Patent Publication Number: US-2015070432-A1

Title: Image forming apparatus capable of conveying a sheet absorbed with an electric charge

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2013-189876, filed on Sep. 12, 2013, and 2014-128785, filed on Jun. 24, 2014, in the Japan Patent Office, the entire contents of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Embodiments discussed herein relate to a conveying device for an image forming apparatus, and in particular, to a conveying device and an image forming apparatus capable of conveying a sheet secured with an electric charge. 
     2. Related Art 
     As an image forming apparatus, such as a printer, a facsimile machine, a copier, a plotter, a multifunctional machine combining these capabilities, etc., an ink-jet printer that employs a droplet ejecting-type printing system with a droplet discharging head that ejects the droplet is known. In such an image forming apparatus, a droplet having landed on a printing medium takes a long time to dry and form an image thereon. For this reason, the printing medium is conveyed with its image forming surface distanced from (i.e., not contacting) a sheet conveying device until the droplet on the printing medium dries. 
     Certain known conveying systems convey the printing medium using electrostatic force generated in a sheet conveying device to attract the printing medium. However, it is difficult to adjust a surface potential of the printing medium to be set to 0V, under a printing head acting as an image forming device. In order to control the surface potential of the printing medium under the printing head, it is effective to arrange a surface potential sensor near the printing head. However, a performance of the surface potential sensor can be affected by humidity, and detection accuracy can decrease under the influence of the ink ejected because of a position of the surface potential sensor being near the printing head. 
     Thereby, although a surface potential sensor is generally arranged so that it is separated from a position of the printing head, the surface potential of the printing medium at a location corresponding to in a position of the surface potential sensor is different from that of the printing medium in a location under the printing head. This is especially the case in low-temperature or low-humidity environments, in which the electrical resistance of the printing medium generally increases, making it difficult to negate the electric field under the printing head caused by the surface potential of the printing medium. As a result, a reverse flow of ink mist toward the printing head arises due to the electric field. 
     SUMMARY 
     Accordingly, one aspect of the present disclosure provides an image forming apparatus that includes an image forming head that ejects droplets and forms an image on a printing medium; a conveyor that conveys the printing medium with the image in a conveying direction; at least one first electric charger that charges the conveyor; a second electric charger that charges the printing medium; a surface potential detector downstream of the second electric charger in the conveying direction detects a surface potential of the printing medium; and a controller that adjusts a voltage applied to the second electric charger. The controller determines a target value for the surface potential of the printing medium at a location corresponding to a position of the surface potential detector based on a detection value detected by the surface potential detector in response to a first voltage being applied to the first electric charger and a second voltage being applied to the second electric charger. The controller adjusts the voltage applied to the second electric charger and the surface potential of the printing medium at the location corresponding to the position of the surface potential detector reaches the target value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present invention and the advantages thereof will be understood by reference to the following detailed description when considered in connection with the accompanying drawings. In the drawings: 
         FIG. 1  is a diagram illustrating the overall configuration of an exemplary image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic plan view illustrating a mechanism according to an embodiment of the present disclosure; 
         FIG. 3  is a side view illustrating a conveyor according to an embodiment of the present disclosure; 
         FIG. 4  is a diagram illustrating a mechanism to engage and disengage a driven roller with a driving roller when a sheet is linearly ejected therefrom according to an embodiment of the present disclosure; 
         FIG. 5  is a diagram illustrating an attraction principle of a conveying roller that adsorbs a sheet as a rotary conveying device according to an embodiment of the present disclosure; 
         FIG. 6  is a block diagram of a controller according to an embodiment of the present disclosure; 
         FIG. 7  is a diagram illustrating exemplary charged states of a sheet and a sheet conveying belt when an electric charging control is executed, according to an embodiment of the present disclosure; 
         FIG. 8  is a chart illustrating exemplary results of measuring a surface potential of a sheet according to an embodiment of the present disclosure; 
         FIG. 9  is a chart illustrating a relationship between an electrical resistance of a sheet and a target value of a surface potential of the sheet at a location along an image forming conveyance path corresponding to a position of a surface potential sensor according to an embodiment of the present disclosure; 
         FIG. 10  is a chart illustrating a relationship between an electrical resistance of a sheet and a target value of a surface potential of the sheet at a location along an image forming conveyance path corresponding to a position of a surface potential sensor for a duplex printing operation according to an embodiment of the present disclosure; 
         FIG. 11  is a chart illustrating a relationship between a thickness of a sheet and a target value of a surface potential of the sheet at a location along an image forming conveyance path corresponding to a position of a surface potential sensor according to an embodiment of the present disclosure; and 
         FIGS. 12A and 12B  are flowcharts illustrating a surface potential control by a controller according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein as reference numerals designate identical or corresponding parts throughout the several views thereof and in particular to  FIGS. 1 to 4 , an exemplary image forming apparatus which has a sheet conveying device according to an embodiment of the present disclosure is described. Specifically, an overall configuration of an exemplary image forming apparatus is illustrated in  FIG. 1 . A mechanism is included in the image forming apparatus as shown in  FIG. 2 . A conveying device is disposed in the image forming apparatus as shown in  FIG. 3 . The mechanism causes a driven roller to engage and disengage with a driving roller when a sheet is linearly ejected from the image forming apparatus as shown in  FIG. 4 . 
     The image forming apparatus includes an image forming head  2  that forms an image by ejecting a droplet onto a printing medium in the form of a sheet  100 . The image forming apparatus includes a conveyor  3  that conveys the sheet  100  inside an apparatus body  10 , a processing liquid coating unit  400  on an upstream side of the image forming head  2  in a sheet conveying direction to coat the sheet  100  with processing liquid  401 , and a sheet-inverting unit  4  to invert the sheet  100  bearing the image thereon. Further, the image forming apparatus includes a sheet-exiting tray  104  to receive the sheet  100  drained therefrom, and a sheet feeding cassette  103  disposed in a lower section of the apparatus body  10  to accommodate multiple sheets  100 . 
     As illustrated in the  FIGS. 1 and 2 , the image forming head  2  supports a carriage  23  by a guiding rod  21  and a guiding tray (not shown). The carriage  23  includes multiple printing heads of respective colors aligned in a main scanning direction, and is supported to move in the main scanning direction. The carriage  23  moves and executes scanning in the main scanning direction when driven by a main scanning motor  27  via a timing belt  29  that is wound around a driving  28 A and a driven pulley  28 B. 
     On the carriage  23 , a printing head  24  including a plurality of droplet discharging printing heads  24   a  that eject droplets of respective colors of black (Bk), cyan (C), magenta (M), and yellow (Y), is mounted. Two the printing heads are used to eject Bk droplets. In this configuration, an image is formed by ejecting an applicable droplet from the printing head  24  onto a sheet  100  while moving the carriage  23  in the main scanning direction, and conveying the sheet  100  from the conveyor  3  in a sheet conveying direction (i.e., a sub-scanning direction), in a manner such as a shuttle type system. 
     Alternatively, a line type printing head including multiple printing heads of respective colors aligned in the sub-scanning direction can be utilized as well. However, the present disclosure is not limited to the above-described alignments of printing heads and nozzle lines of the printing heads, and an alignment order of respective colors. 
     Multiple printing head tanks  25   a  providing a printing head tank unit  25  are mounted on the carriage  23  to supply liquids of multiple colors to the respective discharging printing heads  24   a  of the printing head  24 . Multiple color liquids are respectively supplied to the multiple printing head tank unit  25  from liquid cartridges removably mounted on the apparatus body  10  from a front side thereof. Here, the image forming apparatus is enabled to supply black ink from a single liquid cartridge to a pair of the multiple printing head tanks  25   a.    
     The printing head  24  can employ a pressure generating device, such as a piezoelectric-type actuator, a thermal type actuator, or an electrostatic type actuator, for example. However, the present disclosure is not limited to the above-described exemplary droplet discharging unit. 
