Patent Publication Number: US-8523346-B2

Title: Transport device and recording device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2008-288930, filed Nov. 11, 2008, the entire subject matter and disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The features herein relate to a transport device and a recording device. 
     2. Description of the Related Art 
     A known inkjet recording device applies an AC bias voltage to a conveyor belt for conveying a recording sheet as a recording medium to alternately apply positive charges and negative charges to an insulating layer of the conveyor belt along the moving direction of the conveyor belt. By alternately applying positive charges and negative charges to the conveyor belt in this way, a small AC field is generated near the surface of the conveyor belt. Thus, the recording sheet is attracted to the conveyor belt as a supporting member due to an electrostatic force thereof (Coulomb force), making it possible to prevent the recording sheet from separating from the conveyor belt. 
     SUMMARY OF THE INVENTION 
     In the cases of charging only a conveyor belt, however, the following problems occur. Firstly, charges in the conveyor belt cause dielectric polarization in the recording sheet. In addition, charges having an opposite polarity to the charges in the conveyor belt are induced on the surface of the recording sheet while neutralizing the field generated at the conveyor belt. It takes a certain amount of time to attract a recording sheet with a large attraction force by means of the induced charges. Thus, a force of attraction between the recording sheet and the conveyor belt is insufficient during this time. In addition, ink might adhere to the outer side of the conveyor belt in the long-term use of an inkjet recording device in some cases. Since the ink contains carbon black, an organic pigment, and other such conductive materials, the adhered ink makes the outer side conductive more and more. As described above, if a conductive member (an ink layer adhering to the outer side of the conveyor belt, for example) is positioned between the outer peripheral surface of the conveyor belt and the recording sheet, a field applied by the conveyor belt to cause dielectric polarization on a recording sheet is shielded, leading to an insufficient attraction force. 
     It may be an object of the present invention to provide a transport device and a recording device, which quickly attract a medium to a supporting member for supporting the medium and prevent reduction in attraction force even if a conductive member is positioned between the medium and the supporting member. 
     According to one embodiment herein, a transport device may comprise a transport mechanism comprising a supporting member configured to support a placed medium and transport the medium along a transport path along with movement of the supporting member. The transport device may also comprise a first charging device configured to alternately charge the supporting member with one polarity and an opposite polarity along the transport path to form a plurality of charged segments in the supporting member. The transport device may further comprise a transfer mechanism configured to transfer the medium to the supporting member to place the medium on the supporting member. The transport device may yet further comprise a second charging device configured to alternately charge the medium with one polarity and an opposite polarity along the transfer path for transferring the medium with the transfer mechanism before the transfer mechanism places the medium on the supporting member to form a plurality of charged segments in the medium. The transport device may yet further comprise a control unit configured to control at least one of the transport mechanism, the transfer mechanism, the first charging device, and the second charging device such that a charging pattern of the placed medium and a charging pattern of the supporting member have a predetermined relationship on the supporting member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an inner portion of an inkjet printer including a transport device according to an embodiment. 
         FIG. 2  is a block diagram of the inkjet printer shown in  FIG. 1 . 
         FIG. 3A  is an enlarged view of a transport roller pair and a sheet, and  FIG. 3B  is an enlarged view of a belt charging roller and a conveyor belt. 
         FIGS. 4A to 4C  are schematic diagrams of a relationship between a charged segment formed in a sheet and a charged segment formed in a conveyor belt. 
         FIG. 5  is a flowchart of a series of printing operations of the inkjet printer shown in  FIG. 1 . 
         FIG. 6  is a flowchart of detailed processing in match rate determination step. 
         FIG. 7  is a flowchart of detailed processing in charged voltage determination step according to a modification. 
         FIG. 8  is a timing chart of a series of printing operations shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Various embodiments, and their features and advantages, may be understood by referring to  FIGS. 1-8 , like numerals being used for corresponding parts in the various drawings. 
     Referring to  FIG. 1 , an inkjet printer  1  as a printing apparatus includes a rectangular solid casing  1   a . A discharge recess  6  is formed on the upper side of the casing  1   a . The casing  1   a  includes a control unit  101  for controlling an operation of the printer  1  and a plurality of, e.g., four, inkjet heads  2  for ejecting magenta ink, cyan ink, yellow ink, and black ink. 
     Referring to  FIG. 2 , the control unit  101  is connected to a personal computer (PC)  100 . Each head  2  is driven by a head drive circuit  121 . The head  2  is grounded through the casing  1   a . The lower surface of each head  2  is configured as an ejection surface  2   a  where a plurality of ejection ports for ejecting ink are formed. A transport unit  40  for transporting a sheet P as a printing medium in a transport direction extending from the left to the right in  FIG. 1  is positioned below the plurality of heads  2 . In addition, a feeding cassette  24  capable of storing a plurality of stacked sheets P is positioned below the transport unit  40 . 
     The casing  1   a  includes members such as a pickup roller  25 , the transport unit  40 , transport guides  26  to  28 ,  31 ,  32 , and  71  to  74 , and transport roller pairs  21 ,  22 ,  34 ,  35 , and  75  to  79  to thereby form a transfer path  19 , a transport path  20 , and a retransport path  70 . The sheet P passes through the feeding cassette  24 , the transfer path  19 , and the transport path  20  in this order and reaches the discharge recess  6 . The retransport path  70  retransports the sheet P conveyed with the transport unit  40  such that an opposite surface to the surface that came into contact with ejection surfaces  2   a  of the plurality of heads  2  upon previous transport comes into contact therewith. The retransport path  70  branches off from the transport path  20  on the downstream side of the plurality of heads  2  (more specifically, the transport guide  31 ), and merges into the transfer path  19  in a position between the outlet of the feeding cassette  24  and the plurality of heads  2  (more specifically, the transport guide  26 ). 
