Patent Publication Number: US-9409426-B2

Title: Image forming apparatus, method of controlling tension of recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. patent application Ser. No. 14/204,635 filed on Mar. 11, 2014, which claims priority to Japanese Patent Application No. 2013-058070, filed Mar. 21, 2013, which applications are expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a technology of controlling a tension of a recording medium in an image forming apparatus which detachably holds the recording medium on which an image is formed on a rotational shaft by winding up the recording medium in a roll shape. 
     2. Related Art 
     A recording apparatus in JP-A-10-086472 records an image using a printing unit with respect to continuous paper which is supported by a transport drum which is arranged between two transport rollers, while transporting the continuous paper by rotating the two transport rollers which wind up the continuous paper. In addition, in an apparatus which performs image forming on a recording medium such as continuous paper, it is possible to transport the recording medium using a so-called roll to roll method. In the roll to roll method, rotational shafts are respectively provided at both ends of a transport path of the recording medium, and both ends of the recording medium are supported by the rotational shafts, respectively, by being wound in a roll shape. In addition, the recording medium is transported from a roll which is supported by one rotational shaft to a roll which is supported by the other rotational shaft. In addition, in general, the recording medium is detachably supported by the rotational shaft so as to be easily exchanged. 
     Meanwhile, it is preferable to stabilize a support of a recording medium using a support member by applying a large tension to the recording medium which is supported by the support member in order to perform good image forming with respect to the recording medium which is supported by the support member such as a transport drum. For this reason, the recording medium is stretched between two rollers which interpose the support member therebetween using a large tension. However, the large tension is not only generated by a torque of rollers which interpose the support member therebetween, and is also generated subsidiarily by a torque of rotational shafts which support both ends of the recording medium. Accordingly, when forming an image, such a large torque is not necessary in each of rollers interposing the support member therebetween. 
     On the other hand, when detaching the recording medium from the rotational shafts, since applying of the tension to the recording medium using a rotational shaft from which the recording medium is detached, the rotational shaft does not support a generation of the tension with respect to the recording medium which is supported by the support member any more. As a result, for example, there is a concern that a detaching work of the recording medium may not be smoothly performed, or the like, because the roller on a side of the rotational shaft of which support is lost, and from which the recording medium is detached cannot resist the tension which is originally applied to the recording medium on the support member, and the recording medium may deviate. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a technology in which a deviation of a recording medium when detaching the recording medium from a rotational shaft which supports the recording medium of a roll shape can be suppressed, while executing good image forming by applying a large tension to the recording medium. 
     According to an aspect of the invention, there is provided an image forming apparatus which includes a rotational shaft which detachably holds a recording medium which is wound in a roll shape; a support member which supports the recording medium which comes out from a portion wound in the roll shape; a head which faces the support member, and performs image forming on the recording medium; a driving roller which winds up the recording medium between the rotational shaft and the support member; a holding unit which holds the recording medium by winding up the recording medium on a side opposite to the driving roller with respect to the support member; and a control unit which applies a tension to the recording medium in a first region between the rotational shaft and the driving roller using a torque which is applied to the rotational shaft and the driving roller, and applies a tension to the recording medium in a second region in which the holding member supports the recording medium using a torque which is applied to the driving roller and the holding unit, in which the control unit selectively executes an image forming mode in which the head is caused to execute image forming by applying a tension for image forming to the recording medium in the second region while applying a tension to the recording medium in the first region, and a medium detaching mode in which the tension of the recording medium in the first region is released in a state of stopping the recording medium while applying a tension for detaching a medium which is smaller than the tension for image forming to the recording medium in the second region. 
     According to another aspect of the invention, there is provided a method of controlling a tension of a recording medium in an image forming apparatus which performs image forming using a head which faces a support member with respect to the recording medium which is supported by the support member by coming out from a portion which is wound in a roll shape, while detachably holding the recording medium which is wound in the roll shape on a rotational shaft, the method including applying a tension using a torque which is applied to the rotational shaft and a driving roller with respect to the recording medium in a first region between the driving roller which winds up the recording medium between the rotational shaft and the support member, and applying a tension for image forming with respect to the recording medium in a second region in which the support member supports the recording using a torque which is applied to a holding unit which holds the recording medium by winding up the recording medium on a side opposite to the driving roller with respect to the support member; and releasing the tension of the recording medium in the first region in a state of stopping the recording medium while applying a tension for detaching a medium which is smaller than the tension for image forming to the recording medium in the second region. 
     In the invention which is configured in this manner (image forming apparatus, and method of controlling tension of recording medium), the recording medium which is wound in a roll shape is detachably supported on the rotational shaft. In addition, a recording medium which comes out from a portion wound in the roll shape is supported by the support member. In addition, a driving roller which winds up the recording medium between the rotational shaft and the support member, and a holding unit which holds the recording medium by winding up the recording medium on a side opposite to the driving roller with respect to the support member are provided. Accordingly, a tension with respect to the recording medium in the first region between the driving roller and the rotational shaft is generated by a torque which is applied to the driving roller and the rotational shaft, and a tension with respect to the recording medium in the second region in which the support member supports the recording medium is generated by a torque which is applied to the driving roller and the holding unit. 