     Further, as illustrated in  FIG. 2 , a maintenance and recovery mechanism  121  is positioned in a non-printing region located at one side of the image forming apparatus in a widthwise direction that corresponds to the scanning direction of the carriage  23 . The recovery mechanism  121  recovers and maintains a condition of nozzles of the printing head  24 . 
     The maintenance and recovery mechanism  121  includes moisture retaining caps  123  and a suction cap  122  connected to a suction device (not shown) to cap surfaces of the discharging printing heads  24   a . The maintenance and recovery mechanism  121  includes a wiper  124  to wipe multiple nozzle surfaces of the discharging printing heads  24   a . The maintenance and recovery mechanism  121  includes a trial discharged ink receiver  125  to receive a droplet not contributing to printing (i.e., image formation) discharged thereon (as trial ink discharging). 
     Further, as illustrated in  FIG. 2 , a trial discharged ink receiver  126  is positioned in a non-printing region at an opposite side of the image forming apparatus in the widthwise direction of the image forming apparatus that corresponds to the scanning direction of the carriage  23 . The discharged ink receiver  126  receives droplets discharged from the discharging printing heads  24   a  thereon not contributing to printing (i.e., image formation) as trial ink. In the trial discharged ink receiver  126 , openings  127  are formed corresponding to the discharging printing heads  24   a.    
     As further illustrated in  FIG. 3 , a sheet conveying belt  31  provides an endless sheet conveyor, and is provided in the conveyor  3  to adsorb and send the sheet  100  fed from the bottom while directing the sheet to face the image forming head  2 . 
     The sheet conveying belt  31  is wound around a conveying roller  32  that provides a driving roller, another conveying roller  33  that keeps an image formation region flat in cooperation with the conveying roller  32 , a separating roller  34  arranged downstream of the conveying roller  33  in the sheet conveying direction, and a tension roller  35 . A guide member  40  is also positioned facing the image forming head  2  to guide the sheet conveying belt  31  in a region opposite to the image forming head  2 . 
     The sheet conveying belt  31  is preferably a two-tiered structure. For example, the sheet conveying belt  31  includes a front surface acting as a sheet attraction surface made of pure resin such as ETFE (Ethylene tetrafluoroethylene) pure materials, for example, not subjected to resistance control. A back side layer (e.g. a medium resistance layer, a grounded layer) of the sheet conveying belt  31  may be made of the same material as the front surface, and is subjected to resistance control with carbon. However, the present disclosure is not limited to the above-described configuration, and alternatively, the sheet conveying belt  31  can be constituted as a single layer or as a multilayer structure having three or more layers. 
     The separating roller  34  is provided to separate the sheet  100  with the image adhering to the sheet conveying belt  31  using a curvature separation principle. illustrated in  FIG. 3 , the separating roller  34  is rotatably held by a shaft  36   b  provided at an end of a movably rotatable link  36 . The movably rotatable link  36  is movable around a rotating center of the conveying roller  33  acting as a supporting point  36   a  in a direction as indicated by an arrow (A). The separating roller  34  is also enabled to swing between two corresponding positions in multiple conveying paths as shown by solid and broken lines, respectively. Specifically, the separating roller  34  is positioned to be able to convey the sheet  100  in each sheet conveying path. 
     In moving the separating roller  34  to a position as indicated by a broken line in  FIG. 3 , the separating roller  34  switches a respective position to enter a straight sheet ejecting path  306 , in which the sheet  100  bearing the image thereon is linearly conveyed and sent toward the sheet-exiting tray  104 . In contrast, in moving the separating roller  34  to a position indicated by a solid line in  FIG. 3 , the separating roller  34  switches a respective position to enter a sheet inverting path  311 , in which the sheet  100  bearing the image is sent to a sheet-inverting unit  4 . 
     A sheet conveyance distance between a location of the image forming head  2  and a position in which the sheet  100  is separated from the sheet conveying belt  31  is substantially the same in the straight sheet ejecting path  306  and the sheet inverting path  311 . As a result, with such an arrangement, a drying degree of the sheet  100  can be the same regardless of a type of the sheet conveying path, in which the sheet  100  is conveyed (i.e., the straight sheet ejecting path  306  or the sheet inverting path  311 ), so that the same quality of the image can be obtained regardless of a sheet conveying path being used. 
     Since the separating roller  34  is rotatable around the rotational center of the conveying roller  33  functioning as a fulcrum as described above, a sheet conveying distance between a location where the separating roller  34  separates the sheet  100  from the sheet conveying belt  31  and a position of the image forming head  2  can be substantially the same in the straight sheet ejecting path  306  and the sheet inverting path  311 . 
     The separating roller  34  is positioned at a predetermined position (e.g., at a given minimum distance from the conveying roller  33 ) to enable the sheet conveying belt  31  to always contact the conveying roller  33  with a prescribed tension. Even when the sheet conveying path is switched, a posture of the sheet conveying belt  31  does not change at an image forming region, so that an image can be steadily formed. 
     In a situation where the separating roller  34  is located at the position indicated by the broken line in  FIG. 3 , the separating roller  34  as a whole is positioned below a conveying surface formed by the pair of conveying rollers  32  and  33  that hold the sheet conveying belt  31  facing the image forming head  2 . Specifically, the separating roller  34  is placed lower than the conveying surface by a distance (c) as shown in  FIG. 3 . Accordingly, the sheet conveying belt  31  can contact the conveying roller  33  while ensuring its flatness. 
     Further, as illustrated in  FIG. 3 , a tension roller  35  is held by an arm  37  that is swingable between positions as shown by solid and broken lines in a direction as indicated by arrow (B) in the drawing. Specifically, the arm  37  is swingable around a rotation fulcrum  37   a  acting as a fulcrum, and rotatably supports the tension roller  35  around a holding fulcrum  37   b . The arm  37  is pressed by a pressing device (not shown) in a direction in which the tension roller  35  presses the sheet conveying belt  31  in a prescribed direction. The tension roller  35  moves following the sheet conveying belt  31  even when the sheet conveying belt  31  displaces due to swinging of the separating roller  34 , and accordingly, provides a tension to the sheet conveying belt  31 . By contrast, on an upstream side of the image forming head  2 , a pressing member (e.g., a pressing roller)  38  is provided opposite the conveying roller  32  to press the sheet  100  against the sheet conveying belt  31  at an opposed position. 
     To attract the sheet  100  to the sheet conveying belt  31 , a high power voltage (a power supply voltage), such as a DC (direct current) voltage, or a voltage provided by superimposing a DC voltage and an AC (alternating current) voltage, for example, is supplied from a high voltage power supply  218  (e.g., a DC bias supply unit or a DC and AC superposed bias supply unit and the like) to the pressing roller  38 . 
     On the downstream side of the pressing roller  38 , a surface potential sensor  61  is located in a position to detect a surface potential on the sheet  100  at the upstream side of the image forming head  2 . 
     To charge a surface of the sheet conveying belt  31 , a pair of electric charging rollers  39   a  and  39   b  collectively operate as first electric charge applying devices provided on the upstream side of the pressing roller  38  at different positions on the sheet conveying belt  31  in a belt circulating direction (i.e., a sheet conveying direction). To charge the sheet conveying belt  31 , a high DC voltage or a high voltage provided by superimposing the DC and the AC voltage biases is supplied from the high voltage power supply  217  (, the DC bias supply unit or the DC and AC superimposed bias supply unit and the like) to the pair of electric charging rollers  39   a  and  39   b.    
     On the downstream side of the electric charging roller  39   b , a surface potential sensor  51  is positioned to detect a surface potential of the sheet conveying belt  31 . 