     The sheet P in the feeding cassette  24  is transferred with a transfer unit  30  to the transport unit  40 . The transfer unit  30  includes the pickup roller  25 , the plurality of, e.g., three, transport guides  26 ,  27 , and  28 , and the plurality of, e.g., two, transport pairs  21  and  22 . The pickup roller  25  successively picks up and feeds the uppermost one of the plurality of sheets P stacked in the feeding cassette  24 . The pickup roller  25  is rotated by a pickup motor  132 . The pickup motor  132  is driven by a motor driver  122 . Along with the rotation of the pickup roller  25 , the sheet P fed from the feeding cassette  24  is guided by the plurality of transport guides  26 ,  27 , and  28  that define the transfer path  19  while being transported to the transport unit  40  by the plurality of transport roller pairs  21  and  22 . One roller of the transport roller pair  21  and one roller of the transport roller pair  22  are drive rollers rotating by a driving force of a transfer motor  133  controlled by the control unit  101 . The other roller of the transport roller pair  21  and the other roller of the transport roller pair  22  are driven rollers rotated along with rotation of the one roller of the transport roller pair  21  and one roller of the transport roller pair  22 . The transfer motor  133  is driven by a motor driver  123 . In this embodiment, the transfer unit  30 , the pickup motor  132 , the transfer motor  133 , and the motor drivers  122  and  123  may configure the transfer mechanism. 
     Referring to  FIG. 3A , the transport roller pair  22  includes a plurality of, e.g., two, sheet charging rollers  22   a  and  22   b . The surface of at least one of the plurality of sheet charging rollers  22   a  and  22   b  has an insulating property, and the sheet charting roller is connected to the control unit  101  for determining a charging voltage through a power supply  141 . The sheet charging roller  22   a  includes a metal rotational shaft and a surface layer made of a resin elastic member. The rotational shaft is connected to the power supply  141 . The surface layer is a high-resistance member having substantially the same volume resistivity as the conveyor belt  43 . The rotational shaft is biased to one charging roller, the sheet charging roller  22   b  by a spring  22   c  as a biasing member. The sheet charging roller  22   b  functions as an opposing electrode for the sheet charging roller  22   a  and is grounded. The biasing member including the spring  22   c , the plurality of sheet charging rollers  22   a  and  22   b , and the power supply  141  may function as a sheet charging device. The power supply  141  generates AC potential having a waveform in which +500 V and −500 V repeatedly appear at short intervals (8×10 −3  seconds). The sheet charging roller  22   a  alternately charge the sheet P with a positive polarity and a negative polarity along the transfer path  19  while in come into one surface of the sheet P (opposite surface of the surface in contact with the conveyor belt  43  as described below). The sheet charging roller  22   a  forms a plurality of belt-like charged segments on the sheet P. The segments extend in a direction orthogonal to the transfer direction of the sheet P. A charging potential on the surface of the sheet P is ±400 V. The length of one charged segment in the transfer direction is about 5 mm to 30 mm. This length is much shorter than the length of the sheet P along the transfer path. 
     The transport unit  40  includes a plurality of, e.g., two, parallel belt rollers  41  and  42 , and the endless conveyor belt  43  as a supporting member stretched over the plurality of rollers  41  and  42 . The belt roller  42  is rotated by a belt drive motor  134 . The outer surface of the conveyor belt  43  is made of a polyvinylidene difluoride (PVDF) layer having the thickness of 150 μm, and the volume resistivity of about 10 13  Ω·cm to 10 15  Ω·cm. The transport unit  40  may be configured simply using the plurality of belt rollers  41  and  42  and the conveyor belt  43  as above. The outer surface of the conveyor belt  43  may be made of a high-resistance material such as PET, ETFE, PTFE, or polyimide in addition to PVDF. Further, the conveyor belt  43  may have a two-layer structure. In this case, the outer surface (outer layer) of the belt is preferably given higher resistance than the inner surface (inner layer) to reduce a speed of neutralization of charges between the sheet P and the conveyor belt  43 . For example, a base material, PVDF is used for the outer layer, and carbon-contained PVDF is used for the inner layer to adjust a resistance value. Thus, the resistance of the inner layer is set lower than the outer layer. PET, ETFE, PTFE, and polyimide are preferred as the base material in addition to PVDF. The conveyor belt  43  is a belt made of PVDF as a base material and has one-layer structure. The belt drive motor  134  is driven by the motor driver  124 . The transport unit  40 , the belt drive motor  134 , and the motor driver  124  may configure the transport mechanism. 
     The sheet P transferred with the transfer unit  30  is positioned on the conveyor belt  43  as the supporting member. The transport unit  40  conveys the sheet P in the transport direction such that one surface of the sheet P comes into contact with the plurality of, e.g., four, heads  2  at a predetermined timing along with the driving operation of the belt drive motor  134 . When the sheet P passes below each head  2 , the heads successively eject ink of corresponding color to the sheet P to thereby form a desired color image on the sheet P. 
     A given portion of the conveyor belt  43  on the outer peripheral surface of the belt roller  41  is held between the belt roller  41  and the belt charging roller  136 . The belt charging roller  136  includes a metal-made shaft center, and an elastic layer made of urethane rubber or the like and surrounding the shaft center. On the surface thereof, a protective layer made of nylon or resin fluoride having a high mechanical strength is formed. The protective layer formed on the surface of the elastic layer is an intermediate-resistance (10 6  to 10 10  Ωcm) layer. Further, the shaft center is connected to the control unit  101  through the power supply  142  for determining a charging voltage. The configuration of the belt charging roller  136  is similar to that of the sheet charging roller  22   a . The belt charging roller  136  includes a metal rotational shaft and a resin elastic surface layer. The rotational shaft is connected to the power supply  142 . The belt charging roller  136  is biased to the opposing belt roller  41  by a spring  136   a  as a biasing member. The belt roller  41  functions as a conductive opposing electrode for the belt charging roller  136 , and is grounded through the casing  1   a . The biasing member including the spring  136   a , the belt charging roller  136 , and the power supply  142  inclusive of the belt roller  41  of the transport mechanism function as a belt charging device. The power supply  142  generates AC potential having a waveform in which +1000 V and −1000 V repeatedly appear at short intervals (8×10−3 seconds). Thus, the belt charging roller  136  can alternately charge the conveyor belt  43  with positive and negative polarities along the transport path  20  while in contact with the outer surface of the conveyor belt  43  as shown in  FIG. 3B . The belt charging roller  136  forms a plurality of belt-like charged segments in the conveyor belt  43 . The segments extend in a direction orthogonal to the transport direction of the sheet P. The charging potential at the outer surface of the conveyor belt  43  is ±500 V. In other words, a charging amount of one charged segment of the sheet P is smaller than that of one charged segment of the conveyor belt  43 . The length of one charged segment in the transport direction is about 5 mm to 30 mm like the charged segment formed in the sheet P. This length is much shorter than the length of the sheet P along the transport path. To given an example thereof, if the length of one charged segment is 20 mm in the printer  1  that ejects ink droplets at 20 kHz to realize a dot density of 600 dpi on the sheet P, the ½ period (on time) of the power supply  142  is about 23 ms. 