     In addition, a head performs image forming on the recording medium which is supported by the support member in a state in which the tension for image forming is applied to the recording medium in the second region, while a tension is applied to the recording medium in the first region. At this time, the tension for image forming is not only generated by the torque of the driving roller, but also subsidiarily generated by the torque of the rotational shaft. That is, the tension for image forming is applied to the recording medium when the torques of the driving roller and the rotational shaft resist the torque of the holding unit in collaboration. As a result, it is possible to apply a large tension for image forming to the recording medium which is supported by the support member without applying such a large torque to the driving roller, and to perform good image forming. 
     In addition, according to the aspect of the invention, it is possible to release the tension of the recording medium in the first region in a state of stopping the recording medium while applying a tension for detaching medium which is smaller than the tension for image forming to the recording medium in the second region (medium detaching mode). In the medium detaching mode, releasing of the tension of the recording medium in the first region is performed in a state in which a tension for detaching medium which is relatively small is applied to the recording medium in the second region. Accordingly, a tension to be resisted by the driving roller when releasing the tension of the recording medium in the first region is a tension for detaching medium which is relatively smaller than the tension form image forming. For this reason, it is possible for the driving roller to suppress a deviation of the recording medium by resisting the tension which is applied to the recording medium on the support member (tension for detaching medium), even when the driving roller looses the support from the rotational shaft along with releasing of the tension in the first region. 
     According to such an aspect of the invention, it is possible to perform good image forming by applying a large tension for image forming to a recording medium when performing image forming. On the other hand, when detaching the recording medium from a rotational shaft, it is possible to perform detaching of the recording medium while suppressing a deviation of the recording medium associated with releasing of a tension of the recording medium by executing a medium detaching mode. As a result, it is possible to suppress a deviation of the recording medium when detaching the recording medium from the rotational shaft which supports the recording medium which is wound in a roll shape, while performing good image forming by applying a tension for image forming to the recording medium which is supported by a support member. 
     The image forming apparatus may further include an external member which accommodates the rotational shaft; an opening-shutting door which opens or shuts an opening portion which is open to the rotational shaft, and is provided in the external member; and a locking mechanism which locks the opening-shutting door, in which the control unit may control the locking mechanism so that the opening-shutting door is locked until the tension of the recording medium in the first region is released, and the locking of the opening-shutting door is released after releasing the tension of the recording medium in the first region, during an execution of the medium detaching mode. With such a configuration, it is possible to prevent a worker from accessing the recording medium before the tension thereof is released. For this reason, it is possible to prevent the recording medium or the rotational shaft from being damaged when the worker tries to forcibly detach the recording medium before releasing tension from the rotational shaft, for example. 
     In the image forming apparatus, the control unit may lock the opening-shutting door during the execution of the image forming mode. With such a configuration, it is possible to prevent the worker from accessing the recording medium on which image forming is performed. For this reason, for example, it is possible to prevent the worker from disturbing the image forming on the recording medium by accessing the recording medium in the middle of image forming by mistake. 
     The image forming apparatus may further include an input unit which receives an input from the worker, in which the control unit may execute the medium detaching mode when the input unit receives an input instructing executing of the medium detaching mode. With such a configuration, it is possible to perform releasing of the tension of the recording medium by executing the medium detaching mode at an appropriate timing which corresponds to detaching of the recording medium by the worker. For this reason, it is possible for the worker to improve workability. 
     In the image forming apparatus, the control unit may apply the tension to the recording medium in the first region when the input unit receives an input denoting that exchanging of the recording medium with respect to the rotational shaft is completed, after executing the medium detaching mode. In this manner, for example, when the worker finishes an exchanging work of mounting a new recording medium on the rotational shaft by detaching the previous recording medium from the rotational shaft, it is possible to rapidly apply a tension to the recording medium in the first region, and to be prepared for image forming to be executed thereafter. 
     In the image forming apparatus, the control unit may apply the tension for image forming to the recording medium in the second region when the input unit receives an input denoting that exchanging of the recording medium with respect to the rotational shaft is finished, after executing the medium detaching mode. In this manner, for example, when the worker finishes the exchanging work of mounting a new recording medium on the rotational shaft by detaching the previous recording medium from the rotational shaft, it is possible to rapidly apply a tension to the recording medium in the second region, and to be prepared for image forming to be executed thereafter. 
     In the image forming apparatus, the control unit may transport the recording medium toward the holding unit from the driving roller while feeding the recording medium from the rotational shaft, in the image forming mode. With such a configuration, it is possible to suppress a deviation of the recording medium which is associated with releasing of tension of the recording medium when detaching the recording medium from the rotational shaft which feeds the recording medium. 
     In the image forming apparatus, the control unit may transport the recording medium toward the driving roller from the holding unit while winding the recording medium around the rotational shaft. With such a configuration, it is possible to suppress a deviation of the recording medium which is associated with releasing of tension of the recording medium when detaching the recording medium from the rotational shaft which winds up the recording medium. 