     As illustrated in  FIG. 3 , as the conveying roller  32  is rotated by a sub-scanning motor  331  via a timing belt  332  and a timing roller  333 , the sheet conveying belt  31  circulates in the sheet conveying direction (i.e., a sub-scanning direction) as shown in  FIG. 2 . 
     As illustrated in  FIG. 1 , the sheet-inverting unit  4  includes a conveying roller  136  composed of a conductive elastic member placed on the downstream side of the sheet conveying belt  31 , and provides a rotary sheet conveyor. A driven roller  137  driven by the conveying roller  136  is provided to engage and disengage with the conveying roller  136  in the direction as indicated by arrow (C) to act as a driven rotated member. Further, the sheet-inverting unit  4  includes a path switching nail  41  that switches a sheet conveying path guiding the sheet  100  between a sheet inverting and ejecting path  309  and a double-sided sheet conveying path  304 . Specifically, the sheet-inverting unit  4  inverts the sheet  100  and sends it to one of the sheet inverting and ejecting path  309  and a double-sided sheet conveying path  304 . 
     At least a surface of the conveying roller  136  is composed of a conductive elastic member formed of a conductive elastic material such as conductive rubber, conductive sponge or similar material, for example. In exemplary embodiments in which the conductive elastic member is formed of a conductive rubber, solid rubber, such as EP rubber, chloroprene rubber, and polyurethane rubber, for example; and materials prepared by dispersing conductive carbon or conductive ions into foam rubber, can be used. A volume resistivity of the conductive elastic member is preferably from about 10 2  to about 10 12  (Ω-cm), and is more preferably from about 10 3  to about 10 6  (Ω-cm). 
     The driven roller  137  is placed to engage and disengage with the conveying roller  136  as described above, and presses the sheet  100  against the conveying roller  136  as it engages with the sheet  100 . 
     Accordingly, when a prescribed sheet type (such as cardboard, for example), a condition of an environment, or other operating condition necessitates a prescribed feeding force larger than an attraction power of the sheet conveying roller  136  according to a previous analysis, is detected based on an output from a sheet thickness sensor, that of a temperature and humidity sensor (not shown) or an input from a user, for example, the driven roller  137  is pressed against the conveying roller  136 . As a result, conveying power increases, and a problem, such as sheet jam, for example, may be prevented. 
     In the sheet inverting and ejecting path  309 , into which the sheet  100  is sent from the sheet-inverting unit  4 , a conveying roller  148  having at least a surface composed of a conductive elastic member is deployed to act as a rotary sheet conveyor similar to the conveying roller  136 . A driven roller  149  that is driven by the conveying roller  148 , is positioned as a driven rotated member able to engage and disengage with the conveying roller  148  in a direction as show by arrow (D) in  FIG. 4 . The conveying roller  148  is accordingly located on the downstream side of the sheet conveying belt  31 . 
     To eject the sheet  100  fed out from either the sheet inverting and ejecting path  309  and the straight sheet ejecting path  306  onto the sheet-exiting tray  104 , a conveying roller  143  (i.e., a sheet exit roller) at least having a surface composed of a conductive elastic member is positioned as a rotary conveyor similar to the conveying roller  136 . A driven roller  144  that is driven by the conveying roller  143  provides a driven rotation member and is positioned to engage and disengage the conveying roller  143 . The conveying roller  143  is located on the downstream side of the sheet conveying belt  31 . 
     On the downstream side of the conveying roller  143  and the upstream side of the sheet exit tray  104 , an electric charge removing device  146  (e.g., an electric charge removing brush) is disposed to remove an electric charge remaining on the sheet  100  being discharged. Specifically, the electric charge removing device  146  is provided to eject the sheet  100  onto the sheet-exiting tray  104  while removing the electric charge applied to the sheet  100  by the pressing roller  38  that acts as an electric charge applying device. 
     As illustrated in  FIG. 4 , the driven roller  144  is held by a link  147  capable of swinging between two positions as shown by solid and broken lines in a direction as shown by arrow (D). Specifically, the link  147  is swingable around a rotation fulcrum  147   a  that rotatably supports the driven roller  144  around a holding fulcrum  147   b . The link  147  is pivoted by a driving mechanism (not shown). 
     Respective mechanisms to engage and disengage the above-described driven rollers  137  and  149  with respective driving rollers are similarly configured as in the above-described mechanism. 
     In the double-sided sheet conveying path  304 , various conveying rollers, such as a conveying roller  138   a , a driven roller  138   b , a conveying roller  139   a , a driven roller  139   b , a conveying roller  140   a , and a driven roller  140   b , for example, are arranged. 
     The conveying rollers ( 138   a ,  139   a , and  140   a ) each serve as a rotary conveyor at least having a surface composed of a conductive elastic member similar to the conveying roller  136 . The conveying rollers ( 138   a ,  139   a , and  140   a ) are located on the downstream side of the sheet conveying belt  31 . An engaging and disengaging mechanism that engages and disengages each of the driven rollers ( 138   b ,  139   b , and  140   b ) with respective conveying rollers ( 138   a ,  139   a , and  140   a ), includes the same mechanism as the above-described mechanism that engages and disengages the driven roller  144 . Further, the duplex sheet conveying path  304  is used to re-feed the sheet  100  sent thereto toward the pair of registration rollers  134 . 
     The sheet feeding unit  20  is detachably attached to the apparatus body  10  at a front side thereof. The sheet feeding unit  20  includes a sheet feeding cassette  103  to stack and accommodate multiple sheets  100 , and a pickup roller  141  to separate and feed the multiple sheets  100  stored in the sheet feeding cassette  103  one by one. The sheet feeding unit  20  also includes a pair of conveying rollers  132 . 
     The sheet feeding unit  20  includes a straight manual sheet feeding tray  105  to be manually used, the pickup roller  141  to pick up and feed the sheet  100  one at a time from the straight manual sheet feeding tray  105 , and a pair of conveying rollers  142 . 
     Further, the processing liquid application system  400  includes a deformable bag-shaped processing liquid container, e.g., made of a PET (Poly Ethylene Terephthalate) film (not shown) to contain processing liquid  401  therein, and a pump (not shown) to feed the processing liquid  401  with pressure, when it is supplied from the processing liquid containers. The processing liquid application system  400  also includes a coating unit  410  to coat the sheet  100  acting as a printing medium with the processing liquid  401  or the like. Specifically, the pump pumps up the processing liquid  401  stored in the processing liquid containers, and supplies it to a liquid chamber  402  provided in a coating unit  410  via a supply path (not shown) to prepare for coating of the processing liquid  401 . 
     A liquid level detector (not shown) installed in the liquid chamber  402  detects and confirms that a height of the liquid level and an angle of the liquid plane of the processing liquid  401  supplied to the liquid chamber  402  are within given levels, respectively. The liquid level detector may be an electrode pin system, for example. The electrode pin system is known and is not described in detail here, but detects the liquid level by supplying electricity to electrode pins through the liquid and checking an electrical conductive level between the electrode pins. In this way, a lack of or excessive supplying of the processing liquid  401  more than a prescribed amount to the liquid chamber  402  can be checked and reduced. 
     The coating unit  410  includes a conveying roller  434  that conveys the sheet  100 , a coating roller  432  opposed to the conveying roller  434  to coat the sheet  100  with the processing liquid  401 , and a squeeze roller  433  to supply the processing liquid  401  to the coating roller  432  while thinning the processing liquid  401  as a liquid film thereof. 
     The coating roller  432  is positioned to contact the conveying roller  434 . By contrast, the squeeze roller  433  is positioned to contact the coating roller  432 . Accordingly, a liquid film layer of the processing liquid  401  is formed on the coating roller  432  when it is supplied by the squeeze roller  433  and the coating roller  432 , and conveyed and applied to the sheet  100  as the coating roller  432  rotates in a prescribed direction. 