     Referring to  FIGS. 4A to 4C , a relationship between the charged segment formed in the sheet P and the charged segment formed in the conveyor belt  43  is described. The length of the charged segment along the transport path is L 0  in both of the sheet P and the conveyor belt  43 . The control unit  101  controls the motor driver  122  for the pickup motor  132  and the motor driver  123  for the transfer motor  133  such that a charging pattern of the sheet P and a charging pattern of the conveyor belt  43  have a predetermined relationship (positional relationship) on the conveyor belt  43 . To elaborate, the control unit  101  controls the motor driver  122  for the pickup motor  132  and the motor driver  123  for the transfer motor  133  such that a match rate indicating a rate of a charged segment Rp of an opposite polarity formed in the sheet P and overlapped with the conveyor belt  43  to a charged segment Rb formed in the conveyor belt  43  is set to a predetermined value as well as the same applies to every charged segment Rb formed in the conveyor belt  43 . This control is made based on a belt detection signal output from a belt sensor  137  as described below. 
     The control unit  101  controls the motor driver  122  for the pickup motor  132  and the motor driver  123  for the transfer motor  133  such that the match rate is variable. The match rate takes a plurality of, e.g., three, values as described above, i.e., 0.9 (L 1  shown in  FIG. 4A ), 1.0 (L 2  shown in  FIG. 4B ), and 0.8 (L 3  shown in  FIG. 4C ). The control unit  101  determines the match rate based on the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to form an image on the front side or back side upon two-sided printing. If the match rate is 1.0, the charged segment Rp of an opposite polarity formed in the sheet P opposes the charged segment Rb formed in the conveyor belt  43 , on the conveyor belt  43  with no displacement. 
     A flat platen  44  made of an insulating resin is disposed in a region surrounded by the conveyor belt  43  and opposing the plurality of, e.g., four, heads  2 . The flat platen  44  secures flatness of the sheet P and a predetermined distance between the ejection surface  2   a  and the conveyor belt  43  upon printing. Since the flat platen  44  has an insulating property, the charged conveyor belt  43  is prevented from being neutralized by brining into contact with the flat platen  44 . Here, it is preferred to reduce friction force between the flat platen  44  and the inner side of the conveyor belt  43 . Owing to the friction force between the flat platen and the inner side and the charged conveyor belt  43 , the conveyor belt  43  is attracted to the flat platen  44  with small attraction force. Thus, the conveyor belt  43  is prevented from floating from the flat platen  44 . 
     In an inner space of the conveyor belt  43 , a neutralization plate  45  having an insulating surface is disposed between the flat platen  44  and the belt roller  42 . An upper surface of the neutralization plate  45  is at the same height as the upper surface of the flat platen  44 . The neutralization plate  45  is connected to the control unit  101  through the power supply  143 . The neutralization plate  45  and the power supply  143  function as the neutralization device for the conveyor belt  43 . The power supply  143  for determining a neutralization voltage generates AC potential having a waveform in which +500 V and −500 V appear at short intervals (8×10 −3  seconds). A phase of the AC potential generated with the power supply  143  is opposite to a phase of the charged segment in the conveyor belt  43  on the neutralization plate  45 . Hence, the neutralization plate  45  comes into contact with the inner side of the conveyor belt  43  (an opposite surface to the supported surface of the sheet P on the conveyor belt  43 ) to neutralize the conveyor belt  43 . Thus, the neutralization plate  45  does not hinder the transfer of the sheet P. Here, the length of the neutralization plate  45  in the transfer direction is set not longer than the length of one charged segment in the conveyor belt  43 . In this example, the length is about ¼ of the length of one charged segment. 
     A sheet edge sensor  51  is disposed between the transport roller pair  22  and the inkjet head  2  at the most upstream position. The sheet edge sensor  51  is, for example, an optical reflective or transmission sensor. The sheet edge sensor  51  outputs a sheet edge detection signal indicating that the front edge of the sheet P placed on the conveyor belt  43  reaches a position below the sheet edge sensor  51 . The sheet edge detection signal is supplied to the control unit  101 . 
     The belt sensor  137  as an operation sensor for the conveyor belt  43  is positioned below the conveyor belt  43 . The belt sensor  137  is, for example, an optical reflective or transmission sensor. The belt sensor  137  outputs a belt detection signal indicating that a mark (not shown) formed on the outer peripheral surface of the conveyor belt  43  reaches a position opposite to the belt sensor  137 . The belt detection signal is supplied to the control unit  101 . The mark formed on the conveyor belt  43  is away from a region in which the sheet is placed on the outer surface of the conveyor belt  43 , by a predetermined distance. A plurality of sheet placement regions are formed on the conveyor belt  43  with reference to the mark position. The sheet placement region has the same length as the sheet P along the transport path. 
     The belt detection signal output from the belt sensor  137  is used to control the motor driver  122  for the pickup motor  132  and the motor driver  123  for the transfer motor  133  such that the sheet P conveyed with the transfer unit  30  is placed in the sheet placement region of the conveyor belt  43 . Moreover, the belt detection signal is used to control the power supplies  141  and  142  such that the power supply  142  for the belt charging roller  136  operates only in a period for charging the sheet placement region, and the power supply  141  for the plurality of, e.g., two, sheet charging rollers  22   a  and  22   b  operates only in a period for charging the sheet P. Here, instead of disposing the belt sensor  137 , an encoder as the operation sensor may be connected to the belt roller  41  or  42  to detect an operation of the conveyor belt  43 . 
