while the paper P is conveyed using the first conveyance belt  8 , in accordance with the image data transmitted from the outside (for example, the PC). At this time, the recording heads  17   a  to  17   c  are made to perform the flushing between the sheets of paper P that are conveyed (flushing between the papers) to reduce or prevent clogging of the ink discharge ports  18 . 
     Here, in the present embodiment, the discharge control portion  110   a  determines a pattern (combination) of the plurality of opening groups  82  in the direction A used in the flushing in one cycle of the first conveyance belt  8 , based on the size of the paper P to be used. The size of the paper P to be used can be recognized by the discharge control portion  110   a  in accordance with the information stored in the storage  28  (information on the size of the paper P entered by the operation panel  27   a ) or the image data transmitted with an externally entered printing command. The pattern of the opening groups  82  is literally distinguished from a placement pattern of the paper P, which will be described later. 
       FIGS.  6  to  9    illustrate examples of the pattern of the opening groups  82  for different sized paper P. For example, in the case where the paper P to be used is A4 size (landscape) or letter size (landscape), the discharge control portion  110 A selects the pattern of the opening groups  82  illustrated in  FIG.  6   . In other words, the discharge control portion  110   a  selects, from the six opening groups  82  illustrated in  FIG.  5   , the opening groups  82 A,  82 C, and  82 F as the opening groups  82  used for flushing. In the case where the paper P to be used is A4 size (portrait) or letter size (portrait), the discharge control portion  110   a  selects the opening groups  82 A and  82 D as the opening groups  82  used for flushing from among the six opening groups  82 , as illustrated in  FIG.  7   . In a case where the paper P to be used is A3 size, B4 size or legal size (all in portrait), the discharge control portion  110   a  selects the opening groups  82 A,  82 B, and  82 E as the opening groups  82  used for flushing from among the six groups of openings  82 , as illustrated in  FIG.  8   . In the case where the paper P to be used is 13 inches×19.2 inches in size, the discharge control portion  110   a  selects the opening groups  82 A and  82 D from among the six opening groups  82  as the opening groups  82  used for flushing, as illustrated in  FIG.  9   . In each drawing, the openings  80  in the opening groups  82  that belong to the above patterns are illustrated in black for convenience. 
     The discharge control portion  110   a  then causes the recording heads  17   a  to  17   c  to execute flushing at the timing when the opening groups  82  located according to the determined pattern brought to face the recording heads  17   a  to  17   c  by the running of the first conveyance belt  8 . Here, the running speed of the first conveyance belt  8  (paper conveyance speed) and the positional relationship of the recording heads  17   a  to  17   c  and the opening detection sensor  23  relative to the first conveyance belt  8  are known in advance. Accordingly, when the opening detection sensor  23  detects that the first conveyance belt  8  runs and passes the openings  80  of a predetermined opening group  82 , how many seconds after the detection time point the predetermined opening group  82  passes the position facing the recording heads  17   a  to  17   c  is known. Therefore, the discharge control portion  110   a  can cause the recording heads  17   a  to  17   c  to execute flushing so that the ink passes through the openings  80  of the opening group  82  located in the pattern determined above by having the recording heads  17   a  to  17   c  discharge ink at the discharge timing determined in accordance with the detection result of the openings  80  by the opening detection sensor  23 . 
     2-3. Paper Placement Pattern 
     The paper supply control portion  110   b  controls the supply of the paper P to the first conveyance belt  8  by the registration roller pair  13  such that the paper P is displaced in the direction A from the opening group  82  located in the pattern determined above. In other words, the paper supply control portion  110   b  causes the registration roller pair  13  to supply the paper P between the plurality of opening groups  82  lined up in the direction A in the above pattern on the first conveyance belt  8 . 
     For example, when the paper P to be used is A4 size (landscape) or letter size (landscape), the paper supply control portion  110   b , the registration roller pair  13  is controlled such that, as illustrated in  FIG.  6   , two sheets of paper P are placed between the opening groups  82 A and  82 C on the first conveyance belt  8 , two sheets of paper P are placed between the opening groups  82 C and  82 F, and one sheet of paper P is placed between the opening group  82 F and the opening group  82 A of the next cycle, thus allowing the paper P to be supplied to the first conveyance belt  8  at predetermined supply timings. At this time, the paper supply control portion  110   b  controls the registration roller pair  13  to supply the paper P to the first conveyance belt  8  such that the paper P is placed at least a predetermined distance in the direction A (including both upstream and downstream directions) from the opening groups  82 A,  82 C, and  82 F arranged in the above pattern on the first conveyance belt  8 . An example of the predetermined distance is set to 10 mm herein. 
     Here, the paper supply control portion  110   b  can determine the timing for supplying the paper P by the registration roller pair  13  in accordance with the detection result of the predetermined opening group  82  by the belt sensor  24 . For example, when the belt sensor  24  detects that the first conveyance belt  8  runs and pass the reference opening group  82  (for example, the opening group  82 A), the paper supply control portion  110   b  can determine how many seconds after the detection time point the paper P should be supplied to the first conveyance belt  8  by the registration roller pair  13  to place the paper P at respective positions illustrated in  FIG.  6   . Therefore, the paper supply control portion  110   b  determines the timing for supplying the paper P in accordance with the detection result of the opening group  82  by the belt sensor  24 , and controls the registration roller pair  13  such that the paper P can be supplied at the determined supply timing. In this way, the sheets of paper P can be placed substantially at equally-spaced intervals at respective positions illustrated in  FIG.  6    on the first conveyance belt  8 . In the example illustrated in  FIG.  6   , five sheets of paper P can be conveyed in one cycle of the first conveyance belt  8 , and 150 images per minute (ipm) can be achieved as the number of printed sheets of paper P (productivity) per minute. 
     When the paper P to be used is A4 size (portrait) or letter size (portrait), the paper supply control portion  110   b  controls the registration roller pair  13  to supply the paper P to the first conveyance belt  8  at a predetermined supply timing such that two sheets of paper P are placed between the opening groups  82 A and  82 D and between the opening group  82 D and the opening group  82 A of the next cycle on the first conveyance belt  8 , as illustrated in  FIG.  7   . In the example illustrated in  FIG.  7   , four sheets of paper P can be conveyed in one cycle of the first conveyance belt  8 , achieving the productivity of 120 ipm. 
     When the paper P to be used is A3 size, B4 size or legal size (all in portrait), the paper supply control portion  110   b  controls the registration roller pair  13  to supply the paper P to the first conveyance belt  8  at a predetermined supply timing such that one sheet of paper P is placed between the opening groups  82 A and  82 B, one sheet of paper P is placed between the opening groups  82 B and  82 E, and one sheet of paper P is placed between the opening group  82 E and the opening group  82 A of the next cycle on the first conveyance belt  8 , as illustrated in  FIG.  8   . In the example illustrated in  FIG.  8   , three sheets of paper P can be conveyed in one cycle of the first conveyance belt  8 , achieving the productivity of 90 ipm. 