     In addition, for the control of the driving roller in the image forming mode, various modes can be taken into consideration. Therefore, the control unit may control a torque of the driving roller in the image forming mode. Alternatively, the control unit may control a rotating speed of the driving roller in the image forming mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a front view which illustrates an internal configuration of a printer to which the invention can be applied. 
         FIG. 2  is a perspective view which illustrates an external configuration of the printer illustrated in  FIG. 1 . 
         FIG. 3  is a block diagram which illustrates an electrical configuration which controls the printer illustrated in  FIG. 1 . 
         FIG. 4  is a flowchart which illustrates operations which can be executed in the printer illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a front view which schematically illustrates an example of an internal configuration of a printer to which the invention can be applied. As illustrated in  FIG. 1 , in a printer  1 , one sheet S (web) of which both ends are wound in a roll shape around a feeding shaft  20  and a winding shaft  40  is stretched between the feeding shaft  20  and the winding shaft  40 , and the sheet S is transported to the winding shaft  40  from the feeding shaft  20  along a transport path Pc which stretches in this manner. In other words, a feed roll R 20  and a windup roll R 40  are formed when the both ends of the sheet S in the transport path Pc are wound in a roll shape, respectively, and the sheet S is transported using a roll to roll method from the feed roll R 20  which is pivotally supported by the feeding shaft  20  to the windup roll R 40  which is pivotally supported by the winding shaft  40 . 
     In addition, in the printer  1 , an image is recorded with respect to the sheet S which is transported along the transport path Pc. A type of the sheet S is largely classified into paper types and film types. As a specific example, there is fine quality paper, cast coated paper, art paper, coated paper, or the like, for the paper types, and there is synthetic paper, Polyethylene terephthalate (PET), polypropylene (PP), or the like, for the film types. Schematically, the printer  1  includes a feeding unit  2  (feeding region) which feeds the sheet S from the feeding shaft  20 , a process unit  3  (process region) which records an image on the sheet S which is fed from the feeding unit  2 , and a winding unit  4  (winding region) which winds the sheet S on which an image is recorded in the process unit  3  around the winding shaft  40 . In addition, in the following descriptions, a surface on which an image is recorded is referred to as the front surface, and on the other hand, a surface which is opposite to the front surface is referred to as the rear surface, regarding both surfaces of the sheet S. 
     The feeding unit  2  includes the feeding shaft  20  which winds an end of the sheet S, and a driven roller  21  which winds up the sheet S which is drawn out from the feeding shaft  20 . The feeding shaft  20  supports the end of the sheet S by winding the sheet in a state in which the front surface of the sheet S is caused to face the outside. In addition, when the feeding shaft  20  rotates clockwise as illustrated in  FIG. 1 , the sheet S which is wound around the feeding shaft  20  is fed to the process unit  3  via the driven roller  21 . Incidentally, the sheet S is wound around the feeding shaft  20  through a core tube  22  which is detachable from the feeding shaft  20 . Accordingly, when the sheet S of the feeding shaft  20  is used up, it is possible to exchange the sheet S of the feeding shaft  20  by mounting a new core tube  22  on which a roll shaped sheet S (feed roll R 20 ) is wound on the feeding shaft  20 . 
     The process unit  3  performs recording of an image on the sheet S by appropriately performing processes using each of functional units  51 ,  52 ,  61 ,  62 , and  63  which are arranged along the outer peripheral surface of a platen drum  30  while supporting the sheet S which is fed from the feeding unit  2  on the platen drum  30 . The process unit  3  is provided with a front driving roller  31  and a rear driving roller  32  on both sides of the platen drum  30 , and the sheet S which is transported from the front driving roller  31  to the rear driving roller  32  is supported by the platen drum  30 , and has an image recorded thereon. 
     The front driving roller  31  includes a plurality of minute protrusions which are formed using thermal spraying on the outer peripheral surface, and winds up the sheet S which is fed from the feeding unit  2  from the rear surface side. In addition, the front driving roller  31  transports the sheet S which is fed from the feeding unit  2  to the downstream side of a transport path Pc by rotating clockwise as illustrated in  FIG. 1 . In addition, a nip roller  31   n  is provided with respect to the front driving roller  31 . The nip roller  31   n  comes into contact with the front surface of the sheet S in a state of being urged to the front driving roller  31  side, and interposes the sheet S between the nip roller and the front driving roller  31 . In this manner, a friction force between the front driving roller  31  and the sheet S is secured, and it is possible to reliably transport the sheet S using the front driving roller  31 . 