     A separation plate  5  is positioned on the downstream side of the transport direction of the transport unit  40 . The separation plate  5  separates the sheet P from the conveyor belt  43  by inserting its front edge in between the sheet P and the conveyor belt  43 . 
     The sheet P separated from the conveyor belt  43  by the separation plate  5  is guided by the transport guides  31  and  32  while being transferred to the upstream side by the transport roller pairs  34  and  35 , and then discharged to the discharge recess  6 . One roller of the transport roller pair  34  and one roller of the transport roller pair  35  are drive rollers rotating by a driving force of the transfer motor  133 . The other roller of the transport roller pair  34  and the other roller of the transport roller pair  35  are driven rollers rotated along with the rotation of the one roller of the transport roller pair  34  and the one roller of the transport roller pair  35 . 
     The drive rollers of the transport roller pairs  34  and  35  can rotate in forward and reverse directions. Accordingly, after the rear end of the sheet P conveyed by the transport unit  40  passed a position where the path branches to the retransport path  70 , the transport roller pairs  34  and  35  are rotated in the reverse direction, making it possible to switch back the sheet P. The switched-back sheet P is conveyed to the retransport path  70 . 
     The retransport path  70  includes a plurality of, e.g., four, transport guides  71 ,  72 ,  73 , and  74  and a plurality of, e.g., five, transport roller pairs  75 ,  76 ,  77 ,  78 , and  79 . In the retransport path  70 , the sheet P is guided by the transport guides  71 ,  72 ,  73 , and  74  while being transferred by the transport roller pairs  75 ,  76 ,  77 ,  78 , and  79 . Rollers on one side of the transport roller pairs  75 ,  76 ,  77 ,  78 , and  79  are drive rollers rotating by a driving force of a retransport motor  135 . Rollers on the other side of the transport roller pairs  75 ,  76 ,  77 ,  78 , and  79  are driven rollers rotated along with the rotation of the one rollers. The retransport motor  135  is driven by a motor driver  125 . 
     The sheet P passed through the retransport path  70  and merged into the transfer path  19  at the transport guide  26  is conveyed to the transport unit  40  while a surface opposite to the surface that came into contact with the ejection surfaces  2   a  of the plurality of heads  2  upon previous transport comes into contact therewith. Then, the sheet P passes a portion below the plurality of heads  2 . At this time, the plurality of heads  2  successively eject ink to thereby form a color image on the rear side of the sheet P as well. 
     The humidity sensor  138  is positioned inside the casing  1   a . A humidity signal output from the humidity sensor  138  is supplied to the control unit  101 , and used to adjust the match rate in accordance with the humidity to control a timing for generating AC potential with the power supply  141 . 
     The control unit  101  includes a CPU, an EEPROM for storing programs to be executed by the CPU and data used for the programs in a data rewritable manner, and a RAM for temporarily storing data upon execution of a program. The control unit  101  functions as a head control unit  102 , a transport control unit  103 , a belt charging control unit  105 , a sheet charging control unit  106 , a neutralization control unit  107 , a printed data storage unit  111 , a sheet thickness storage unit  112 , a front/rear side specifying unit  113 , and a match rate determination table storage unit  114  by executing these hardware components and software components in the EEPROM in cooperation with each other. 
     The printed data storage unit  111  stores printed data regarding an image to be formed on the sheet P, which is transmitted from the PC  100 . The head control unit  102  controls the head drive circuit  121  to eject ink from each inkjet head  2  at a desired timing based on the printed data stored in the printed data storage unit  111 . 
     The transport control unit  103  controls the motor drivers  122  to  125  to transport the sheet P along the transfer path  19 , the transport path  20 , and the retransport path  70  at a desired timing. The transport control unit  103  controls the motor driver  122  for the pickup motor  132  and the motor driver  123  for the transfer motor  133  so as to set the match rate to one of 0.8, 0.9, and 1.0 based on the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to print an image on the front side or the back side, and so as to place the sheet P on the conveyor belt  43  based on the determined match rate. 
     The belt charging control unit  105  controls an operation timing of the power supply  142  connected to the belt charging roller  136  based on a belt detection signal output from the belt sensor  137 . The sheet charging control unit  106  controls an operation timing of the power supply  141  connected to the sheet charging roller  22   a ( 22   b ) based on a belt detection signal output from the belt sensor  137 . 
     The neutralization control unit  107  controls an operation timing of the power supply  143  for the neutralization plate  45  based on a sheet edge detection signal output from the sheet edge sensor  51 . To elaborate, the power supply  143  is operated only for a predetermined period from the time when the sheet edge detection signal was output. In this predetermined period, the neutralization plate  45  opposes the plurality of charged segments formed in the conveyor belt  43 . 
     The sheet thickness storage unit  112  stores one or more identification numbers for identifying the sheet P in association with the thickness of each sheet. The data stored in the sheet thickness storage unit  112  can be rewritten based on data input by a user. Moreover, the sheet thickness storage unit  112  specifies a so-called active identification number (sheet type of the sheet P stored in the feeding cassette  24 ) indicating which of one or more identification numbers correspond to the target sheet P together with the thickness thereof. The specified information can be changed by user operation on buttons. 
     The front/rear side specifying unit  113  stores a front/rear side specifying flag indicating whether to print an image on a surface of the sheet P upon the single-sided printing, a first surface (front surface) upon the two-sided printing, or a second surface (rear surface) upon the two-sided printing in the next operation. The front/rear side specifying flag stored in the front/rear side specifying unit  113  is rewritten each time an operation of printing an image on one side of the sheet P is completed. 
     The match rate determination table storage unit  114  stores a match rate determination table storing a combination of a plurality of consecutive humidity ranges that do not overlap each other, a plurality of consecutive thickness ranges of the sheet P that do not overlap each other, and whether to print an image on the front/rear side upon single-sided printing and two-sided printing in association with three match rates (0.8, 0.9, and 1.0). In other words, a match rate can be uniquely determined in accordance with the internal humidity of the casing  1   a , and whether to print an image on the front/rear side of the sheet P upon single-sided printing and two-sided printing. The higher the match rate, the thicker the sheet P. Further, the higher the internal humidity of the casing  1   a , the higher the match rate. Then, the match rate is higher upon single-sided printing and printing on the first surface than upon printing on the second surface. 