     When the paper P to be used is 13″×19.2″ in size, the paper supply control portion  110   b  controls the registration roller pair  13  to supply the paper P to the first conveyance belt  8  at a predetermined supply timing such that one sheet of paper P is placed between the opening groups  82 A and  82 D, and one sheet of paper P is placed between the opening group  82 D and the opening group  82 A of the next cycle on the first conveyance belt  8 , as illustrated in  FIG.  9   . In the example illustrated in  FIG.  9   , two sheets of paper P can be conveyed in one cycle of the first conveyance belt  8 , achieving the productivity of 60 ipm. 
     Thus, as illustrated in  FIGS.  6  to  9   , the pattern of the opening groups  82  used for flushing is determined according to the size of the paper P to be used and, accordingly, the placement pattern of the paper P shifted in the direction A from the opening group  82  is determined. Thus, the placement pattern of the paper P placed on the first conveyance belt  8  can be determined according to the size of the paper P to be used. 
     Here, the supply speed of the paper P (running speed of the first conveyance belt  8 ), and the positional relationship between the recording heads  17   a  to  17   c  and the paper detection sensor  22  are known in advance. Therefore, when the paper detection sensor  22  detects the passage of the paper P, how many seconds after the detection time point the paper P passes the positions facing the recording heads  17   a  to  17   c  is known. Therefore, the discharge control portion  110   a  causes the ink to be discharged from the recording heads  17   a  to  17   c  at the discharge timing determined in accordance with the detection result of the paper P by the paper detection sensor  22 , so that the ink discharged from the recording heads  17   a  to  17   c  lands on the paper P and the image is recorded on the paper P. 
     3. Flushing Timing Adjustment 
     In normal printing, as described in (2-3) above, the discharge control portion  110   a  causes the ink to be discharged from the recording heads  17   a  to  17   c  at the first discharge timing determined in accordance with the detection time point when the paper P is detected by the paper detection sensor  22 . Here, it is assumed that the first discharge timing is set, for example, at the time when time T1 (unit: second) has elapsed after the detection time point of the paper P by the paper detection sensor  22 . When executing flushing, as described in (2-2) above, the discharge control portion  110   a  causes the ink to be discharged from the recording heads  17   a  to  17   c  at the second discharge timing determined in accordance with the detection time point of the openings  80  by the opening detection sensor  23 . Here, it is assumed that the second discharge timing is set, for example, at the time when time T2 (unit: second) has elapsed after the detection time point of the paper P by the opening detection sensor  23 . Since the paper detection sensor  22  is located upstream of the opening detection sensor  23  with respect to the recording heads  17   a  to  17   c  (recording portion  9 ), T1&gt;T2 is satisfied. 
     The second discharge timing mentioned above varies for each device (printer  100 ), depending on the detection tolerance and the installation tolerance of the opening detection sensor  23 . To facilitate handling of such variations in the second discharge timing from device to device, the second discharge timing is adjusted by means described later for each device in the present embodiment. Before describing the adjustment of the second discharge timing or the flushing timing, the opening detection sensor  23  described above used to adjust the flushing timing is described in details. 
     3-1. Details of Opening Detection Sensor 
     In the present embodiment, an optical analog sensor (optical sensor) that detects both regularly reflected light and diffusely reflected light is used as the opening detection sensor  23 . Such an optical sensor is also called an image density sensor (ID sensor).  FIG.  10    schematically illustrates a light amount distribution of reflected light (regularly and diffusely reflected lights) when an object is irradiated with light. As illustrated in the drawing, when an object is irradiated with light, the light is reflected on the surface of the object. At this time, the distribution of the amount of light of the reflected light (ratio of the light amount of the regularly reflected light to the diffusely reflected light) is determined by the characteristics of the object (for example, refractive index) when the light incident angle is constant. The ID sensor uses such a change in the distribution of the light amount of the reflected light according to the characteristics of the object and is usually used as sensors to detect the state of the detected object (for example, image density). 
       FIG.  11    is a cross-sectional view schematically illustrating the structure of the opening detection sensor  23 . The opening detection sensor  23  includes a light source  23   a  which is, for example, made up of an LED, a first light receiving portion  23   b  that detects the regularly reflected light, a second light receiving portion  23   c  that detects the diffusely reflected light, and a housing  23   d . The first and second light receiving portions  23   b  and  23   c  include, for example, photodiodes. The light emitted from the light source  23   a  is transmitted through the lens  23   e  provided on the housing  23   d  and irradiates the first conveyance belt  8 . Of the light reflected from the first conveyance belt  8 , the regularly reflected light passes through the lens  23   e  again and is received by the first light receiving portion  23   b . Thus, the first light receiving portion  23   b  outputs a signal corresponding to the intensity of the regularly reflected light. Of the light reflected by the first conveyance belt  8 , the diffusely reflected light that passes through the lens  23   e  again and is received by the second light receiving portion  23   c . Therefore, the second light receiving portion  23   c  outputs a signal corresponding to the intensity of the diffusely reflected light. 
       FIG.  12    schematically illustrates variations in the signals output from the opening detection sensor  23 . In  FIG.  12   , the signal output from the first light receiving portion  23   b  is referred to as “output  1 ”, and the signal output from the second light receiving portion  23   c  is referred to as “output  2 .” 
     As illustrated on the left-hand side of  FIG.  12   , in a case where the object is the first conveyance belt  8 , the level (intensity) of the output  1  is relatively high and the level of the output  2  is relatively low. As illustrated in the center of  FIG.  12   , when the light enters the opening  80  of the first conveyance belt  8 , the light passes through the opening  80 , so that the levels of both output  1  and output  2  are low. 
     On the other hand, as illustrated on the right-hand side of  FIG.  12   , when the light is incident on the paper P on the first conveyance belt  8 , the level of the output  1  is lower and the level of the output  2  is higher, because the degree of diffusion of the incident light increases compared to the case where light is incident directly on the first conveyance belt  8 . The levels of the output  1  and the output  2  should change depending on the type of the paper P to be used (glossiness, whiteness, or the like) and the sensitivity of the light receiving portions. 
     Thus, by using the ID sensor that detects the reflected light and the diffusely reflection light as the opening detection sensor  23 , the controller  110  (for example, the discharge control portion  110   a ) can determine, in accordance with the output signal from the opening detection sensor  23 , whether the object passing through the position facing the opening detection sensor  23  is (i) the first conveyance belt  8 , (ii) the opening  80  of the first conveyance belt  8 , or (iii) the paper P on the first conveyance belt  8 . The output signal gain may be adjusted according to the sensitivity of the first and second light receiving portions  23   b  and  23  of the opening detection sensor  23 , so that the controller  110  can clearly distinguish the above three states. 
     3-2. Flushing Timing Adjustment Method 
     Next, a flushing timing adjustment method using the opening detection sensor  23  described above is described.  FIG.  13    is a flowchart illustrating the flow of the operation of the flushing timing adjustment method according to the present embodiment.  FIG.  14    is a timing chart illustrating the output signal of the paper detection sensor  22 , the output signal of the opening detection sensor  23  (regularly reflected light detection signal, diffusely reflected light detection signal), an ink discharge timing (during image recording, during flushing), and a flushing executable timing, when the paper P is supplied on the first conveyance belt  8  having the openings  80 . The paper P is adsorbed and conveyed on the first conveyance belt  8 , so that the conveyance speed of the first conveyance belt  8  is the same as the conveyance speed of the paper P. 