     The platen drum  30  is a cylindrical drum of which a diameter is 400 mm, for example, is rotatably supported by a not shown support mechanism, and winds up the sheet S which is transported from the front driving roller  31  to the rear driving roller  32  from the rear surface side. The platen drum  30  supports the sheet S from the rear surface side while rotating in the transport direction Ds of the sheet S in a driven manner by receiving a friction force between the platen drum and the sheet S. Incidentally, the process unit  3  is provided with driven rollers  33  and  34  which replicate the sheet S on both sides of a winding portion with respect to the platen drum  30 . The driven roller  33  in the driven rollers replicates the sheet S by winding up the front surface of the sheet S between the front driving roller  31  and the platen drum  30 . On the other hand, the driven roller  34  replicates the sheet S by winding up the front surface of the sheet S between the platen drum  30  and the rear driving roller  32 . In this manner, it is possible to secure a long winding portion of the sheet S with respect to the platen drum  30  by replicating the sheet S on the respective upstream side and downstream side in the transport direction Ds with respect to the platen drum  30 . 
     The rear driving roller  32  includes a plurality of minute protrusions which are formed using thermal spraying on the outer peripheral surface, and winds up the sheet S which is transported from the platen drum  30  through the driven roller  34  from the rear surface side. In addition, the rear driving roller  32  transports the sheet S to the winding unit  4  by rotating clockwise as illustrated in  FIG. 1 . In addition, a nip roller  32   n  is provided with respect to the rear driving roller  32 . The nip roller  32   n  comes into contact with the front surface of the sheet S in a state of being urged to the rear driving roller  32  side, and interposes the sheet S between the nip roller and the rear driving roller  32 . In this manner, a friction force between the rear driving roller  32  and the sheet S is secured, and it is possible to reliably transport the sheet S using the rear driving roller  32 . 
     In this manner, the sheet S which is transported from the front driving roller  31  to the rear driving roller  32  is supported by the outer peripheral surface of the platen drum  30 . In addition, the process unit  3  is provided with a plurality of recording heads  51  corresponding to colors which are different from each other in order to record a color image on the front surface of the sheet S which is supported by the platen drum  30 . Specifically, four recording heads  51  corresponding to a yellow color, a cyan color, a magenta color, and a black color are aligned in the transport direction Ds in this order of the colors. Each recording head  51  faces the front surface of the sheet S which is wound around the platen drum  30  with a little clearance, and ejects ink (color ink) of a corresponding color from nozzles in an ink jet method. In addition, a color image is formed on the front surface of the sheet S when each recording head  51  ejects ink on the sheet S which is transported in the transport direction Ds. 
     Incidentally, Ultraviolet (UV) ink (photocurable ink) which is cured by being irradiated with ultraviolet rays (light) is used as ink. Therefore, UV irradiators  61  and  62  (light irradiation unit) are provided in the process unit  3  in order to fix ink onto the sheet S by curing the ink. In addition, curing of the ink is executed in two stages of temporary curing and main curing. The UV irradiator  61  for temporary curing is arranged between each of the plurality of recording heads  51 . That is, the UV irradiator  61  cures ink to an extent of not being collapsed in shape (temporary curing) by irradiating weak UV rays, and does not cure ink completely. On the other hand, the UV irradiator  62  for main curing is provided on the downstream side in the transport direction Ds with respect to the plurality of recording heads  51 . That is, the UV irradiator  62  completely cures (main curing) ink by irradiating UV rays which are stronger than those of the UV irradiator  61 . 
     In this manner, the UV irradiator  61  which is arranged between each of the plurality of recording heads  51  temporarily cures color ink which is ejected onto the sheet S from the recording head  51  on the upstream side in the transport direction Ds. Accordingly, ink which is ejected onto the sheet S from one recording head  51  is temporarily cured until reaching a recording head  51  which is neighboring to the one recording head  51  on the downstream side in the transport direction Ds. In this manner, it is possible to suppress occurrence of color mixing in which color inks of different colors are mixed. A color image is formed on the sheet S when the plurality of recording heads  51  eject ink of colors which are different from each other in a state of suppressing the color mixing in this manner. In addition, the UV irradiator  62  for main curing is provided on the further downstream side in the transport direction Ds than the plurality of recording heads  51 . For this reason, the color image which is formed using the plurality of recording heads  51  is fixed onto the sheet S by being subject to main curing by the UV irradiator  62 . 
     In addition, a recording head  52  is provided on the downstream side in the transport direction Ds with respect to the UV irradiator  62 . The recording head  52  faces the front surface of the sheet S which is wound around the platen drum  30  with a little clearance, and ejects transparent UV ink onto the front surface of the sheet S from nozzles in an ink jet method. That is, the transparent ink is further ejected onto the color image which is formed using the recording heads  51  of four colors. The transparent ink is ejected onto the entire surface of the color image, and gives the color image texture such as glossiness, or a mat look. In addition, a UV irradiator  63  is provided on the downstream side in the transport direction Ds with respect to the recording head  52 . The UV irradiator  63  completely cures (main curing) the transparent ink which is ejected from the recording head  52  by irradiating strong UV rays. In this manner, it is possible to fix the transparent ink onto the front surface of the sheet S. 
     In this manner, the color image which is coated with the transparent ink is formed on the sheet S which is wound around the outer peripheral portion of the platen drum  30  by appropriately performing ejecting and curing of ink in the process unit  3 . In addition, the sheet S on which the color image is formed is transported to the winding unit  4  by the rear driving roller  32 . 