     It is to be noted that the processing liquid  401  serves as quality modification material to modify the quality of the surface of the sheet  100  when applied to the surface of the sheet  100 . For example, the processing liquid  401  serves as a fixative (e.g., a setting agent) when uniformly coated onto the sheet  100  in advance, because water in the ink is urged to quickly penetrate into the sheet  100  and a color component of ink is thickened while hastening the ink to dry to avoid blurring (e.g., feathering, bleeding, etc.) and striking through of the ink to a rear surface of the sheet, so that the productivity (number of images outputted per unit of time) can be enhanced. 
     As a chemical composition of the processing liquid  401 , solution prepared by adding both cellulose (hydroxypropyl cellulose, for example,) that promotes penetration of moisture and a base agent such as talc fine powder, for example, to surfactants (e.g., anion, cationic, nonionic, and mixture of two or more of these, for example) is used in exemplary embodiments of the disclosure. The chemical composition can further contain fine particles. 
     The sheets  100  housed in the sheet feeding cassette  103  are separated and fed one at a time by a pickup roller  131  and sent by a pair of conveying rollers  133  to the pair of registration rollers  134 . Subsequently, the sheet  100  is sent from the pair of registration rollers  134  at a predetermined time toward the processing liquid coating unit  400  along a sheet conveying path  300 . The processing liquid  401  is then coated onto the sheet  100  by the process fluid coating unit  400 . 
     Now, an attraction principle of the conveying roller attracting a sheet thereto as a rotary conveyor in the image forming apparatus is described with reference to  FIG. 5  and applicable drawings. Here, only the conveying roller  143  is mainly described. However, each of the other conveying rollers ( 136 ,  148 , and  138   a  to  140   a ) has substantially the same configuration and executes substantially the same operation as well. 
     Since the DC voltage (or an AC voltage superimposed DC voltage) is supplied to the pressing roller  38  as described above, a negative (−) electric charge  700 , for example, is applied onto the surface of the sheet  100  (e.g., an image forming surface) sandwiched between the sheet conveying belt  31  and the pressing roller  38 . Since a positive (+) electric charge  701  appears on the sheet conveying belt  31  due to electrostatic induction when the negative charge  700  is applied onto the sheet  100 , the sheet  100  may be attracted by the sheet conveying belt  31  thereonto by Coulomb force. 
     At this moment, an attraction force may be further enhanced by previously applying a positive electric charge onto the sheet conveying belt  31  using the pair of electric charging rollers  39   a  and  39   b.    
     As a result, an image is formed on the sheet  100  by the image forming head  2  while the sheet  100  is secured to and intermittently conveyed by the sheet conveying belt  31  as the sheet conveying belt  31  circulates. Subsequently, as illustrated in  FIG. 5 , the sheet  100  with the image thereon is separated due to curvature of the separating roller  34  from the sheet conveying belt  31 . 
     The sheet  100  separated from the sheet conveying belt  31  is conveyed toward the conveying roller  143  composed of an electrically conductive elastic member. Since a vertex of the conveying roller  143  is lower than a sheet conveying surface formed by the sheet conveying belt  31 , the sheet  100  is hardly peeled off from both the conveying roller  143  and the sheet conveying belt  31 , even after the sheet  100  is attracted onto the conveying roller  143 . The negative electric charge  700  has been applied onto the sheet  100 , and a positive electric charge  701  is electrostatically generated on the surface of the conveying roller  143  composed of an electrically conductive elastic member. Since the negative electric charge  700  in the sheet  100  and the positive electric charge  701  in the conveying roller  143  attract each other, the sheet  100  is attracted onto the conveying roller  143  by the Coulomb force. 
     A contact area between the conveying roller  143  and the sheet  100  is apparently smaller than that between the sheet conveying belt  31  and the sheet  100 , and a stronger sheet attraction force is needed to constantly convey the sheet  100  than when it is conveyed by the sheet conveying belt  31 . In this regards, it is necessary to raise the electric attraction force of the conveying roller  143  having a different construction from the sheet conveying belt  31 . The sheet conveying belt  31  is a two-tier structure composed of an insulating layer on its surface and a resistance controlled (conductive) layer with its resistance controlled by carbon on its backside. On the other hand, the surface of the conveying roller  143  is composed of a conductive member. 
     The sheet  100  attracted onto the conveying roller  143  is then sent and ejected onto the sheet-exiting tray  104  by the conveying roller  143 . 
     A charge removing device  146  is positioned between the conveying roller  143  and the sheet-exiting tray  104  to remove the negative electric charge  700  remaining on the sheet  100 , the sheet  100  can exit onto the sheet-exiting tray  104  without bearing the negative electric charge  700  thereon. Accordingly, multiple sheets  100  exiting onto the sheet-exiting tray  104  are likely to avoid sticking to each other due generally to the electrostatic charge remaining thereon. 
     Heretofore, in this embodiment, the conductive elastic member is employed as the exemplary rotary conveyor, because it has a relatively high friction coefficient, a large adsorption force, and is prepared at a low cost. However, the present disclosure is not limited thereto, and a similar conveying force can be also obtained by utilizing a belt or a roller at least having a surface composed of a conductive member as well. 
     Now, with reference to  FIG. 1 , an aspect of when the sheet  100  bearing the image formed in the image forming head  2  is linearly ejected onto the sheet-exiting tray  104  is described. 
     As described earlier, the sheet  100  coated with the processing liquid  401  is conveyed into the sheet conveying path  305  via the pair of conveying rollers  145 . Subsequently, in the sheet conveying path  305 , the sheet  100  is fed onto the sheet conveying belt  31 , in which a DC electric field is formed. The sheet  100  is then given an electric charge having a reverse polarity to that of the sheet conveying belt  31  by the pressing roller  38 . Consequently, the sheet  100  is electrostatically attracted onto the sheet conveying belt  31  and is held thereon. 
     Then, the printing head  24  is driven based on an image signal while moving the carriage  23  with respect to the sheet  100  and executes printing on the sheet  100  by ejecting droplets thereon to form an image of one line when the sheet  100  reaches and stops at a starting position for starting image formation. When one line printing is completed, the sheet  100  is sent by an amount of one line to execute printing on the next line. Thus, by intermittently conveying the sheet  100 , an image is sequentially formed on the sheet  100  (line by line). When receiving either a signal indicating that printing is completed or that the end of sheet  100  has reached an end of a printing region, the printing is terminated. 
     The separating roller  34  is moved to a position as shown by the broken line in  FIG. 1  (a position as shown by the solid line in  FIG. 4 ), at the latest, before the tip of the sheet  100  in the process of image formation reaches the conveying roller  33 . 
     By this, the sheet  100  bearing the image is conveyed and is attracted and further conveyed by the conveying roller  143  along the straight sheet ejecting path  306  as the sheet conveying belt  31  moves and circulates. The sheet  100  bearing the image finally exits onto the sheet-exiting tray  104  with the printing surface facing upward. Further, also in this situation, as described earlier, since the electric charge is applied onto the sheet  100 , an electric charge having a reverse polarity to that of the sheet  100  is excited (generated) on the conveying roller  143 , and the sheet  100  is electrostatically attracted thereon and is further conveyed by the conveying roller  143 . 
     Now, an exemplary operation executed when the sheet  100  bearing the image formed in the image forming head  2  is inverted and is ejected onto the sheet-exiting tray  104  in the image forming apparatus is described. 
     Specifically, similar to the situation in which the sheet  100  is linearly ejected, the printing head  24  is driven based on an image signal while moving the carriage  23  with respect to the sheet  100  and executes printing on the sheet  100  by ejecting droplets thereon to form an image of one line when the sheet  100  reaches and stops at a starting position for starting image formation. When the one line is printed, the sheet  100  is sent by an amount of one line to execute printing on the next line. Thus, by intermittently conveying the sheet  100 , an image is sequentially formed on the sheet  100  (line by line). When receiving either a signal indicating that the printing is completed or indicating that the end of sheet  100  reaches the end of a printing region, the printing is terminated. 