     First, for example, in the factory settings of the device (printer  100 ), an operator involved in manufacturing the device operates the operation panel  27  to select a flushing timing adjustment mode (S 1 ). This selection of the mode allows the opening detection sensor  23 , which normally detects the openings  80  moved by the running of the first conveyance belt  8 , to be used as a sensor to detect the paper P, and allows the ink to be discharged from the recording heads  17   a  to  17   c  in accordance with the detection of the paper P by the opening detection sensor  23 . 
     Next, when the paper detection sensor  22  detects the paper P fed to the first conveyance belt  8  by the registration roller pair  13  (S 2 ), and the opening detection sensor detects the paper P conveyed by the first conveyance belt  8  (S 3 ), the discharge control portion  110   a  controls the discharge of the ink by the recording heads  17   a  to  17   c  to record a check pattern (first chart, second chart) on the paper P (S 4 ). 
     Specifically, in S 4 , the discharge control portion  110   a  causes the recording heads  17   a  to  17   c  to discharge inks from predetermined nozzles (ink discharge ports  18 ) at a plurality of predetermined discharge timings set in advance using the detection time point t0 of paper P as a reference time point by the paper detection sensor  22  to record a first chart C 1  on the paper P. The discharge control portion  110   a  also causes the recording heads  17   a  to  17   c  to discharge ink from other nozzles at the plurality of discharge timings set in advance using the detection time point t2 of the paper P by the opening detection sensor  23  as the reference time point, thus recording the second chart C 2  on the paper P alongside the first chart C 1 . 
       FIG.  15    schematically illustrates an example of a first chart C 1  and a second chart C 2 . The first chart C 1  includes a first reference line Cr 1  and a plurality of first auxiliary lines Cs 1 . The first reference line Cr 1  is a line formed by the ink discharged at a reference discharge timing (for example, after time T1 has elapsed from the detection time point t0 of the paper P by the paper detection sensor  22 ) from predetermined nozzles of the recording heads  17   a  to  17   c  in accordance with the detection of the paper P by the paper detection sensor  22 . The first auxiliary lines Cs 1  are lines formed by the ink discharged at a plurality of timings before and after the reference discharge timing from the predetermined nozzles. The plurality of the first auxiliary lines Cs 1  are arranged with the first reference line Cr 1  on the paper P in the conveyance direction of the paper P. The first reference line Cr 1  and the plurality of first auxiliary lines Cs 1  are formed on the paper Pin the longitudinal direction of the recording heads  17   a  to  17   c , that is, in parallel with the width direction (belt width direction) of the paper P. 
     In the drawing, “A to I, 0 to 9” are signs given for convenience corresponding to each position of the first reference line Cr 1  and the plurality of first auxiliary lines Cs 1 . Of these signs, the sign “0” corresponds to the position of the first reference line Cr 1 , and the signs “A-I, 1 to 9” correspond to the positions of the first auxiliary line Cs 1 . In particular, the signs “1 to 9” are given to the first auxiliary lines Cs 1  located on the upstream side of the first reference line Cr 1  in this order upstream from the first reference line Cr 1 . The signs “A to I” are given to the first auxiliary lines Cs 1  located on the downstream side of the first reference line Cr 1  in this order downstream from the first reference line Cr 1 . 
     It is assumed that the interval between the discharges of ink to form the first reference line Cr 1  and the plurality of the first auxiliary lines Cs 1  is, for example, 50 μsec. Therefore, using the discharge timing of the first reference line Cr 1  as a reference timing, the ink discharge timing for forming each line of the first chart C 1  is, for example, T1−50 (μsec), T1−50×2 (ρsec), T1−50×3 (μsec), . . . , upstream from the first reference line Cr 1 , and, for example, T1+50 (μsec), T1+50×2 (μsec), T1+50×3 (μsec), . . . , downstream from the first reference line Cr 1 . 
     On the other hand, the second chart C 2  includes a second reference line Cr 2  and a plurality of second auxiliary lines Cs 2 . The second reference line Cr 2  is a line formed by the ink discharged at a reference discharge timing (for example, after time T2 has elapsed from the detection point t2 of the paper P by the opening detection sensor  23 ) from other nozzles of the recording heads  17   a  to  17   c  in accordance with the detection of the paper P by the opening detection sensor  23 . The second auxiliary lines Cs 2  are lines formed by the ink discharged at a plurality of timings before and after the reference discharge timing from the other nozzles. The plurality of the second auxiliary lines Cs 2  are lined up with the second reference line Cr 2  on the paper P in the conveyance direction of the paper P. The second reference line Cr 2  and the plurality of second auxiliary lines Cs 2  are formed on the paper P in the longitudinal direction of the recording heads  17   a  to  17   c , that is, in parallel with the width direction (belt width direction) of the paper P. 
     It is assumed that the ink discharge interval to form the second reference line Cr 2  and the plurality of the second auxiliary lines Cs 2  is, for example, 60 μsec. Therefore, using the discharge timing of the second reference line Cr 2  as a reference discharge timing, the ink discharge timing to form the lines of the second chart C 2  is, for example, T1−60 (μsec), T1−60×2 (μsec), T1−60×3 (μsec), . . . , upstream from the second reference line Cr 2 , and, for example, T1+60 (μsec), T1+60×2 (μsec), T1+60×3 (μsec), . . . , downstream from the second reference line Cr 2 . 
     Thus, the ink discharge interval to form the lines is different between the first chart C 1  and the second chart C 2 . Therefore, an interval D 1  between all lines included in the first chart C 1  in the conveyance direction of the paper P (first reference line Cr 1 , first auxiliary lines Cs 1 ) is different from a distance D 2  between all lines included in the second chart C 2  (second reference line Cr 2 , second auxiliary lines Cs 2 ) in the conveyance direction of the paper P. For example, D 1 &lt;D 2  is satisfied in the present embodiment. 
     Once the check pattern illustrated in  FIG.  15    is recorded on the paper P, the operator can see the first chart on the paper P, C 1  and the second chart C 2 , and then determine whether and to what extent there is a discrepancy between the ink discharge timing based on the paper detection by the paper detection sensor  22  and the ink discharge timing based on the paper detection by the opening detection sensor  23 . For example, in the example of  FIG.  15   , the first reference line Cr 1  of the first chart C 1  is aligned with the second reference line Cr 2  of the second chart C 2 , meaning that the timings match up, so that the operator can determine that the ink discharge timings of the two charts match up (no discrepancy in discharge timing). 