     The winding unit  4  includes a driven roller  41  which winds up the sheet S from the rear surface side between the winding shaft  40  and the rear driving roller  32 , in addition to the winding shaft  40 . The winding shaft  40  supports an end of the sheet S by winding up in a state in which the front surface of the sheet S faces outside. That is, when the winding shaft  40  rotates clockwise as illustrated in  FIG. 1 , the sheet S which is transported from the rear driving roller  32  is wound around the winding shaft  40  through the driven roller  41 . That is, the sheet S is wound around the winding shaft  40  through a core tube  42  which is detachable from the winding shaft  40 . Accordingly, when the sheet S which is wound around the winding shaft  40  (windup roll R 40 ) is full, it is possible to detach the sheet S together with the core tube  42 . 
       FIG. 2  is a perspective view which illustrates an example of an external configuration of the printer in  FIG. 1 . In  FIG. 2 , only the feeding shaft  20 , the winding shaft  40 , the feed roll R 20 , and the windup roll R 40  are denoted by dotted lines in the internal configuration of the printer  1 . As illustrated in  FIG. 2 , the printer  1  includes a housing member  7  (external member) which accommodates each unit illustrated in  FIG. 1 . The housing member  7  is configured of a feeding unit cover  72  which is provided on the left side in  FIG. 2 , and mainly covers the feeding unit  2 , a process unit cover  73  which is provided in a center in  FIG. 2 , and mainly covers the process unit  3 , and a winding unit cover  74  which is provided on the right side in  FIG. 2 , and mainly covers the winding unit  4 . 
     The feeding unit cover  72  accommodates the feeding shaft  20  and the feed roll R 20 . An opening portion  721  opens on the front side of the feeding unit cover  72  by facing the feeding shaft  20  and the feed roll R 20 . In addition, a feeding unit door  723  which opens and shuts the opening portion  721  is provided on the front side of the feeding unit cover  72 . Accordingly, when opening the feeding unit door  723 , a worker can perform a work such as exchanging of the feed roll R 20  with respect to the feeding shaft  20  by accessing the feeding unit  2 . On the other hand, by shutting the feeding unit door  723 , access to the feeding unit  2  by the worker can be prevented. 
     The winding unit cover  74  accommodates the winding shaft  40  and the windup roll R 40 . An opening portion  741  opens on the front side of the winding unit cover  74  by facing the winding shaft  40  and the windup roll R 40 . In addition, a winding unit door  743  which opens and shuts the opening portion  741  is provided on the front side of the winding unit cover  74 . Accordingly, when opening the winding unit door  743 , a worker can perform a work such as exchanging of the windup roll R 40  with respect to the winding shaft  40  by accessing the winding unit  4 . On the other hand, by shutting the winding unit door  743 , access to the winding unit  4  by the worker can be prevented. 
     Hitherto, a schematic apparatus configuration of the printer  1  has been described. Subsequently, an electrical configuration for controlling the printer  1  will be described.  FIG. 3  is a block diagram which schematically illustrates an electrical configuration for controlling the printer in  FIG. 1 . As illustrated in  FIG. 3 , a printer control unit  200  which controls each unit of the printer  1  is provided in the printer  1 . Specifically, the printer control unit  200  is a computer which is configured of a Central Processing Unit (CPU), or a memory. 
     The printer  1  is provided with a monitor  210  which is configured of a liquid crystal display, or the like, as an interface between a worker and the printer control unit  200 , and an operation unit  220  which is configured of a keyboard, a mouse, or the like. A menu screen is displayed on the monitor  210 , in addition to an image as a printing target. Accordingly, a worker can set various printing conditions such as a type of a printing medium, a size of the printing medium, and a printing quality by opening a printing setting screen from the menu screen, by operating the operation unit  220  while confirming on the monitor  210 . In addition, the worker can also perform an input of instructing an execution of image forming, an input which informs of performing an exchanging work of the feed roll R 20  or the windup roll R 40 , or the like, with respect to the printer control unit  200  through the monitor  210 , or the operation unit  220 . In addition, a specific configuration of the interface with the worker can be variously modified, and the operation unit  220  may be configured of a touch panel of the monitor  210 , using a touch panel display as the monitor  210 , for example. In addition, the printer control unit  200  controls each unit of the printer  1  as follows according to an input from the worker. 
     The printer control unit  200  controls an ink ejecting timing of each of the recording heads  51  which forms a color image according to a transport of the sheet S. Specifically, the control of the ink ejecting timing is performed based on an output of a drum encoder E 30  (detection value) which is attached to a rotational shaft of the platen drum  30 , and detects a rotating position of the platen drum  30 . That is, since the platen drum  30  rotates along with the transport of the sheet S in a driven manner, it is possible to ascertain a transport position of the sheet S by referring to an output of the drum encoder E 30  which detects a rotating position of the platen drum  30 . Therefore, the printer control unit  200  forms a color image by causing ink which is ejected from each recording head  51  to be landed on a target position of the transported sheet S, by generating a print timing signal (pts) from the output of the drum encoder E 30 , and controlling an ink ejecting timing of each recording head  51  based on the pts signal. 