     The separating roller  34  is moved to a position as shown by the solid line in  FIG. 1 , at the latest, before the tip of the sheet  100  in the process of image formation reaches the conveying roller  33 . 
     By this, the sheet  100  bearing the image formed in this way is subsequently conveyed and diagonally sent downward and is further sent into the sheet-inverting unit  4  through the sheet inverting path  311  by the sheet conveying belt  31  as it circulates. 
     Since an electric charge has been given to the sheet  100 , an electric charge having a reverse polarity to that of the sheet  100  is excited (generated) in the conveying roller  136  as described earlier, and the sheet  100  is electrostatically attracted and conveyed by the conveying roller  136 , and taken in by the sheet-inverting unit  4 . 
     Further, the sheet  100  conveyed into the sheet-inverting unit  4  subsequently evacuates from the sheet-inverting unit  4  as the conveying roller  136  reversely rotates. At this moment, a path switching nail  41  is located at a position as shown by a solid line in the drawing, and accordingly, the sheet  100  fed out by the conveying roller  136  is conveyed toward the sheet inverting and ejecting path  309 . 
     In the sheet inverting and ejecting path  309 , since the electric charge has been applied to the sheet  100 , an electric charge having a reverse polarity to that of the sheet  100  is applied to the conveying roller  148  as described earlier. Thus, the back side of the sheet  100  opposite a front side bearing the image formed in this way, is electrostatically attracted by the conveying roller  148  and is thereby conveyed downstream. 
     The sheet  100  is subsequently sent to the conveying roller  143  from the sheet inverting and ejecting path  309 . Subsequently, since the electric charge is given to the sheet  100 , and an electric charge having a reverse polarity to that of the sheet  100  is excited in the conveying roller  143  as described earlier, the sheet  100  is electrostatically attracted and conveyed by the conveying roller  143 . The sheet  100  consequently exits onto the sheet-exiting tray  104  with its printing surface facing down. 
     Since the conveying roller  143  is also used in executing the straight sheet ejection, the conveying roller  143  attracts the image printed surface of the sheet  100  when the sheet inverting and ejecting process is executed. However, since the sheet  100  passes through the sheet-inverting unit  4  in the sheet inverting and ejecting process, an ink drying and settling time can be relatively sufficiently ensured before the sheet  100  reaches the conveying roller  143 , and accordingly, the ink almost never adheres to the conveying roller  143 . 
     By supposing that the sheet  100  having a property of poor ink drying fixative is conveyed, the driven roller  144  positioned in a location opposed to the conveying roller  143  can also be composed of a conductive elastic member as well, so that the sheet  100  can be attracted onto the driven roller  144  and conveyed in the sheet inverting and ejecting process. 
     All of the conveying rollers placed downstream of the sheet conveying belt  31 , while facing the back side of the sheet  100 , are not necessarily conductive to attract the sheet  100 . Further, only some of the conveying rollers need to be conductive to attract the sheet  100  as well. In particular, a prescribed conveying roller disposed closer to the sheet conveying belt  31  is preferably enabled to attract the sheet  100 . 
     Now, an operation of forming multiple images on both sides of the sheet  100  respectively is described. 
     As described above, the sheet  100  coated with the processing liquid  401  is conveyed to sheet conveying path  305  via the pair of rollers  145 . In the sheet conveying path  305 , the sheet  100  is fed onto the sheet conveying belt  31 , in which a DC electric field is formed. The sheet  100  is subsequently given an electric charge having a reverse polarity to that of the sheet conveying belt  31  by the pressing roller  38 . Accordingly, the sheet  100  can be electrostatically attracted onto the sheet conveying belt  31  and is held thereon. 
     Then, the printing head  24  is driven based on an image signal while moving the carriage  23  with respect to the sheet  100  and executes printing on the sheet  100  by ejecting droplets thereon to form an image of one line when the sheet  100  reaches and stops at a starting position for starting image formation. Hence, by intermittently conveying the sheet  100 , an image is sequentially formed on the sheet  100  (line by line). When the one line is printed, the sheet  100  is sent by one line to execute printing on the next line. When receiving either a signal indicating that the printing is completed or indicating that the end of sheet  100  reaches the end of a printing region, the printing is terminated. 
     The separating roller  34  is moved to a position as shown by the solid line in  FIG. 1 , at latest, before the tip of the sheet  100  in the process of image formation reaches the conveying roller  33 . As a result, the sheet  100  bearing the image formed in this way is subsequently conveyed and diagonally sent downwardly and is further sent into the sheet-inverting unit  4  through the sheet inverting path  311  by the sheet conveying belt  31  as it circulates. 
     Since the electric charge has been given to the sheet  100 , and an electric charge having a reverse polarity to that of the sheet  100  is generated in the conveying roller  136  as described earlier, the sheet  100  is electrostatically attracted to and is conveyed by the conveying roller  136 . The sheet  100  is subsequently taken in by the sheet-inverting unit  4 . The sheet  100  conveyed into the sheet-inverting unit  4  subsequently evacuates from the sheet-inverting unit  4  as the conveying roller  136  reversely rotates. At this moment, a path switching nail  41  is located at a position as shown by a broken line in the drawing, and accordingly, the sheet  100  sent by the conveying roller  136  is conveyed toward the double-sided sheet conveying path  304 . The sheet  100  is subsequently conveyed by multiple conveying rollers ( 138   a ,  139   a , and  140   a ) and sent to the pair of registration rollers  134  again. 
     As described earlier, since the electric charge has been applied onto the sheet  100  again, and a reverse polarity to that in the sheet  100  is generated on the multiple conveying rollers ( 138   a  to  140   a ), the sheet  100  is electrostatically attracted thereon and further conveyed by these multiple conveying rollers  138   a  to  140   a . Subsequently, the sheet  100  sent to the pair of registration rollers  134  is resent therefrom at a predetermined time toward the processing liquid coating unit  400  via the sheet conveying path  300 . The processing liquid  401  is subsequently coated onto the other side (in which the image has not formed yet) of sheet  100  by the process fluid coating unit  400  as described above. Subsequently, after an image is formed on the other side of the sheet in the image forming head  2 , the sheet  100  is further conveyed as the sheet conveying belt  31  shown by a broken line circulates and exits onto the sheet-exiting tray  104  along the straight sheet ejecting path  306 , with its printing side facing upward as the conveying roller  143  rotates. 
     Now, an operation of a straight sheet ejection process in which the sheet  100  is almost linearly fed and conveyed from the manual sheet feeding tray  105  is described. 
     Specifically, by using the manual sheet feeding tray  105 , an image can be easily formed on a special sheet, such as cardboard, or a sticker release paper sheet, for example. A path extended from the manual sheet feeding tray  105  joins the sheet conveying path downstream of the processing liquid coating unit  400  in the conveying direction, a sheet such as a coated sheet, for example, not requiring coating of the processing liquid is preferably fed from the manual sheet feeding tray  105  as well. For this reason, the manual sheet feeding tray  105  is enabled to load several sheets thereon while enabling the pickup roller  141  to pick up and supply the sheets  100  one at a time. 
     Specifically, the sheets  100  housed in the manual sheet feeding tray  105  are separated and fed one at a time by the pickup roller  141 , and conveyed by the conveying roller  142  toward the printing sheet conveying path  305 . Subsequently, as described above, the sheet  100  is intermittently conveyed by the sheet conveying belt  31  again, and an image is formed thereon in the image forming head  2 . Subsequently, the sheet  100  bearing the image is further conveyed as the sheet conveying belt  31  shown by a broken line circulates and exits onto the sheet-exiting tray  104  through the straight sheet ejecting path  306 , with its printing side facing upward as the conveying roller  143  rotates. 