     Thus, when the operator sees the paper P on which the check pattern is recorded in S 4  and determines that both ink discharge timings match up (No in S 5 ), there is no need to change the flushing timing, and the operator operates the operation panel  27  to enter the end of the flushing timing adjustment (S 6 ). In this case, the discharge control portion  110   a  sets (maintains), as the flushing timing, the reference discharge timing immediately before the end of the flushing timing adjustment, that is, the ink discharge timing to form the second reference line Cr 2  (after time T2 from the detection time point), in accordance with the detection time point t1 of the openings  80  by the opening detection sensor  23 , and stores the flushing timing in the storage  28 . 
     When the flushing is executed in the installation environment on the user side after shipment of the device, the discharge control portion  110   a  causes the nozzles of the recording heads  17   a  to  17   c  to discharge ink after time T2 has elapsed from the detection time point t1 of the opening  80  by the opening detection sensor  23 . With the flushing timing set as described above, the ink discharged from the recording heads  17   a  to  17   c  can pass through the openings  80 . 
     When the normal printing is performed in the installation environment on the user side, the discharge control portion  110   a  can allow the image to be recorded on the paper P by discharging the ink from the recording heads  17   a  to  17   c  after time T1 has elapsed from the detection time point t0, using the detection time point t0 when the paper P is detected by the paper detection sensor  22  as the reference time point. 
     On the other hand,  FIG.  16    schematically illustrates a state in which a positional discrepancy has occurred between the first chart C 1  and the second chart C 2 . As illustrated in the drawing, when the first reference line Cr 1  and the second reference line Cr 2  are misaligned in the paper conveyance direction, the operator can determine that there is a discrepancy between the ink discharge timing based on paper detection by the paper detection sensor  22  and the ink discharge timing based on paper detection by the opening detection sensor  23 . In the example of  FIG.  16   , the second reference line Cr 2  of the second chart C 2  is shifted in the direction of earlier discharge timing by five tick marks of the first auxiliary lines Cs 1  of the first chart C 1 , meaning that the amount of the temporal discrepancy of the ink discharge timing is expressed by one tick mark of the ink discharge interval (50 μsec)×number of tick marks (−5)=−250 μsec (negative signs indicate that the discharge timing is earlier than the reference timing). 
     Thus, when the operator sees the paper P on which the check pattern is recorded in S 4  and determines that there is a discrepancy between the ink discharge timing based on the paper detection by the paper detection sensor  22  and the ink discharge timing based on the paper detection by the opening detection sensor  23  (Yes at S 5 ), the flushing timing needs to be changed, and the operator operates the operation panel  27  to enter the flushing timing adjustment value (S 8 ). For example, in the example of  FIG.  16   , the operator enters a value corresponding to the amount of temporal discrepancy of the ink discharge timing as the adjustment value. As the adjustment value, for example, the number of tick marks on the scale indicating the amount of discrepancy (for example, “−5”) or a value of the discrepancy amount itself (for example, “−250”) can be used. 
     Thus, the discharge control portion  110   a  changes the ink discharge timing to form the lines of the second chart C 2  according to the discrepancy amount. For example, the discharge control portion  110   a  changes the ink discharge timing to form the second reference line Cr 2  from “after time T2 from the detection time point of the openings  80  by the opening detection sensor  23 ” to “after time T2′ from the detection time point of the openings  80  by the opening detection sensor  23 ”. In the above example, T2′ (sec)=T2 (sec)+250 (μsec)=T2+0.25 (sec). In addition, the discharge control portion  110   a  changes the ink discharge timing to form the plurality of second auxiliary lines Cs 2  by back calculation in accordance with the ink discharge timing (after the change) to form the second reference line Cr 2 . The process then returns to S 2  and repeats the process from S 2  onward. 
     Subsequently, if the operator sees the paper P with the check pattern recorded in S 4  and there is a discrepancy between the ink discharge timing based on the paper detection by the paper detection sensor  22  and the ink discharge timing based on the paper detection by the opening detection sensor  23 , the operator enters the adjustment value again in S 8  and repeats the above process. 
     On the other hand, if the operator determines in S 4  that there is no discrepancy, the operator proceeds to S 6  and operates the operation panel  27  to enter the end of flushing timing adjustment. In this case, the discharge control portion  110   a  sets, as the flushing timing, the reference discharge timing immediately before the end of the flushing timing adjustment, that is, the ink discharge timing to form the second reference line Cr 2  (after time T2′ from the detection time point of the openings  80  by the opening detection sensor  23 ), in accordance with the detection time point of the openings  80  by the opening detection sensor  23 ), and stores the flushing timing in the storage (S 7 ). 
     When executing the flushing in the installation environment on the user side after the device is shipped, the discharge control portion  110   a  causes the ink to be discharged from the nozzles of the recording heads  17   a  to  17   c  after time T2′ has elapsed from the detection time point t1 of the openings  80  by the opening detection sensor  23 . As a result, the ink discharged from the recording heads  17   a  to  17   c  at the adjusted flushing timing passes through the openings  80  in the first conveyance belt  8 . 
     The example of adjusting the flushing timing for any one of the openings  80  in each of the opening groups  82  has been described above, but as in the present embodiment, in the structure in which the openings  80  of each opening group  82  are arranged in the staggered manner, a time difference in flushing timing needs to be provided between the first row (for example, the opening row  81   a ) and the second row (for example, the opening row  81   b ) of each opening group  82 . The positional information of the openings  80  of one opening group  82  (positional information of the openings  80  of the opening row  81   a  and openings  80  of the opening row  81   b ) is stored in the storage  28  in advance, and the running speed of the first conveyance belt  8  is constant, so that the above time difference can be obtained by calculation. 
     For example, when the flushing timing for the openings  80  of the opening row  81   a  is set (maintained or changed) in the above manner, the flushing timing for the openings  80  of the next opening row  81   b  can be set (maintained or changed) by adding the above time difference to the flushing timing for the openings  80  of the opening row  81   a . By causing the ink to be discharged from the corresponding nozzles of the recording heads  17   a  to  17   c  at the flushing timing set above, it is possible to execute flushing to allow the ink to pass through the openings  80  of opening row  81   b.    
     4. Effect 
     As described above, the present embodiment uses the opening detection sensor  23 , which originally detects the openings  80 , as a sensor to detect the paper P, thus discharging the ink at the timing in accordance with the detection of the paper P by the paper detection sensor  22  to record the first chart C 1  on the paper P, while recording the second chart C 2  on the paper P by discharging the ink at the discharge timing in accordance with the detection of the paper P by the opening detection sensor  23  (S 2  to S 4 ). After the first chart C 1  and the second chart C 2  are recorded on the paper P, the flushing timing, which is set using the detection time point of the openings  80  by the opening detection sensor  23 , is adjusted in accordance with the input of instruction from the operation panel  27  (S 6  to S 8 ). The flushing timing adjustment may include changing the flushing timing in accordance with the input of the adjustment value in S 8 , and also include maintaining the initial setting of the flushing timing without change in accordance with the input of the end of adjustment in S 6 . 