     In addition, also a timing of ejecting the transparent ink by the recording head  52  is controlled by the printer control unit  200  based on an output of the drum encoder E 30 , similarly. In this manner, it is possible to accurately eject the transparent ink with respect to a color image which is formed by the plurality of recording heads  51 . In addition, a timing of ON-OFF of the UV irradiators  61 ,  62 , and  63 , or an irradiation light amount is also controlled by the printer control unit  200 . 
     The printer control unit  200  also controls locking states of the feeding unit door  723  and the winding unit door  743  which are illustrated in  FIG. 2 . That is, a locking mechanism for feeding unit door  725  for executing locking and unlocking of the feeding unit door  723  is provided in the feeding unit cover  72 . Accordingly, an operation of opening a shut feeding unit door  723  is prevented while the locking mechanism for feeding unit door  725  is locking the feeding unit door  723 , and is possible only when the locking mechanism for feeding unit door  725  has unlocked the feeding unit door  723 . In addition, locking and unlocking of the feeding unit door  723  can be performed when the printer control unit  200  controls the locking mechanism for feeding unit door  725 . Similarly, a locking mechanism for winding unit door  745  for executing locking and unlocking of the winding unit door  743  is provided in the winding unit cover  74 . Accordingly, an operation of opening a shut winding unit door  743  is prevented while the locking mechanism for winding unit door  745  is locking the winding unit door  743 , and is possible only when the locking mechanism for winding unit door  745  has unlocked the winding unit door  743 . In addition, locking and unlocking of the winding unit door  743  is performed when the printer control unit  200  controls the locking mechanism for winding unit door  745 . 
     In addition, the printer control unit  200  conducts a function of controlling the transport of the sheet S which is described in detail in  FIG. 1 . That is, a motor is connected to the respective feeding shaft  20 , the front driving roller  31 , the rear driving roller  32 , and the winding shaft  40  among members which configure a sheet transport system. In addition, the printer control unit  200  controls the transport of the sheet S by controlling a speed or a torque of each motor while rotating these motors. The control of the transport of the sheet S will be described in detail below. 
     The printer control unit  200  supplies the sheet S from the feeding shaft  20  to the front driving roller  31  by rotating a feeding motor M 20  which drives the feeding shaft  20 . At this time, the printer control unit  200  controls a torque of the feeding motor M 20 , and adjusts a tension of the sheet S from the feeding shaft  20  to the front driving roller  31  (feeding tension Ta). That is, a tension sensor S 21  which detects the feeding tension Ta is attached to the driven roller  21  which is arranged between the feeding shaft  20  and the front driving roller  31 . The tension sensor S 21  can be configured of, for example, a load cell which detects a power which is received from the sheet S. In addition, the printer control unit  200  performs a feedback control of the torque of the feeding motor M 20  based on a detection result of the tension sensor S 21 , and adjusts the feeding tension Ta of the sheet S. 
     In addition, the printer control unit  200  rotates a front driving motor M 31  which drives the front driving roller  31 , and a rear driving motor M 32  which drives the rear driving roller  32 . In this manner, the sheet S which is fed from the feeding unit  2  passes through the process unit  3 . At this time, a speed control is performed with respect to the front driving motor M 31 , and on the other hand, a torque control is performed with respect to the rear driving motor M 32 . That is, the printer control unit  200  constantly adjusts a rotating speed of the front driving motor M 31  based on an encoder output of the front driving motor M 31 . In this manner, the sheet S is transported at a constant speed by the front driving roller  31 . 
     On the other hand, the printer control unit  200  controls a torque of the rear driving motor M 32 , and adjusts a tension of the sheet S from the front driving roller  31  to the rear driving roller  32  (process tension Tb). That is, a tension sensor S 34  which detects the process tension Tb is attached to a driven roller  34  which is arranged between the platen drum  30  and the rear driving roller  32 . The tension sensor S 34  can be configured of, for example, a load cell which detects a power which is received from the sheet S. In addition, the printer control unit  200  performs a feedback control of the torque of the rear driving motor M 32  based on a detection result of the tension sensor S 34 , and adjusts the process tension Tb of the sheet S. 
     In addition, the printer control unit  200  rotates a winding motor M 40  which drives the winding shaft  40 , and winds the sheet S which is transported by the rear driving roller  32  around the winding shaft  40 . At this time, the printer control unit  200  controls a torque of the winding motor M 40 , and adjusts a tension of the sheet S from the rear driving roller  32  to the winding shaft  40  (winding tension Tc). That is, a tension sensor S 41  which detects the winding tension Tc is attached to the driven roller  41  which is arranged between the rear driving roller  32  and the winding shaft  40 . The tension sensor S 41  can be configured of, for example, a load cell which detects a power which is received from the sheet S. In addition, the printer control unit  200  performs a feedback control of the torque of the winding motor M 40  based on a detection result of the tension sensor S 41 , and adjusts the winding tension Tc of the sheet S. 