     Here, in the following modified example, in place of the match rate determination table storage unit  114 , a charging voltage determination table storage unit is disposed. The charging voltage determination table storage unit stores a combination of a plurality of consecutive humidity ranges that do not overlap each other, a plurality of consecutive thickness ranges of the sheet P that do not overlap each other, and whether to print an image on the front/rear side upon single-sided printing and two-sided printing in association with a plurality of, e.g., three, levels of charging voltage for each of the power supplies  141  and  142 . In the charging amount determination table, the thicker the sheet P, the higher the charging voltage for each of the power supplies  141  and  142 . In addition, the higher the internal humidity of the casing  1   a  is, the higher the charging voltage is. Further, the charging voltage is lower upon the signal-sided printing and printing on the first surface than upon printing on the second surface. 
     Next, a series of printing operations of the inkjet printer  1  is described with reference to flowcharts in  FIGS. 5 and 6  and a timing chart in  FIG. 8 . 
     First, in step S 1 , the processing waits for reception of a print command including print data from the PC 100 . Upon receiving the print command (S 1 : YES), in step S 2 , print data included in the received print command is stored in the printed data storage unit  111 . 
     In step S 3 , the transport control unit  103  controls the motor driver  124  to drive the belt drive motor  134  to start rotating the belt roller  42 . As a result, the conveyor belt  43  is rotated at a constant speed. 
     In step S 4 , a match rate used for printing with the transport control unit  103  is determined. The match rate is determined in accordance with a procedure shown in  FIG. 6 . First, in step S 101 , a humidity signal output from the humidity sensor  138  is obtained. Then, in step S 102 , a thickness of the sheet P corresponding to an active identification number specified by the sheet thickness storage unit  112  is acquired. Further, in step S 103 , a front/rear side specifying flag stored in the front/rear side specifying unit  113  is acquired. Then, in step S 104 , the transport control unit  103  determines a match rate in accordance with a combination of the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to print an image on the front/rear side upon two-sided printing using the match rate determination table stored in the match rate determination table storage unit  114 . At this time, the charging voltage applied to the sheet P and the charging voltage applied to the conveyor belt  43  are kept constant. 
     A modified example of the processing in step S 4  is described with reference to  FIG. 7 . In this modified example, the match rate is kept constant (for example, match rate=1) and in addition, the charging voltage of the power supplies  141  and  142  is set variable. Then, an appropriate charging voltage is determined in accordance with a combination of the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to print an image on the front/rear side upon single-sided printing and two-sided printing. First, in step S 201 , a humidity signal output from the humidity sensor  138  is acquired. Then, in step S 202 , the thickness of the sheet P corresponding to an active identification number specified by the sheet thickness storage unit  112  is acquired. Moreover, in step S 203 , the front/rear side specifying flag stored in the front/rear side specifying unit  113  is acquired. Then, in step S 204 , the transport control unit  103  determines the charging voltage in accordance with a combination of the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to print an image on the front/rear side upon two-sided printing using the charging voltage determination table stored in the charging voltage determination table storage unit for each of the power supplies  141  and  142 . 
     Subsequently, in step S 5 , the processing waits until the belt sensor  137  detects a mark formed on the conveyor belt  43 . 
     After the detection of the mark, in step S 6 , the processing waits until the time t 1  after the elapse of time T 1  from the time t 0  at which the belt sensor  137  detected the mark. At time t 1 , the front edge of the sheet placement region formed in the conveyor belt  43  reaches the belt charging roller  136 . The time T 1  is determined based on a distance between the sheet placement region and the mark, a distance between the belt charging roller  136  and the belt sensor  137 , and a transport speed of the sheet P transported by the conveyor belt  43 . Then, after the elapse of the time T 1  (S 6 : YES), in step S 7 , the belt charging control unit  105  controls the power supply  142  to start charging the conveyor belt  43  by use of the belt charging roller  136 . Referring to  FIG. 3B , plural charged segments are formed from the front edge of the sheet placement region on the conveyor belt  43 . Further, the printer  1  can be easily controlled using the belt sensor  137 . 
     On the other hand, after the detection of the mark, in step S 8 , the processing waits until time T 2  elapses from the time t 0  at which the belt sensor  137  detected the mark. The time T 2  is a time period necessary for the front edge of the sheet placement region (plural charged segments) reaches the placement position for the sheet P on the conveyor belt  43  (as denoted by P 0  in  FIG. 1 ) from the time t 0 . This time is determined based on a distance between a position of the front edge of the sheet placement region and the placement position and the transport speed at time t 0 . After the elapse of the time T 2  (S 8 : YES), in step S 9 , the transport control unit  103  controls the motor drivers  122  and  123  to start driving the pickup motor  132  and the transfer motor  133 , and the pickup roller  25  and the transport roller pairs  21  and  22  start rotating. Thus, the top sheet P in the feeding cassette  242  is fed and transferred through the transfer path  19 . In parallel therewith, the front edge of the sheet placement region passes the placement position and moves toward the inkjet head  2 . A certain amount of time is further necessary for the sheet P to reach the placement position. Thus, the sheet P is placed in any charged segment without fail. Although the time t 2  is not shown in  FIG. 8 , it is a predetermined value smaller than the time T 3 . 
     Subsequently, the processing waits until time t 2  after the elapse of time T 3  from time t 0  in step S 10 . At time t 2 , the front edge of the sheet P transferred through the transfer path  19  reaches the roller pair  22 . The time T 3  is the sum of the time T 2  and the quotient of the distance between the feeding cassette  24  and the roller pair  22  divided by the transfer speed for the sheet P transferred through the transfer path  19 . Then, after the elapse of the time T 3  (S 10 : YES), in step S 11 , the sheet charging control unit  106  controls the power supply  141  to start charging the sheet P by use of the sheet charging rollers  22   a  and  22   b . Referring to  FIG. 3A , plural charged segments are formed in the sheet P. The processing in steps S 6  and S 7  is performed in parallel to the processing in steps S 8  to S 11 . From the viewpoint of certainly placing the sheet P on the charged segment, the front edge of the charged sheet P only needs to reach the placement position after the front edge of the sheet placement region. To satisfy the condition, the start time or duration of each step is adjusted. 