     By recording the first chart C 1  and the second chart C 2  on the paper P as described above, the operator who is involved in the manufacturing of the device can see the first chart C 1  and the second chart C 2  to easily determine whether the ink discharge timing based on the detection by the opening detection sensor  23  matches the normal timing. In other words, the operator can easily determine whether the ink discharge timing during flushing in accordance with the detection of opening  80  by the opening detection sensor  23  is correct (whether the discharged ink can pass through the openings  80 ) by comparing the image (second chart C 2 ) obtained by actually discharging ink onto the paper P at the timing based on the detection of paper P by the opening detection sensor  23  (second chart C 2 ) and another image (first chart C 1 ) obtained by discharging ink at the timing based on the detection of the paper P by another sensor (paper detection sensor  22 ). The operator can then enter the instruction to adjust (change or maintain) the flushing timing in accordance with the determination result. 
     In addition, the user of the manufactured printer  100  can also recognize the state of the printer  100  by letting the printer  100  to record the check pattern (first chart C 1  and second chart C 2 ). 
     When the user feels that the printer  100  may be malfunctioning, the user can operate the operation panel  27  to have the check pattern recorded. The user can find information about recording such check patterns from the help screen stored in the storage  28  and displayed on the operation panel  27  or in the manuals that come with the printer  100 . By viewing the check pattern according to the help screen or the manuals, the user can determine the state of the printer  100  with respect to the ink discharge timing during flushing. Alternatively, the user can send the check pattern to the manufacturer, the distributor, or the like of the printer  100  to check the state of the printer  100 . 
     When the check pattern indicates that the printer  100  should be adjusted, the user may adjust the flushing timing in the same way as the operator described above, or may ask someone who manufactured or sold the printer  100  to make the adjustment. 
     The main control portion  110   c  may record the check pattern at predetermined intervals to allow the user to check the state of the printer  100 . The interval between recordings may be by the number of sheets printed or by time, such as the elapsed time or the operating time of the printer  100 . Instead of recording the check pattern, the main control portion  110   c  may cause the operation panel  27  to display to urge recording of the check pattern at predetermined intervals. 
     On the device side, the flushing timing is adjusted for each device in accordance with the input of the instruction. Even when the opening detection sensor  23  has the tolerance (detection tolerance, installation position tolerance), the ink can be discharged from the recording heads  17   a  to  17   c  at an appropriate flushing timing for each device and made to accurately pass through the openings  80  to execute the flushing in the installation environment on the user side after the device is shipped. This eliminates the need to form larger openings  80  in the first conveyance belt  8  to reduce the effect of the tolerance of the opening detection sensor  23 . As a result, the following effects can further be obtained. 
     Specifically, the size of the openings  80  can be kept to the minimum necessary, thus suppressing the decrease of the strength of the first conveyance belt  8 . Even when the paper P is placed to overlap the openings  80  that are not used for flushing on the first conveyance belt  8  during the normal image formation, the deflection of the paper P toward the inside of the openings  80  can be suppressed due to the small size of the openings  80 . Thus, the degradation in quality of the recorded image caused by the deflection of the paper P can be suppressed. 
     Since the effect of the tolerance of the opening detection sensor  23  is reduced, it is possible to expand the design margin of the device regarding the installation and assembly of the components including the opening detection sensor  23 . 
     According to the flushing timing adjustment method of the present embodiment, the flushing timing can be easily adjusted in accordance with the input of instruction by the operator, thus simplifying the work required for the adjustment. 
     In particular, in S 6 , the flushing timing is adjusted in accordance with the adjustment value entered after the first chart C 1  and the second chart C 2  are recorded on the paper P. This makes it possible to change the flushing timing appropriately in accordance with the adjustment value. 
     The first chart S 1  includes the first reference line Cr 1  and the second chart S 2  includes the second reference line Cr 2 . By recording the first chart C 1  and the second chart C 2  on the paper P, the operator can easily recognize whether there is a discrepancy between the ink discharge timing based on the detection of the paper P by the paper detection sensor  22  and the ink discharge timing based on the detection of the paper P by the opening detection sensor  23  in accordance with the positional discrepancy between the first reference line Cr 1  of the first chart C 1  and the second reference line Cr 2  of the second chart C 2  in the conveyance direction of the paper P. Therefore, the operator can easily determine whether it is necessary to adjust the flushing timing (ink discharge timing based on the detection of the openings  80  by the opening detection sensor  23 ). 
     The first chart C 1  further includes the plurality of the first auxiliary lines Cs 1 , and the second chart C 2  further includes the plurality of the second auxiliary lines Cs 2 . In this case, the operator can recognize and recognize easily the amount of discrepancy between the first reference line Cr 1  of the first chart C 1  and the second reference line Cr 2  of the second chart C 2  in the paper P conveyance direction by corresponding it to the time interval between the first reference line Cr 1  and the first auxiliary line Cs 1  formed by matching the discharge timing with the second reference line Cr 2 . Specifically, in the example of  FIG.  15   , the operator can easily recognize the amount of discrepancy of the discharge timing between the first reference line Cr 1  and the second reference line Cr 2  by corresponding it to the time interval (for example, 50 μsec×5 (absolute value)) between the first reference line Cr 1  and the first auxiliary line Cs 1  marked with the number “5” formed by matching the discharge timing with the second reference line Cr 2 . 
     The interval D 1  between all lines of the first chart C 1  in the conveyance direction of the paper P is different from the interval D 2  between all lines of the second chart C 2  in the conveyance direction of the paper P (see  FIG.  14   ). In this case, the operator can easily find the lines formed at the matched discharge timing in the first chart C 1  and the second chart C 2 . This further allows the operator to easily recognize the discrepancy amount between the first reference line Cr 1  and the second reference line Cr 2 . 
     The opening detection sensor  23  is the optical sensor that detects both the regularly reflected light and the diffusely reflected light. By using such an opening detection sensor  23 , the discharge control portion  110   a  can reliably distinguish and recognize whether the detection target is the first conveyance belt  8  (area on which the paper P is not placed), the openings  80  of the first conveyance belt  8 , or the paper P on the first conveyance belt  8 , in accordance with the output signals from the opening detection sensor  23 , that is, the detection signal of the regularly reflected light and the detection signal of the diffusely reflected light. This ensures the implementation of the flushing timing adjustment method described above using the opening detection sensor  23 . The ID sensor that detects both the regularly reflected light and the diffusely reflected light can be used as the opening detection sensor  23 , thus effectively utilizing the ID sensor. 
     The opening detection sensor  23  may be a reflective optical sensor that detects the regularly reflected light. The reflective optical sensor outputs the detection signal such as the output  1  illustrated in  FIG.  12   , so that the discharge control portion  110   a  can distinguish and recognize, in accordance with the detection signal of the regularly reflected light, whether the detection target is the first conveyance belt  8 , the openings  80  of the first conveyance belt  8 , or the paper P on the first conveyance belt  8 , thus implementing the flushing timing adjustment method of the present embodiment. The flushing timing adjustment method of the present embodiment is applicable to the case where the conveyance of paper is controlled to be completely separated from the openings of the conveyance belt, as in Patent Literature 3, for example. In this case, the reflective optical sensor can be used as the opening detection sensor  23 . 