       FIG. 4  is a flowchart which illustrates an example of operations which can be executed in the printer in  FIG. 1 , and specifically exemplifies a case in which the printer control unit  200  executes an image forming mode, or a roll exchanging mode from a standby state. That is, the printer control unit  200  confirms a presence or absence of an input denoting an instruction of executing an image forming mode, or an input denoting an instruction of executing a roll changing mode in the standby state (steps S 101  and S 102 ). In addition, when there is any input, the printer control unit  200  executes the mode. Incidentally, in the standby state, either the feeding unit door  723 , or the winding unit door  743  is shut and locked. 
     In step S 101 , whether or not there is the input of instructing executing of the image forming mode from a worker through the monitor  210 , or the operation unit  220  is confirmed. In addition, when there is the input of instructing executing of the image forming mode (Yes in step S 101 ), steps S 103  to S 105  are sequentially executed. Specifically, in step S 103 , the transport of the sheet S is started. The transport of the sheet is executed while applying predetermined tensions Ta, Tb, and Tc to the sheet S in each of the feeding unit  2 , the process unit  3 , and the winding unit  4 . In this manner, it is possible to stably transport the sheet S from the feeding shaft  20  to the winding shaft  40 , and it is possible to securely support the sheet S using the platen drum  30  by causing the sheet S to come into close contact with the platen drum  30 , in the process unit  3 . As a result, it is possible to stably perform image forming on the sheet S. Specifically, the process tension Tb which is applied to the sheet S in the process unit  3  is set to a tension for image forming Th which is relatively large. In this manner, it is possible to perform image forming on the sheet S in a state in which the sheet S is stably supported by the platen drum  30  by causing the sheet S to come into close contact with the platen drum  30 . 
     When a speed of transporting the sheet S which is started in step S 103  becomes stable at a predetermined transport speed for image forming, the image forming mode in step S 104  is executed, and image forming is performed when the recording heads  51  and  52 , and the UV irradiators  61  to  63  are operated using the above described manner. In addition, when the image forming mode in step S 104  is completed, the transport of the sheet S is stopped in step S 105 . At this time, each tension Ta, Tb, and Tc which is applied to the sheet S after stopping is the same as each tension Ta, Tb, and Tc which is applied to the sheet S while executing the image forming mode. Accordingly, in a standby state, the process tension Tb which is applied to the sheet S becomes the tension for image forming Th. When steps S 103  to  105  are finished, the printer control unit  200  returns to the standby state (steps S 101  and S 102 ). In addition, both the feeding unit door  723  and the winding unit door  743  are shut and locked during executing of steps S 103  to S 105 . 
     In step S 101 , when it is determined that there is no input of instructing executing of the image forming mode (determining as No), the process proceeds to step S 102 , and whether or not there is an input of instructing executing of the roll changing mode from a worker through the monitor  210 , or the operation unit  220  is confirmed. When there is no input of instructing executing of the roll changing mode (No in step S 102 ), the printer control unit  200  returns to step S 101 , and is in a standby state. 
     On the other hand, when there is the input of instructing executing of the roll changing mode (Yes in step S 102 ), the printer control unit  200  executes the roll changing mode in steps S 106  to S 109 . That is, a worker can inform the printer control unit  200  of performing exchanging of the feed roll R 20  or the windup roll R 40  by performing the input. In addition, when the printer control unit  200  which received the information executes the roll changing mode, a preparation for exchanging the feed roll R 20  or the windup roll R 40  is completed. 
     Incidentally, a worker can instruct the roll exchange while specifying a roll to be exchanged between the feed roll R 20  and the windup roll R 40 . With respect to this, a series of operations which is executed by receiving the instruction (steps S 106  to S 115 ) is basically the same even though operation targets are different between the roll R 20  and the roll R 40 . Therefore, in the following descriptions, points when a worker informs of exchanging of the windup roll R 40  will be described, after describing a case in which the worker informs of exchanging of the feed roll R 20 . 
     In step S 106 , the process tension Tb is changed from the tension for image forming Th to a tension for detaching medium Tl. The tension for detaching medium Tl is set to a value which is smaller than the tension for image forming Th, and is larger than zero (Th&gt;Tl&gt;0). By applying the tension for detaching medium Tl to the sheet S of the process unit  3 , it is possible to suppress the process tension Tb of the sheet S to be low while causing the sheet S to come into close contact with the platen drum  30 . In step S 107 , an output of the feeding motor M 20  is stopped. In this manner, the torque which is applied to the feeding shaft  20  by the feeding motor M 20  is lost (become zero), and the feeding tension Ta is released (Ta=0). In addition, even after the releasing of the feeding tension Ta, the process tension Tb which is set to the tension for detaching medium Tl is continuously applied to the sheet S in the process unit  3 . In addition, subsequent to cutting of the excitation of the feeding motor M 20  in step S 108 , locking of the feeding unit door  723  is released in step S 109  (that is, unlocked). In this manner, when the roll exchanging mode is completed, the worker can access the feeding unit  2  by opening the feeding unit door  723 . 