     After a while from steps S 7  and S 11 , the front edge of the sheet P reaches a position on the conveyor belt  43  (placement position: P 0 ). From then on, the sheet P is successively placed on the conveyor belt  43  from the front edge of the sheet P to the rear edge. A match rate of the plurality of charged segments formed in the sheet P and the plurality of charged segments in the conveyor belt  43  is 0.8 or more, and an attraction force is generated between each charged segment formed in the sheet P and each charged segment formed in the conveyor belt  43 . In this way, both of the sheet P and the conveyor belt  43  are charged in advance, making it possible to quickly attract the sheet P to the conveyor belt  43 . Thus, high-speed transport and high-speed printing are realized using the printer  1 . Further, even if an ink layer (containing a conductive material such as carbon black) is formed on the lower surface of the sheet P and the outer surface of the conveyor belt  43 , an attraction force is rarely reduced. Hence, it is possible to prevent the sheet P from floating from the conveyor belt  43 . Accordingly, a gap between the head  2  and the conveyor belt  43  can be reduced down to, for example, about 0.5 mm. Thus, an accuracy of ejection of ink droplets on the sheet P is improved. 
     In particular, all the charged segments formed on the conveyor belt  43  have the same match rate, and an attraction force applied to the sheet P on the conveyor belt  43  can become uniform. Accordingly, a large attraction force can be obtained with a small charging amount. Further, the charged segments have the same length, which simplifies control. Further, since the match rate is set to 0.8 or more, a large attraction force can be obtained. In particular, if the match rate is 1.0, the sheet P is held on the conveyor belt  43  with a very large attraction force. 
     Further, the match rate is set variable, making it possible to adjust an attraction force. The match rate is determined in accordance with a combination of the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to form an image on one surface upon single-sided printing and on the front/rear side upon two-sided printing in step S 4 . Thus, the sheet P can be transported with an appropriate attraction force. In addition, changing the match rate to adjust an attraction force is advantageous in that the power supply circuit can be simplified compared with the case of setting the charging voltage of the power supplies  141  and  142  illustrated in  FIG. 7  variable. Further, an attraction force can be adjusted also by setting the charging voltage of the power supplies  141  and  142  illustrated in  FIG. 7  variable. In the above modified example, a charging voltage of the power supplies  141  and  142  is determined in accordance with a combination of the internal humidity of the casing  1   a , the thickness of the sheet P, and whether to form an image on the front/rear side upon two-sided printing in step S 4 . Hence, the sheet P can be conveyed with an appropriate attraction force. 
     As described above, a charging amount of one charged segment on the sheet P is smaller than a charging amount of one charged segment on the conveyor belt  43 . Thus, a leak field to the outside of the sheet P (in a direction opposite to the direction from the sheet P to the conveyor belt  43 ) is suppressed. Therefore, occurrences of an electric disturbance outside of the sheet P during the transfer can be suppressed. As a result, a change in fly path of ink droplets due to the leak field can be suppressed. Further, this contributes to reduction of noise on the electric signal. 
     Moreover, in this embodiment, the sheet charging roller  22   a  charges an opposite surface of the sheet P to the surface in contact with the conveyor belt  43 . Therefore, a dry surface is charged all the time even during printing on the second surface. Accordingly, neutralization of charges rarely occurs due to humidity of an ink droplet ejected from the head  2 , and an attraction force is less changed. As a modified example thereof, the sheet charging roller  22   b  may charge the surface of the sheet P in contact with the conveyor belt  43 . At this time, electric bonding between charges in the conveyor belt  43  and charges in the sheet P becomes firm, and occurrences of electric disturbance outside of the sheet P during the transfer can be suppressed considerably. 
     Moreover, after the detection of the mark, in step S 12 , the processing waits until time t 4  after the elapse of time T 4  from time t 0 . At time t 4 , the rear edge of the sheet placement region formed in the conveyor belt  43  reaches the belt charging roller  136 . The time T 4  is the sum of the time T 1  and the quotient of the distance of the sheet placement region along the transport path  20  divided by the transfer speed for the sheet P conveyed on the conveyor belt  43 . Then, after the elapse of the time T 4  (S 12 : YES), in step S 13 , the belt charging control unit  105  controls the power supply  142  to complete charging the conveyor belt  43  with the belt charging roller  136 . 
     On the other hand, in step S 14 , the processing waits until time t 5  after the elapse of time T 5  from time t 0 . At time t 5 , the rear edge of the sheet P transferred through the transfer path  19  reaches the roller pair  22 . The time T 5  is the sum of the time T 3  and the quotient of the distance of the sheet P along the transport path  19  divided by the transfer speed for the sheet P transferred through the transfer path  19 . Then, after the elapse of the time T 5  (S 14 : YES), in step S 15 , the sheet charging control unit  106  controls the power supply  141  to complete charting the sheet P with the sheet charging rollers  22   a  and  22   b . As described above, only for a period in which the sheet charging rollers  22   a  and  22   b  hold the sheet P, the power supply  141  operates and in addition, the power supply  142  operates to charge only a region of the conveyor belt  43  on which the sheet P is to be placed, as a charged segment. This makes it possible to shorten an operation time for the power supplies  141  and  142 . Hence, the service life thereof can be increased, and power consumption thereof can be reduced. 
     Further, the processing waits until the sheet edge sensor  51  outputs a sheet edge detection signal in step S 16 . Then, if the sheet edge detection signal is output at time t 3  (S 16 : YES), after the elapse of a predetermined time from then, in step S 17 , the head control unit  102  controls the head drive circuit  121  to start printing on the sheet P with the head  2 . 