     In the example described above, the operator directly operates the operation panel  27  to enter the instruction on the flushing timing (enter the instruction to end the adjustment or enter the adjustment value). Alternatively, the operator may enter the instructions on the PC. In this case, the device (printer  100 ) receives the input of the instruction via the communication portion  29 , and the flushing timing is adjusted on the device side in accordance with the input of the instruction. 
     According to the above description, the inkjet recording device (printer  100 ) that is directly used to implement the flushing timing adjustment method described above can be expressed as below. Specifically, the inkjet recording device includes the recording heads  17   a  to  17   c  each having the plurality of nozzles, the endless first conveyance belt  8  that conveys the paper P and includes the openings  80  at the plurality of locations in the conveyance direction of the paper P, the ink passing through the openings  80  during the execution of the flushing to discharge the ink at the timing different from the timing at which the recording heads  17   a  to  17   c  contribute to the image formation, the registration roller pair  13  that supplies the paper P to the first conveyance belt  8 , the paper detection sensor  22  that detects the paper P to be supplied to the first conveyance belt  8  through the registration roller pair  13 , the opening detection sensor  23  that detects the openings  80  that move by the running of the first conveyance belt  8  and detects the paper P on the first conveyance belt, and the discharge control portion  110   a  that controls the ink discharge by the recording heads  17   a  to  17   c . The discharge control portion  110   a  causes the ink to be discharged from the predetermined nozzles of the recording heads  17   a  to  17   c  at the plurality of discharge timings set in advance using the detection time point of the paper P by the paper detection sensor  22  as the reference time point to record the first chart C 1  on the paper P, while causing the ink to be discharged from other nozzles of the recording heads  17   a  to  17   c  at the plurality of discharge timings set in advance using the detection time point of the paper P by the opening detection sensor  23  as the reference time point to record the second chart C 2  alongside the first chart C 1  on the paper P. The printer  100  further includes the input accepting portion  30  (operation panel  27 , communication portion  29 ) that accepts the input of the instruction for the flushing timing adjustment which indicates the timing for discharging the ink when the recording heads  17   a  to  17   c  execute the flushing. The discharge control portion  110   a  adjusts the flushing timing set by using the detection time point of the openings  80  by the opening detection sensor  23  as the reference time point, in accordance with the input of instruction accepted by the input accepting portion  30 . 
     5. Another Example of Check Pattern 
       FIG.  17    schematically illustrates another example of the first chart C 1  and the second chart C 2  which are recorded on the paper P during the flushing timing adjustment. As illustrated in the drawing, the interval of all lines included in the first chart C 1  (first reference line Cr 1 , first auxiliary lines Cs 1 ) in the paper conveyance direction may be identical to the interval of all lines included in the second chart C 2  (second reference line Cr 2 , second auxiliary lines Cs 2 ) in the paper conveyance direction. The line widths of all lines in the first chart C 1  are different from each other, and the line widths of all lines in the second chart C 2  are different from each other. The first chart C 1  may include lines having the line widths identical to the line widths of the lines in the second chart C 2 . 
       FIG.  18    schematically illustrates the state in which the positional discrepancy has occurred between the first chart C 1  and the second chart C 2  formed as illustrated in  FIG.  17   . When such first and second charts C 1  and C 2  are recorded on the paper P, the operator can also recognize the positional discrepancy between the first chart C 1  and the second chart C 2  in the flushing timing adjustment mode. In other words, in accordance with such first and second charts C 1  and C 2 , the operator can recognize the discrepancy between the ink discharge timing based on the detection of the paper P by the paper detection sensor  22  and the ink discharge timing based on the detection of the paper P by the opening detection sensor  23 . Therefore, even when these first and second charts C 1  and C 2  are recorded on the paper P, the operator can easily determine whether the flushing timing adjustment is necessary according to the first and second charts C 1  and C 2 . 
     6. Identifying Flushing Executable Timing (Part 1) 
     In the printer  100  according to the present embodiment, as illustrated in  FIGS.  6  to  9   , in which the flushing is performed in a manner that the ink passes through the openings  80  of the opening groups  82  (portions painted in black) provided in the pattern corresponding to the size of the paper P, the sheets of paper P are supplied to the first conveyance belt  8  such that each paper P is displaced from the openings  80  in the paper conveyance direction. To control such paper conveyance, the discharge control portion  110   a  may identify the flushing executable timing in accordance with the detection result of the openings  80  by the opening detection sensor  23  and the detection result of the paper P by the opening detection sensor  23 . The flushing executable timing is the ink discharge timing at which the ink discharged from the recording heads  17   a  to  17   c  in the execution of flushing passes through at least part of any one of the openings  80  among the openings  80  of the opening groups  82  located at the plurality of positions in the paper conveyance direction. 
     For example, as illustrated in  FIG.  6   , when the sheets of A4 size (landscape) paper P are supplied to the first conveyance belt  8  at equally-spaced intervals, the opening detection sensor  23  detects the openings  80  of the opening groups  82 A,  82 C, and  82 F. The paper P is placed overlapping the openings  80  of opening groups  82 B,  82 D, and  82 E, so that the opening detection sensor  23  cannot detect these openings  80  (the opening detection sensor  23  detects the paper P). Therefore, the discharge control portion  110   a  can identify the timing at which the openings  80  of the opening groups  82 A,  82 C, and  82 F detected by the opening detection sensor  23  by the running of the first conveyance belt  8  are brought to face the recording heads  17   a  to  17   c  as the flushing executable timing as described above. 
     By identifying the flushing executable timing by the discharge control portion  110   a , the ink can be discharged, in the actual execution of the flushing, from the recording heads  17   a  to  17   c  at least at one identified flushing executable timing to pass through the openings  80  that do not overlap the paper P. Thus, the flushing can be performed with accuracy. 
     Even when the supply of the paper P to the first conveyance belt  8  is controlled such that the paper P is shifted from the opening groups  82 A,  82 C, and  82 F on the first conveyance belt  8 , it may be possible that the conveyance of the paper P is delayed for some reason (for example, slippage on the surface of the rollers on the way to the first conveyance belt  8 ) and the paper P is placed on top of the openings  80  of opening groups  82 A,  82 C, and  82 F. In this case, the flushing executable timing is identified in the upstream vicinity of the recording heads  17   a  to  17   c  in accordance with the detection of the actual openings  80  and the paper P by the opening detection sensor  23 . Therefore, the opening detection sensor  23  does not detect the openings  80  where the paper P overlaps the openings  80 , and the timing when the recording heads  17   a  to  17   c  face such openings  80  is not identified as the flushing executable timing. This avoids the execution of flushing at the timing when such openings  80  face the recording heads  17   a  to  17   c , and prevents soiling of the paper P overlapping the openings  80  with ink during flushing. 
     In a case where the paper P is placed overlapping the openings  80  to be flushed for some reason as described above, the discharge control portion  110   a  may avoid the flushing for such openings  80 , or may also control to perform the following alternative measures (1) to (3). 