     Heretofore, a conveying operation of the multiple driven rollers  137 ,  149 ,  144 , and  138   b  to  140   b  ha not been described. However, as described earlier, in accordance with a sheet type and environmental conditions (such as temperature, humidity, for example) or the like, the driven rollers ( 137 ,  149 ,  144 , and  138   b  to  140   b ) are moved to contact the respective conveying rollers ( 136 ,  148 ,  144 , and  138   a  to  140   a ) to press the sheet  100  thereagainst. 
     Now, an overview of a controller provided in the image forming apparatus is described with reference to  FIG. 6 . 
     Specifically, the controller  200  is comprised of a CPU (central processing unit)  201  that generally controls the image forming apparatus, a ROM (read only memory)  202  that stores programs and the other fixed data implemented by the CPU  201 , and a RAM (random access memory)  203  that temporarily stores image data (printing data), for example. 
     The controller  200  also includes a non-volatile memory (NVRAM)  204  that holds data even when a power supply is interrupted. Further, the controller  200  includes an ASIC (application specific integrated circuit)  205  that applies various signal processes to image data, executes image forming processes such as sorting, for example, and handles input and output signals other than those of processes to generally control the image forming apparatus. 
     Further, the controller  200  also includes a scanner control unit  206  that controls an image reading unit  11  to read an image and processes image data read by the image reading unit  11  and so forth. 
     An external I/F  207  (Interface) of the controller  200  used to receive data from an external device is enabled to send and receive data and signals. A printing head-driving control unit  208  and a printing head driver  209  of the controller  200  collectively control the printing head  24  included in the image forming head  2  to operate. 
     Further, included in the controller  200  are a motor driving unit  211  that drives a main scanning motor  27  to execute main scanning of the carriage  23 , and a motor-driving unit  212  that drives the sub-scanning motor  331  to rotate the conveying roller  32  and accordingly circulate the sheet conveying belt  31 . A motor driving unit  213  drives a sheet feeding motor  45 , and a motor driving unit  214  that drives a sheet ejection motor  271  to operate and rotate various rollers, such as the sheet conveying roller  143 , for example, are provided. 
     A motor driving unit  215  of the control unit  200  that drives a double-sided sheet conveying motor  291  to drive and rotate various rollers located in a duplex sheet conveying path  304 , and a motor-driving unit  317  that drives a conveying motor  318  to drive and rotate the conveying roller  136  located in the sheet-inverting unit  4 . 
     The controller  200  includes a motor driving unit  320  that drives a separating motor  319  to move the separating roller  34 . 
     The controller  200  further includes a clutch driving unit  216  that drives a clutch group  241 . The clutch group  241  includes multiple sheet feeding-electromagnetic clutches that independently drive and rotate the pickup roller  131  and the pair of conveying rollers  132 , and the pickup roller  141  and the pair of conveying rollers  142 , respectively. Further, the clutch group  241  includes an electromagnetic clutch that independently drives the sheet conveying paths and a path switching plate solenoid that pivots the path switching nail  41  to switch the sheet conveying path. 
     The controller  200  includes the high voltage power supply  217  that supplies a high voltage to the pair of electric charging rollers  39   a  and  39   b . The high voltage power supply  217  can independently control each of the high voltages applied to the pair of electric charging rollers  39   a  and  39   b , respectively. 
     The controller  200  includes a high voltage power supply  218  that supplies a high voltage to the pressing roller  38 . 
     The controller  200  also includes an I/O (Input and Output port)  221  that captures detection signals from various sensors. Specifically, a detection signal is inputted to the I/O  221  from the temperature humidity sensor  500  that detects temperature and humidity as an environmental condition. Also inputted to the I/O  221  are detection signals from an image formation starting sensor (not shown) and an image formation end sensor (not shown). Further, measuring signals from the respective surface potential sensors  51 ,  61  are inputted to the I/O  221 . 
     Further, an operation panel  222  is connected to the controller  200  to input and display information necessary for the apparatus. 
     Accordingly, the controller  200  processes and stores read image data in a buffer included in the scanner control unit  206  when the image reading unit  11  reads an image of an original document. By contrast, the controller  200  stores printing data or the like in a buffer included in an external I/F  207  upon receiving it from an external host, such as an information processing device (e.g., a personal computer), an image reader (e.g., an image scanner), or an imaging device (e.g., a digital camera), for example, via the external I/F  207 . 
     The CPU  201  reads image data from the scanner control unit  206  or the external I/F  207 , and analyzes the image data. The ASIC  205  then executes necessary image processing and data reordering processing or the like and transfers printing image data to a printing head-driving control unit  208 . Here, dot pattern data for outputting an image based on data sent from the external device can be generated by storing font data in the ROM  202 , for example. Otherwise, image data can be spread as bitmap data by a printer driver provided in the external host, and is transferred to the image forming apparatus. 
     Upon receiving the image data (e.g., the dot pattern data) corresponding to one line of each printing head of the printing head  24 , the printing head-driving control unit  208  transfers the one line dot pattern data to a printing head driver  209 . Based on the dot pattern data, the printing head driver  209  selectively provides a driving waveform and drives an actuator included in the printing head  24  and lets a prescribed nozzle of the printing head of the of the printing head  24  discharge a droplet therefrom. 
     As a result, in the image forming apparatus configured in this way, the sheet  100  is fed one by one from either the sheet feeding unit  20  or the double-sided sheet conveying path  310  and is pressed against the sheet conveying belt  31  by the pressing roller  38 . As a result, a conveying direction of the sheet  100  is changed by an angle of about 90°. The sheet  100  is then electrostatically attracted onto the sheet conveying belt  31  and further conveyed in the sub-scanning direction as the sheet conveying belt  31  circulates. 
     Then, the printing head  24  is driven based on an image signal and executes printing an image of one line on the sheet  100  which is currently stopped, by ejecting a droplet thereonto while moving the carriage  23 . When one line printing is completed, the sheet  100  is sent by one line to execute printing on the next line. In this way, by intermittently conveying the sheet  100 , an image is sequentially formed on the sheet  100  (e.g., line by line). 
     Upon receiving either a signal indicating that the printing is completed or indicating that the end of sheet  100  reaches the end of a printing region, the printing is terminated. 
     At this moment, by moving the separating roller  34  between positions in accordance with usage of the sheet conveying path as shown by solid and broken lines in  FIGS. 1 and 4  as described above, the sheet conveying path for conveying the sheet  100  bearing the image is switched. The sheet  100  is accordingly sent onto the sheet-exiting tray  104  via a prescribed conveying path. 
     Charging control applied to the sheet  100  via control of power supplying to the pressing roller  38  according to an embodiment of the present disclosure is described with reference to  FIG. 7 . 
       FIG. 7  is a diagram illustrating a charged state of each of the sheet  100  and the conveying belt  31  when charging control is implemented thereon via control of power supplying to the pressing roller  38 . 
     Initially, as shown in  FIG. 7 , the high voltage power supply  217  provides a high voltage to the electric charging roller  39   a . The electric charging roller  39   a  provides positive electric charge to the sheet conveying belt  31 . Thus, the sheet conveying belt  31  bears the positive electric charge thereon. Similarly, the high voltage power supply  217  supplies a high voltage to the electric charging roller  39   b . The electric charging roller  39   b  then supplies the positive electric charge to the sheet conveying belt  31  to electrically positively charge the sheet conveying belt  31  uniformly so that it bears the positive electric charge thereon. 
     Thus, with electric charging rollers  39   a  and  39   b , the sheet conveying belt  31  is charged with the same polarity electric charge in two steps, and, finally is charged in the required amount of electric charges. Here, the number of electric charging rollers is not limited to two; there may be more than two. 
     Such a positively charged state of the sheet conveying belt  31  is detected by the surface potential sensor  51 . The control unit  200  subsequently adjusts the high voltage (the power supply voltage) supplied from the high voltage power supply  217  to at least one of electric charging rollers, preferably  39   b  based on the detection result to render the surface potential to be a given value. 