     Subsequently, in step S 18 , the processing waits until time t 6  after the elapse of T 6  from the time t 3 . The time T 6  is the sum of the quotient of the distance between the sheet edge sensor  51  and the neutralization plate  45  divided by the transfer speed for the sheet P conveyed on the conveyor belt  43 . Then, after the elapse of time T 6  (S 18 : YES), in step S 19 , the neutralization control unit  107  controls the power supply  143  to start neutralization of the conveyor belt  43  with the neutralization plate  45 . As a modified example thereof, the time to start and terminate neutralization may be controlled based on the elapsed time from time t 0 . In addition, in step S 19 , the neutralization is started only when the front edge of the sheet P reaches the neutralization plate  45 . At this time, the front edge of the charged sheet placement region first reaches a position on the upstream side of the front edge of the sheet P. However, charges are neutralized until this region is charged again with the belt charging roller  136 . 
     The processing in steps S 12  and S 13 , the processing in steps S 14  and S 15 , and the processing in steps S 16  to S 19  are performed in parallel with each other. The sequence of the time t 3 , the time t 4 , and the time t 5  as shown in  FIG. 8  is determined for illustrative purposes only, and the time t 3  may follow the time t 4  or the time t 5 . From the viewpoint of certainly placing the sheet P on the charged segment until the image formation on the sheet P is completed, the rear edge of the charged sheet P only needs to reach the placement position ahead of the rear end of the sheet placement region. The start time or duration of each step is adjusted to satisfy the above condition. 
     If the rear edge of the sheet P passes the head  2  at the most downstream position along the transport path, in step S 20 , the head control unit  102  completes printing on the sheet P with the head  2 . In step S 21 , the processing waits until time t 7  after the elapse of time T 7  from time t 3 . The time T 7  is the sum of the quotient of the total distance of the distance between the sheet edge sensor  51  and the neutralization plate  45  and the length of the sheet P divided by the transfer speed for the sheet P conveyed on the conveyor belt  43 . Then, after the elapse of the time T 7  (S 21 : YES), in step S 22 , the neutralization control unit  107  completes neutralization of the conveyor belt  43  with the neutralization plate  45 . 
     During a period from step S 19  to step S 22 , the neutralization plate  45  is charged with an opposite phase to the phase of the charged segment in the conveyor belt  43  on the neutralization plate  45 . Thus, the charged segment formed in the conveyor belt  43  during previous charging operation can disappear. Accordingly, a charged segment can be formed in the conveyor belt  43  in the next charging operation without considering a position of the charged segment formed in the conveyor belt  43  in the previous charging operation. In other words, a charged state becomes uniform at any time, and the degree of freedom of position at which a charged segment is formed in the conveyor belt  43  can be increased. Moreover, a potential of the conveyor belt  43  can be kept from gradually increasing. In particular, the neutralization plate  45  is charged with an opposite phase to the phase of the charged segment in the conveyor belt  43 . Thus, the charged segment formed in the conveyor belt  43  can disappear with high reliability. Moreover, the neutralization plate  45  is disposed between the separation position of the sheet P and the plurality of heads  2 , which facilitates separation of the sheet P from the conveyor belt  43 . 
     Subsequently, it is determined whether to print the rear side of a sheet having an image printed on the front side in step S 23 , and steps S 17  to S 20 , that is, whether to print an image on a second surface after printing an image on a first surface of the sheet P (two-sided printing). If printing on the second surface is not performed or an image is printed on the second surface of the sheet in steps S 17  to S 20  (S 23 : NO), the processing advances to step S 24 , and the sheet P is directly discharged to the discharge recess  6  to terminate the printing processing. Since each sheet P is alternately charged with two polarities, the charged segment formed in the sheet P can easily disappear in a short time after the transfer. Accordingly, handling property of the plural transferred sheets P stacked in the discharge recess  6  is rarely reduced. 
     In the case of printing an image on the second surface (S 23 : YES), in step S 25 , the front/rear side specifying flag stored in the front/rear side specifying unit  113  is rewritten to a value corresponding to an instruction to print an image on the second surface upon two-sided printing. Moreover, in step S 26 , the sheet P is switched back. In other words, after the rear edge of the sheet P fed from the transport unit  40  passed a portion where the path branches off to the retransport path  70 , the transport roller pairs  34  and  35  are rotated in the reverse direction to thereby feed the sheet P to the retransport path  70 . The sheet P retransported through the retransport path  70  is stopped with its front edge being in contact with the transport roller pair  21 . Then, the processing returns to step S 4 , and the match rate is determined upon printing on the second surface. 
     After a while, an image is printed on the second surface in accordance with the above procedure. In this case as well, an operation of charging the sheet placement region or the start time of the operation of charging the sheet P are set with reference to the detection time of the mark on the conveyor belt  43  as a starting point. The start time of the transfer of the sheet P with the transport roller  21  is set after the elapse of a predetermined time from when the sheet placement region reaches a placement position. The predetermined time corresponds to a time necessary for the sheet P fed from the feeding cassette  24  reaches the transport roller  21 . With this construction, the sheet P is placed and conveyed on the conveyor belt  43 , subjected to image formation with the inkjet heads  2 , and separated from the conveyor belt  43  similar to the first surface. In the case of printing an image on the second surface of the sheet P upon two-sided printing, if an ink layer formed through printing on the first surface is not dried, a resistance value of the sheet P is reduced, and charges in the sheet P are easily neutralized in a short time. However, by forming the charged segment in the sheet P using the sheet charging rollers  22   a  and  22   b , the sheet P can be attracted to the conveyor belt  43  with a large attraction force. In particular, since the sheet charging roller  22   a  is brought into contact with the second surface on which an ink layer is not formed to thereby form the charged segment, an attraction force is rarely reduced. Further, the match rate is higher upon printing on the second surface than upon printing on the first surface. Therefore, it is possible to suppress reduction in attraction force due to the first surface having an ink layer containing a conductive material formed on the surface, which opposes the conveyor belt  43 , upon printing on the second surface. Also in the modified example shown in  FIG. 7 , a charging voltage of the power supplies  141  and  142  upon printing on the second surface is higher than the charging voltage upon printing on the first surface. This makes it possible to suppress reduction in attraction force upon printing on the second surface. 