     (1) When the opening detection sensor  23  detects, subsequent to the paper P placed on top of the openings  80 , openings  80  (openings  80  other than the openings  80  located in the pattern corresponding to the size of the paper P) that are not scheduled to perform the flushing, the discharge control portion  110   a  may cause the recording heads  17   a  to  17   c  to execute the flushing for such openings  80  (at the timing when such openings  80  are brought to face the recording heads  17   a  to  17   c ). 
     (2) The discharge control portion  110   a  may cause the recording heads  17   a  to  17   c  to execute the flushing (stir flushing) for the paper P that overlaps the openings  80  (at the timing when the paper P faces the recording heads  17   a  to  17   c ). In this case, it is desirable to discharge the ink in a dispersed manner to the paper P so as not to affect the quality of the recorded image on the paper P. 
     (3) The paper supply control portion  110   b  may control the registration roller pair  13  to delay the conveyance of the next paper P to be supplied, so that the openings  80  appear between the paper P that is placed on top of the openings  80  and the next paper P to be supplied. Then, the discharge control portion  110   a  may cause the recording heads  17   a  to  17   c  to execute the flushing for such openings  80  located between the two sheets of paper P (at the timing when the openings  80  face the recording heads  17   a  to  17   c ). 
     When the flushing is performed for the paper P, the main control portion  110   c , for example, displays an error on the operation panel  27  to notify to the outside that such flushing has been performed. It may also be possible that, depending on the frequency that the paper P overlaps the openings  80  to be flushed, the paper supply control portion  110   b  may control the registration roller pair  13  to control the supply timing of the paper P and correct the placement position of the paper P on the first conveyance belt  8 . In addition, the main control portion  110   c  may cause the operation panel  27  to display to urge cleaning of the first conveyance belt  8  according to the frequency, so as to reduce the possibility that the back side of the paper P (side that is in contact with the first conveyance belt  8 ) that overlaps the openings  80  is soiled with ink that is adhered around the openings  80 . 
     In the above, the first conveyance belt  8  may be provided using the belt having the plurality of opening groups  82  which are irregularly positioned in the paper conveyance direction (the interval between the opening group  82  in the paper conveyance direction changes). Alternatively, a belt having the opening groups  82  or the openings  80  located at equally-spaced intervals in the paper conveyance direction and at intervals equal to or smaller than the length of the smallest size paper P in the paper conveyance direction. When using such a first conveyance belt  8 , for example, the paper supply control portion  110   b  can control the registration roller pair  13  such that the plurality of sheets of paper P are placed on the first conveyance belt  8  at constant intervals and supply the sheets of paper P sequentially to the first conveyance belt  8  regardless of the positions of the openings  80  of the first conveyance belt  8 . In this case, the discharge control portion  110   a  can also use the same method as above, that is, in accordance with the detection results of the openings  80  and the paper P by the opening detection sensor  23 , identifying the flushing executable timing and causing the recording heads  17   a  to  17   c  to execute the flushing, thus obtaining the same effect as above. 
     If the plurality of flushing executable timings are identified, the discharge control portion  110   a  may cause the recording heads  17   a  to  17   c  to execute flushing at all of the identified flushing executable timings, or may cause the recording heads  17   a  to  17   c  to execute the flushing at some of the identified flushing executable timings. For example, the discharge control portion  110   a  may select the timing to actually discharge ink from the recording heads  17   a  to  17   c  from the plurality of flushing executable timings depending on the usage state of each nozzle of the recording heads  17   a  to  17   c , and cause the recording heads  17   a  to  17   c  to execute flushing at the selected timing. In this case, the flushing can be executed by the recording heads  17   a  to  17   c  at the minimum necessary timings to obtain the effect of preventing nozzle clogging by flushing. The usage state of each nozzle may be determined by the discharge control portion  110   a  in accordance with, for example, the image data used for printing (image data of the image recorded on the paper P). Therefore, by executing flushing according to the usage state of each nozzle, unnecessary flushing can be avoided and the increase in ink consumption due to unnecessary flushing can be avoided. 
     7. Identifying Flushing Executable Timing (Part 2) 
     As illustrated in  FIG.  5   , the first conveyance belt  8  of the present embodiment has the plurality of openings  80  arranged in the staggered manner in one opening group  82 . More specifically, the first conveyance belt  8  has the opening groups  82  at the plurality of locations in the paper conveyance direction. Each opening group  82  includes the plurality of opening rows  81  each including the openings  80  arranged in the belt width direction perpendicular to the paper conveyance direction. The openings  80  in each opening row  81  of each opening group  82  are shifted from the openings  80  of the adjacent opening row  81  in the belt width direction in a manner that the openings of the opening row partially overlap the openings of the adjacent opening row when viewed in the conveyance direction. 
     The opening detection sensor  23  used in the present embodiment is sized to detect one opening  80  (see  FIG.  5   ). In this case, the opening detection sensor  23  can be regarded as detecting a particular opening  80  of a particular opening row  81  (for example, the opening row  81   a ) of each opening group  82 . As described above, the storage  28  stores the position information of all openings  80  of the first conveyance belt  8  in advance. 
     In the structure in which the printer  100  includes such a first conveyance belt  8  and an opening detection sensor  23 , the discharge control portion  110   a  may perform the following control process. Specifically, in accordance with the position information of each opening  80  set in advance, the detection result of the opening  80  of the particular opening row  81   a  detected by the opening detection sensor  23 , and the detection result of the paper P detected by the opening detection sensor  23 , the discharge control portion  110   a  may determine whether there are the openings  80  that overlap with the paper P in the other opening row  81   b  of the opening group  82  to which the particular opening row  81   a  belongs and, in accordance with the determination result, identify the flushing executable timing for the openings  80  of the other opening row  81   b.    
     For example, when it is determined that no paper P overlaps the other opening row  81   b , the discharge control portion  110   a  may identify the ink discharge timing for the openings  80  of the other opening row  81   b  as the flushing executable timing. When it is determined that the paper P completely overlaps the openings  80  of the other opening row  81   b , the discharge control portion  110   a  may exclude the ink discharge timing for the openings  80  of the other opening row  81   b  from the flushing executable timing. When it is determined that the paper P overlaps part of the openings  80  of the other opening row  81   b , the discharge control portion  110   a  may identify the ink discharge timing for the remaining area of the opening row  81   b  (area that is not overlapped by the paper P) as the flushing executable timing. 
       FIG.  19    is a timing chart illustrating the output signals of the various sensors and the relationship between the positions of the openings  80  of two opening rows  81   a  (first opening row) and  81   b  (second opening row), both of which belong to the opening group  82 , and the flushing executable timing. In the example illustrated in the chart, of the opening row  81   b  of the opening group  82 , the discharge control portion  110   a  identifies the ink discharge timing for the openings  80  of the opening row  81   b  of the opening group  82  that are not overlapped by the paper P (or the paper P is shifted in the conveyance direction) as the flushing executable timing (see tm3, tm4). When the paper P overlaps part of the openings  80  of the opening row  81   b , the discharge control portion  110   a  identifies the ink discharge timing to discharge ink for the remaining area other than the part of the openings  80  as the flushing executable timing (see tm1, tm2). 