     By contrast, the sheet  100  is conveyed onto the sheet conveying belt  31  bearing the positive electric charge thereon. At this moment, by receiving the negative electric charge  700 , the sheet  100  is negatively electrically charged by the pressing roller  38  to which a high voltage is supplied from the high voltage power supply  218 . 
     By negatively charging the sheet  100  from above the sheet  100 , since the electric charge on the sheet  100  and that on the sheet conveying belt  31  are balanced, the surface potential on the sheet  100  can be reduced. 
     Next, a change of the surface potential of the sheet  100 , which arises between locations along an image forming conveyance path  301  corresponding to positions of the surface potential sensor and the printing head  24 , is described with reference to  FIG. 8 . 
     As a result of the electrical resistance of the sheet  100 , the negative electric charge  700  borne thereon takes a prescribed time to reach a back side of the sheet  100 . In particular, in a low temperature and low humidity environment, since the electrical resistance of the sheet  100  is relatively high, such as 10 12  Ω-cm for example, and the negative electric charge  700  moves slowly, the surface potential of the sheet  100  when the sheet is near the surface potential sensor  61  is different than when the sheet  100  is below the printing head  24  as shown in  FIG. 8 . 
     During operation, a positive surface potential of about 200V is produced on the surface of the sheet  100  along the image forming conveyance path  301  because the negative electric charge  700  on the surface of the sheet  100  moves toward the sheet conveying belt  31  until the sheet  100  arrives at the location along the image forming conveyance path  301  corresponding to the position of the printing head  24 . This is the case, even if the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  is 0V as shown in  FIG. 8 . 
     Therefore, it is necessary to take this charge transfer into account, and maintain the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  at an amount so the surface potential at the location corresponding to the position of the printing head  24  is approximately 0V. 
     The charge transfer is affected by the electrical resistance of the sheet  100  as described above. In addition, it is advantageous to measure the electrical resistance of the sheet just before a printing operation, in order to account for the influence of the environmental transformation such as temperature and humidity, or of the moisture content resulting from storage environment. 
     Therefore, in this embodiment, the electrical resistance of the sheet  100  is estimated or calculated using a value measured with the surface potential sensor  61  just before the printing operation. 
     In other words, if the applied voltage to the sheet conveying belt  31  (the first applied voltage) and the applied voltage to the sheet  100  (the second applied voltage) are fixed values, a transfer speed of the negative electric charge  700  can be estimated or calculated by a value measured with the surface potential sensor  61 . This is because the speed of the sheet  100  from the pressing roller  38 , which applies the voltage to the sheet  100 , to the surface potential sensor  61 , is constant. In addition, if the transfer speed of the negative electric charge  700  is known, the electrical resistance of the sheet  100  can be calculated. 
     Then, the electrical resistance of a subsequent sheet  100  can estimated or calculated by referring to the value of the second applied voltage applied to a previous sheet  100 , if the second applied voltage is changed according to a type of the subsequent sheet  100 , for example. 
     Moreover, if the electrical resistance of the sheet is known, a difference of the surface potential of the sheet  100  between the locations corresponding to the positions of the surface potential sensor  61  and the position of the printing head  24  can be predicted. In other words, adjustment to 0V of the surface potential of the sheet in the position of the printing head  24  is enabled by deciding a value (the target value) of the surface potential of the sheet in the position of the surface potential sensor  61  based on the predicted difference of the surface potential. 
     An example of the relationship between the electrical resistance of the sheet  100  and the target value of the surface potential (target value) of the sheet  100  at the location corresponding to the position of the surface potential sensor  61 , in order for the surface potential at the location corresponding to the position of a printing head  24  to 0V, is illustrated in  FIG. 9 . In addition, the value of the applied voltage to the sheet conveying belt  31  (the first applied voltage) is 2000V, and the value of the applied voltage to the sheet  100  (the second applied voltage) is 3000V in  FIG. 9 . The value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  determined using the electrical resistance of the sheet  100  that is estimated or calculated. If the electrical resistance of the sheet is 10 13  (Ω-cm), the surface potential of the sheet at the location corresponding to the position of the printing head  24  can be 0V by if the surface potential of the sheet  100  is 400V at the location corresponding to the surface potential sensor  61  as shown in  FIG. 9 . Thus, the applied voltage to the pressing roller  38  is controlled so that the value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  becomes 400V. Thereby, the value of the surface potential of the sheet  100  at the location corresponding to the position of the printing head  24  can be reliably adjusted to be 0V, without requiring the surface potential sensor  61  to be positioned under the printing head  24  to measure the surface potential of the sheet  100 . 
     The first applied voltage and the second applied voltage are generally fixed for each of the pressing roller  38 , and the pair of the electric charging rollers ( 39   a ,  39   b ). Therefore, the relationship between the target value of the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  and the measured value of the surface potential first measured with the surface potential sensor  61 , may be previously measured and stored in a data table. Using this data table, the target value can be determined based on the value of the second applied voltage and the measured value of the surface potential by the surface potential sensor  61  directly, without calculating the electrical resistance of the sheet. 
     Moreover, if the second applied voltage is changed according to a type of sheet as described above, relationships between the target value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61 , the second applied voltage to the pressing roller  38 , and the measured value of the surface potential first measured with the surface potential sensor  61 , may be measured and stored in a data table. 
     In this way, based on the measured value of the surface potential sensor  61 , when the first applied voltage is applied to electric charging rollers  39   a  and  39   b , and the second applied voltage is applied to the pressing roller  38 , the target value of the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  is determined, and the second applied voltage to the pressing roller  38  is adjusted so that surface potential detected by the surface potential sensor  61  becomes the target value. Hereby, the electric field under the printing head  24  can be restrained and the degradation of image quality by a reverse flow of ink mist toward the printing head  24  can be prevented. 
     Next, the duplex printing operation of the image forming apparatus is explained with reference to  FIG. 10 . Further, in duplex printing, “the first page” is defined as a side of the sheet  100  on which an earlier image is formed and “the second page” is defined as a side of the sheet  100  on which a later image is formed. A solid line of  FIG. 10  shows a relationship between an initial electrical resistance of the sheet (before printing the first page), and the target value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  when the first page is printed. On the other hand, the dot-dash line of  FIG. 10  shows the a relationship between the initial electrical resistance of the sheet (before printing the first page), and the target value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  when the second page is printed. 
     In this case, the electrical resistance of the sheet  100  is estimated or calculated before printing the first page, and the target values of both the first page and the second page are determined based on the electrical resistance. The detection of the electrical resistance of the sheet  100  may be carried out just before printing the second page. However, it is desirable that the target value of the second page is decided using an electrical resistance before printing the first page, because the electrical resistance of the sheet  100  becomes uneven under the influence of ink that is used in printing the first page. 
     The target value of the second page is set to a value (200V in  FIG. 10 ) that is lower than a target value of the first page (400V in  FIG. 10 ). A time when the second page is printed, includes ink from printing the first page, and the electrical resistance of the sheet  100  is decreased from an electrical resistance that is detected before printing the first page. As a result, the electric charge on the second page of the sheet  100  moves more easily, even if surface potential is low. 
     Next, a relationship between the thickness of the sheet  100  and a target value of the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  is explained with reference to  FIG. 11 . Specifically, examples of the relationship according to the thickness of a sheet  100  between the electrical resistance of the sheet  100  and the surface potential (target value) of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  so the surface potential at the location corresponding to the position of the printing head  24  is 0V, are shown in  FIG. 11 . The value of the applied voltage to the sheet conveying belt  31  (the first applied voltage) is 2000V, and the value of the applied voltage to the sheet  100  (the second applied voltage) is 3000V in  FIG. 11 . 