     As described above, both of the sheet P and the conveyor belt  43  are charged, and charging patterns of the sheet P and the conveyor belt  4  have a predetermined relationship. Thus, the sheet P can be quickly attracted to the conveyor belt  43  by means of an attraction force applied between each charged segment formed in the sheet P and each charged segment formed in the conveyor belt  43 . Accordingly, a mechanism for pressing the sheet P to the conveyor belt  43  until the sheet P is certainly attracted to the conveyor belt  43  can be omitted. Moreover, it is unnecessary to take a measure for reducing an amount of ink applied to the first surface that would reduce an attraction force generated through charging upon printing on the second surface or a measure for drying ink adhering to the first surface before printing on the second surface. 
     The embodiments of the present invention are described above, but the present invention is not limited to the above embodiments, and various design changes may be performed on the embodiments. For example, in the above embodiments, the transport mechanism transports a medium on a belt, but the transport mechanism may transport a medium with a drum. In the above embodiments, the sheet thickness storage unit is provided as the thickness specifying unit. A sensor for measuring the thickness of the sheet P may be provided instead. 
     Further, in the above embodiment, an operation timing of the pickup motor  132  and the transfer motor  133  is controlled to adjust the match rate of charged segments formed in the sheet P to the charged segments formed in the conveyor belt  43 . However, the match rate may be adjusted by controlling not only the pickup motor  132  and the transfer motor  133  but also at least one of the pickup motor  132 , the transfer motor  133 , the belt drive motor  134 , the power supply  141 , and the power supply  142 . 
     In the above embodiments, the charging voltage increases as the thickness of the sheet P increases for each of the power supplies  141  and  142 , and the charging voltage increases as the internal humidity of the casing  1   a  increases. Moreover, the charging voltage upon printing on the second surface is higher than upon printing on the first surface. However, the charging voltage may increase as the thickness of the sheet P increases for either the power supply  141  or  142 . In addition, the charging voltage may increase as the internal humidity of the casing  1   a  increases for either the power supply  141  or  142 . The charging voltage upon printing on the second surface may be higher than upon printing on the first surface for either the power supply  141  or  142 . Further, a power supply may be provided independently for the sheet charging roller  22   a  and the sheet charging roller  22   b , and the above control may be performed for only one of the two power supplies. 
     In the above embodiments, the power supply  143  is connected to the neutralization plate  45 , but the neutralization plate  45  may be simply grounded. In the above embodiments, the neutralization plate  45  is positioned in the internal space of the conveyor belt  43  but may be positioned outside the conveyor belt  43 . Further, in the above embodiments, the conveyor belt  43  is charged by the belt charging roller  136  in the direction from the outer surface. However, the conveyor belt  43  may be charged in the direction from the inner surface. Moreover, in the above embodiment, the roller pair  22  functions as both of the charging device and the transfer unit  30 , but the charging device may be disposed independently of the transfer unit  30 . 
     Further, the inkjet printer of the above embodiments enables two-sided printing, but the present invention is applicable to an inkjet printer not having retransport path  70 , which enables only single-sided printing. 
     Further, in the above embodiments, the medium conveyed with the conveyor belt  43  is the sheet P. However, the present invention is applicable to a transport device or recording device for transferring a transparent plastic film (sheet member) containing PET, polycarbonate, polyvinyl chloride, or polyethylene, a white opaque film, or cloth in addition to the paper including high-quality paper. Moreover, the flat platen  44  that secures the flatness of the sheet P and a head gap is disposed independently of the neutralization plate  45  but may be integrally formed. At this time, a neutralization electrode is buried in a resin layer of the flat platen  44 . 
     Further, in the above embodiments, the match rate determination table storage unit  114  determines a match rate depending on the humidity, the sheet thickness, and whether to print an image on two sides. In addition, the match rate may be determined while considering the type of sheet. For example, as for plain paper, non-coated paper such as high-quality paper, and coated paper, a match rate is higher in the coated paper. In addition, in the case of disposing the charging voltage determination table storage unit, the charging voltage may be determined while considering the type of sheet as well. The charging voltage is higher in the coated paper. 
     Further, in the above embodiment, as a method for adjusting an attraction force between the sheet P and the conveyor belt  43 , either a method of adjusting the match rates for the charged segments or a method for changing the charging voltage for forming each charged segment is adopted. However, a combination thereof may be adopted. At this time, a combination of the match rate and the charging voltage is determined according to a combination of the humidity of the casing  1   a , the thickness of the sheet P, and whether to print an image upon single-sided printing or on the front/rear side upon two-sided printing. 
     In the above embodiments, as in step S 8 , when the front edge of the sheet placement region reaches the placement position, the operation of feeding the sheet P and subsequent operation of transferring the sheet P along the transfer path  19  are started. However, from the viewpoint of certainly placing the sheet P on the charged segment, the front edge of the sheet P only needs to reach the placement position ahead of the front edge of the sheet placement region. The sheet feeding operation is started from time t 0 , and a time necessary for the front edge of the sheet P to reach the placement position through the transfer path  19  may be set shorter than the time T 2 . As long as this condition is satisfied, the start time of the sheet feeding operation may be set before or after the time t 1  or set to the time t 0 . 
     Further, as described in step S 19 , the start time of neutralization of the neutralization plate  45  is adjusted to the time at which the front edge of the sheet P reaches the placement position, but may be set to the time at which the front edge of the sheet placement region reaches the position of the neutralization plate  45 . With this construction, each time the belt charging roller  136  charges the sheet placement region, uniform charged state can be obtained at any time. 
     Moreover, as described in step S 22 , the stop time of neutralization is adjusted to the time at which the rear end of the sheet P reaches the placement position but may be set to the time at which the rear edge of the sheet placement region reaches the position of the neutralization plate  45 . With this construction, each time the belt charging roller  136  charges the sheet placement region, uniform charged state is obtained at any time. 
     Moreover, the present invention is applicable to a recording device of another type in place of the inkjet printer  1  including the inkjet head  2 , such as a printing having a thermal head for thermal transferring ink on the sheet P. Further, the present invention is applicable to a transport device for transporting a medium including a reading device such as a scanner in place of the recording device.