     In the first conveyance belt  8  in which the openings  80  are arranged in the staggered manner and the opening detection sensor  23  detects the openings  80  of the particular opening row  81   a , even when the opening detection sensor  23  does not detect the openings  80  of the other opening row  81   b , the discharge control portion  110   a  can determine whether the paper P is placed on top of the openings  80  of the other opening row  81   b , in accordance with the position information of each opening  80 , the detection result of the openings  80  in the particular opening row  81   a , and the detection result of the paper P. Therefore, by setting the flushing executable timing in accordance with the determination result, the discharge control portion  110   a  can execute the flushing appropriately for the openings  80  of the other opening row  81   b  even though the opening detection sensor  23  does not detect the openings  80 . 
     When it is determined that the paper P is shifted from the openings  80  of the other opening row  81   b  as described above, the discharge control portion  110   a  identifies the ink discharge timing for the openings  80  as the flushing executable timing. In this case, the discharge control portion  110   a  can perform the flushing for the openings  80  of the opening row  81   b  by causing the recording heads  17   a  to  17   c  to discharge ink at the identified flushing executable timing. 
     When it is determined that the paper P overlaps part of the openings  80  of the other opening row  81   b , the discharge control portion  110   a  identifies the ink discharge timing for the remaining area of the openings  80  as the flushing executable timing. In this case, the discharge control portion  110   a  can perform the flushing for the area of the openings  80  of the opening row  81   b  where the paper P does not overlap by causing the recording heads  17   a  to  17   c  to discharge ink at the identified flushing executable timing. The discharge control portion  110   a  can determine whether the flushing is actually performed at the identified flushing executable timing. 
     When it is determined that there are the openings  80  of the opening row  81   b  that overlap with the paper P, it is desirable that the discharge control portion  110   a  stops the ink discharge from the recording heads  17   a  to  17   c  for the openings  80 . In other words, it is desirable that the discharge control portion  110   a  excludes such an ink discharge timing for the openings  80  from the flushing executable timing. In this case, the ink is not discharged onto the paper P placed on top of the openings  80  during flushing according to the flushing executable timing, so that the possibility of soiling the paper P with ink can be avoided. 
     When a length of the remaining area of the openings  80  (area that does not overlap with the paper P) in the paper conveyance direction is equal to or smaller than a predetermined value (for example, several millimeters) due to the paper P overlapping part of the openings  80 , the execution of flushing for the remaining area of the openings  80  may cause soiling of the paper P by the ink proceeding to the area other than the remaining area of the openings  80  and landing on and soiling the paper P due to variations of an ink discharge angle of the ink discharged during flushing. 
     Therefore, it is desirable that the discharge control portion  110   a  stops the ink discharge toward the openings  80 . In other words, when it is determined that the paper P overlaps part of the openings  80  of the opening row  81   b  and the length of the remaining area of the openings  80  in the paper conveyance direction is equal to or smaller than the predetermined value, the discharge control portion  110   a  desirably stops the ink discharge by the recording heads  17   a  to  17   c  for the openings  80 . This avoids the possibility of soiling the paper P due to the variations in the ink discharge angle when executing the flushing for the openings  80  (especially for the remaining area that does not overlap the paper P). 
     Even when the paper P is separated from the openings  80  of each opening group  82  (opening row  81   a , opening row  81   b ) on the first conveyance belt  8 , if the conveyance of the paper P is delayed, for example, for some reason and the separation distance is equal to or smaller than the predetermined distance (for example, several millimeters), if flushing is performed on the above-mentioned openings  80 , a risk of causing soiling the paper P with the ink proceeding in the direction other than the direction of the openings  80  and landing on and soiling the nearby paper P due to variations in the discharge angle of the ink discharged during the flushing. 
     Therefore, it is desirable that the discharge control portion  110   a  calculates the separation distance between the openings  80  and the paper P in the paper conveyance direction in accordance with the detection results of the openings  80  and the paper P by the opening detection sensor  23 , and stops the ink discharge from the recording heads  17   a  to  17   c  for the openings  80  when the separation distance is equal to or smaller than the predetermined distance. This avoids the possibility of soiling the paper P that is placed near the openings  80  due to the variations of the ink discharge angle during the flushing for the openings  80 . 
     In the above description, one opening detection sensor  23  is used to detect one opening  80  in the particular opening row  81   a . In addition, another opening detection sensor that detects the openings  80  of the other opening row  81   b  may also be provided. Then, such an opening detection sensor detects the paper P, and the discharge control portion  110   a  may identify the flushing executable timing for the openings  80  of the other opening row  81   b  in accordance with the detection results of the openings  80  of the other opening row  81   b  and the paper P by the opening detection sensor. 
     As described above, the printer  100  that functions as the inkjet recording device according to the present embodiment includes the opening detection sensor  23  that detects both regularly reflected light and diffusely reflected light. In this case, as described above, the discharge control portion  110   a  can recognize the first conveyance belt  8 , the openings  80 , and the paper P separately in accordance with the output signal from the opening detection sensor  23 . Therefore, in the factory settings of the printer  100 , the flushing timing adjustment method described above can be carried out in accordance with the output results of the opening detection sensor  23 . After shipment from the factory, the flushing control described above can be performed by the discharge control portion  110   a  in accordance with the output results of the opening detection sensor  23 . In other words, the opening detection sensor  23  used for the flushing timing adjustment in the factory settings can also be effectively used to control the flushing after shipment from the factory. 
     8. Others 
     In the above, the case where the paper P is suctioned onto and conveyed on the first conveyance belt  8  by the negative-pressure suction has been described. Alternatively, the first conveyance belt  8  may electrically be charged, and the paper P is electrostatically suctioned onto and conveyed on the first conveyance belt  8  (electrostatic suctioning). 
     In the above, the case where the color printer that records color images using four-color inks has been described as an example of the inkjet recording device has been described. Alternatively, the flushing timing adjustment method and the flushing control are applicable to the case where a monochrome printer that records monochrome images with black ink is used. 
     INDUSTRIAL APPLICABILITY 
     The invention is applicable to inkjet printers and other inkjet recording devices. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               8  First conveyance belt 
               13  Registration roller pair (recording medium supply portion) 
               17   a  to  17   c  Recording head 
               18  Ink discharge port (nozzle) 
               22  Paper detection sensor (recording medium detection sensor) 
               23  Opening detection sensor 
               27  Operation panel (input accepting portion) 
               29  Communication portion (input accepting portion) 
               30  Input accepting portion 
               80  Opening 
               81 ,  81   a ,  81   b  Opening row 
               82 ,  82 A to  82 F Opening group 
               100  Printer (inkjet recording device) 
               110   a  Discharge control portion 
             C 1  First chart 
             C 2  Second chart 
             Cr 1  First reference line 
             Cr 2  Second reference line 
             Cs 1  First auxiliary line 
             Cs 2  Second auxiliary line 
             P Paper (recording medium)