     When the electrical resistance of a sheet is 10 13  (Ω-cm), if the thickness of the sheet is 0.2 mm, the target value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  is set to 500V, and if the thickness of the sheet is 0.06 mm, the target value of the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  is set to 300V. Because it is difficult to move an electric charge in the sheet  100  from a surface facing the printing head  24  to a rear surface when the thickness of the sheet  100  is increased, it is necessary to set the target value of the surface potential to a higher value. 
     Therefore, adjustment to a more accurate surface potential is enabled by detecting thickness of the sheet  100 , and determining the applied voltage for the pressing roller  38  based on the thickness of the sheet  100 . In addition, a detection of the thickness of the sheet  100  may be judged from the input to the operation panel  222  of the image forming apparatus, or may be measured directly. 
     Next, an example of the surface potential control by the controller  200  is explained with reference to a flowchart in  FIGS. 12A and 12B . 
     The control of the surface potential of the sheet is started (S 1 ), then, upon receiving a sheet-feeding signal (S 2 ), the controller  200  applies a predetermined voltage value (for example, 2000V) to electric charging rollers  39   a  and  39   b (S 3 ), which charge the sheet conveying belt  31 , and applies a predetermined voltage value (for example, 3000V) (S 4 ) to the pressing roller  38 , which charges the sheet  100 . 
     Next, it is determined if the leading edge of the sheet  100  reaches the surface potential sensor  61  (S 5 ). If the leading edge has reached the surface potential sensor  61 , the surface potential of the sheet  100  is measured using the surface potential sensor  61  (S 6 ). 
     Then, the electrical resistance of the sheet  100  is estimated based on the predetermined voltage value applied to electric charging rollers  39   a  and  39   b , the predetermined voltage value applied to the pressing roller  38 , and a value of the surface potential measured by the surface potential sensor  61  (S 7 ). Furthermore, with reference to a data table stored beforehand, the target value of surface potential of the sheet  100  at the location along the image forming conveyance path  301  corresponding to the position of the surface potential sensor  61  is determined so that surface potential of the sheet at the location along the image forming conveyance path  301  corresponding to the position of the printing head  24  becomes 0V. 
     The voltage value applied to the pressing roller  38  (the second applied voltage), the surface potential of the sheet  100  measured by the surface potential sensor  61 , and the target value of the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  by which the surface potential of the sheet  100  at the location corresponding to the position of the printing head  24  becomes 0V, is stored in the data table. In addition, a correction value for the target value of the surface potential of the sheet based on information related to the thickness of a sheet  100  is also stored in the data table (S 8 ). 
     If the applied voltage to electric charging rollers  39   a  and  39   b  and the applied voltage to the pressing roller  38  are fixed values and do not fluctuate, the target value of the surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61  may be determined only based on a value of the surface potential measured by the surface potential sensor  61 . 
     Next, the controller  200  reads the data about the thickness of a sheet that is detected or was stored beforehand (S 8 ), the target value is revised based on the thickness of the sheet  100  (S 9 ), and a final target value is determined (S 10 ). 
     Then, the controller  200  controls the voltage applied to the pressing roller  38  the surface potential of the sheet  100  at the location corresponding to the position of the surface potential sensor  61  reaches the final target value (S 11 ). The value of surface potential of the sheet  100  at the location corresponding to the position of the printing head  24  can be adjusted to 0V by the controlled voltage applied to the pressing roller  38  in this way. 
     Next, the controller  200  determines whether a trailing edge of the sheet  100  arrives at the location corresponding to the surface potential sensor  61  (S 12 ), and controls a voltage applied to the pressing roller  38  until the trailing edge of the sheet  100  arrives at the surface potential sensor  61  so that surface potential of the sheet at the location corresponding to the position of the surface potential sensor  61 , is remains at the final target value. If there is no sheet-feeding signal after the trailing edge of the sheet reaches the surface potential sensor  61  (S 13 ), the control of the surface potential is ended. 
     The value of the voltage applied to the pressing roller  38  is controlled so that the surface potential of the sheet  100  at the location corresponding to the position of the printing head  24  is 0V as mentioned previously. However the value of the surface potential at the location corresponding printing head  24 , for which the target value is determined in order to provide, is not limited to a value of 0V. Rather, the target value can be determined so the surface potential at the location corresponding to the position of the printing head  24  is a value that is sufficient to prevent a reverse flow of ink mist toward the printing head  24 . 
     In the present disclosure, the material of the sheet is not limited to just paper and rather includes an OHP (overhead projector) sheet, cloth, glass, and a baseboard or the like. Further, the sheet includes material capable of attracting an ink drop and other liquid or the like, such as a direct printing medium, an indirect printing medium, a printing sheet, or a printing form, for example. Further, it is noted that in the present disclosure, image formation, recording, printing, imaging, and duplicating are used interchangeably. 
     It is also noted that the image forming apparatus represents a system that executes image formation by ejecting droplets onto a medium made of such as paper, yarn, fiber, fabric, leather, metal, plastic, glass, wood, and ceramics, for example. The image formation onto the medium includes forming specific images, such as a character or a figure, for example, as well as a non-specific image that is formed simply by landing droplets on a medium. 
     It is also noted that the ink is not particularly limited to so called ink unless particularly so described, and includes a DNA sample, resist, pattern material, and resin or the like. Specifically, the ink is a general term that represents liquid capable of forming an image, such as so called printing liquid, fixing operation processing liquid, or ordinary liquid, for example. 
     Further, the image forming apparatus includes both a serial type image forming apparatus and a line type image forming apparatus, unless otherwise described herein. 
     According to v present disclosure, an electric field generated under an image forming device (e.g. a printing head) can be effectively reduced with a simple configuration. That is, an image forming apparatus includes an image forming head that ejects droplets and forms an image on a printing medium; a conveyor that conveys the printing medium with the image in a conveying direction; at least one first electric charger that charges the conveyor; a second electric charger that charges the printing medium; a surface potential detector downstream of the second electric charger in the conveying direction that detects a surface potential of the printing medium; and a controller that adjusts a voltage applied to the second electric charger. The controller determines a target value for the surface potential of the printing medium at a location corresponding to a position of the surface potential detector based on a detection value detected by the surface potential detector in response to a first voltage being applied to the first electric charger and a second voltage being applied to the second electric charger. The controller adjusts the voltage applied to the second electric charger and the surface potential of the printing medium at the location corresponding to the position of the surface potential detector reaches the target value. 
     According to another aspect of the present disclosure, an electric field generated under an image forming device (e.g. a printing head) can be more effectively reduced according to a type of a printing medium. That is, the controller determines the target value based on the detection value and a value of the second voltage when the detection value is detected. 
     According to another aspect of the present disclosure, an electric field generated under an image forming device (e.g. a printing head) can be more effectively reduced without computing an electrical resistance value. That is a data table defines a relationship that is stored in the data table between the detection value, a voltage value of the second voltage at a time when a detection value is detected, and the target value. The controller determines the target value based on the voltage value of the second voltage, the detection value, and the relationship defined by the data table. 
     According to another aspect of the present disclosure, an electric field generated under an image forming device (e.g. a printing head) can be more effectively reduced without influence of an image formed previously on one side of a printing medium. That is the image forming unit forms a first image on a first side of the printing medium, a path switching nail switches the path of printing medium, and the image forming unit forms a second image on a second side of the printing medium. The controller determines a target value for a second side of the printing medium is less than a target value of a first side. 
     According to another aspect of the present disclosure, an electric field generated under an image forming device (e.g. a printing head) can be more effectively reduced even if a thickness of a printing medium varies. That is a thickness detector detects a thickness of the printing medium, and the target value is determined the detection value and the thickness of the printing medium. 
     Numerous additional modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present disclosure may be executed otherwise than as specifically described herein. For example, the order of steps for forming in the image forming apparatus is not limited to the above-described various embodiments and may be altered as appropriate.