     That is, in step S 110 , the worker detaches the sheet S from the feeding shaft  20  by accessing the feeding shaft  20  of the feeding unit  2 , and mounts a new feed roll R 20  on the feeding shaft  20 . In addition, the worker connects an end of the sheet S which is detached from the feeding shaft  20  to an end of the feed roll R 20  which is newly mounted on the feeding shaft  20 , and shuts the feeding unit door  723 . In this manner, exchanging of the feed roll R 20  which is supported by the feeding shaft  20  is completed. In the subsequent step S 111 , the worker performs an input denoting that the exchanging of the feed roll R 20  is completed with respect to the printer control unit  200  through the monitor  210 , or the operation unit  220 . 
     When receiving the input denoting the completion of roll exchanging from the worker, the printer control unit  200  excites the feeding motor M 20  in step S 113 , after locking the feeding unit door  723  in step S 112 . In the subsequent step S 114 , an output of the feeding motor M 20  is started, and the process tension Tb is applied to the sheet S of the feeding unit  2 . A feeding tension Ta at this time has the same magnitude as the feeding tension Ta in the image forming mode. In addition, the printer control unit  200  changes the process tension Tb from the tension for detaching medium Tl to the tension for image forming Th (step S 115 ), and returns to a standby state. 
     Hitherto, contents of steps S 106  to S 115  in a case of being informed of exchanging of the feed roll R 20  by the worker has been described. As described above, contents of steps S 106  to S 115  in a case of being informed of exchanging of the windup roll R 40  by the worker are also the same. That is, the output of the winding motor M 40  is stopped in step S 107  after the process tension Tb is reduced to the tension for detaching medium Tl in step S 106 , and then the winding tension Tc is released. In addition, the excitation of the winding motor M 40  is cut (step S 108 ), and the winding unit door  743  is unlocked (step S 109 ). In this manner, the roll exchanging mode is executed. 
     When the roll exchanging mode is completed, and access to the winding unit  4  becomes possible, the worker detaches the windup roll R 40  from the winding shaft  40  in step S 110 , attaches the end of the sheet S which is drawn in from the process unit  3  to the winding shaft  40  in a roll shape, and shuts the winding unit door  743 . Subsequently, when the worker performs an input denoting that the roll exchange is completed (step S 111 ), the winding unit door  743  is locked (step S 112 ), and the winding motor M 40  is excited (step S 113 ). In addition, an output of the winding motor M 40  is started, and the same winding tension Tc as the image forming mode is applied to the sheet S of the winding unit  4  (step S 114 ). Finally, the printer control unit  200  increases the process tension Tb to the tension for image forming Th (step S 115 ), and returns to a standby state. 
     As described above, in the printer  1  according to the embodiment, the recording heads  51  and  52  perform image forming on the sheet S which is supported by the platen drum  30  in a state in which the tension for image forming Th is applied to the sheet S of the process unit  3 , while the feeding tension Ta is applied to the sheet S of the feeding unit  2 . At this time, the tension for image forming Th is not only generated by the torque of the front driving roller  31 , and is also subsidiarily generated by the torque of the feeding shaft  20 . That is, the tension for image forming Th is applied to the sheet S when the torques of the front driving roller  31  and the feeding shaft  20  resist the torques of the rear driving roller  32  and the winding shaft  40  in collaboration. As a result, it is possible to apply a large tension for image forming Th to the sheet S which is supported by the platen drum  30  without applying such a large torque to the front driving roller  31 , and to perform good image forming. 
     In addition, in the printer  1  according to the embodiment, it is possible to release the tension of the sheet S in the feeding unit  2 , in a state in which the sheet S is stopped, while applying the tension for detaching medium Tl which is smaller than the tension for image forming Th to the sheet S in the process unit  3  (roll exchanging mode). In the roll exchanging mode, releasing of the tension of the sheet S in the feeding unit  2  is executed, in a state in which a tension for detaching medium Tl which is relatively small is applied to the sheet S in the process unit  3 . Accordingly, when releasing the tension of the sheet S in the feeding unit  2 , a tension to be resisted by the front driving roller  31  is a tension for detaching medium Tl which is smaller than the tension for image forming Th. For this reason, it is possible for the front driving roller  31  to suppress a deviation of the sheet S by resisting the tension which is applied to the sheet S on the platen drum  30  (tension for detaching medium Tl), even when the front driving roller  31  looses the support from the feeding shaft  20  along with releasing of the tension in the feeding unit  2 . 
     In addition, in the printer  1  according to the embodiment, the recording heads  51  and  52  perform image forming on the sheet S which is supported by the platen drum  30  in a state in which the tension for image forming Th is applied to the sheet S in the process unit  3  while the winding tension Tc is applied to the sheet S in the winding unit  4 . At this time, the tension for image forming Th is not only generated by the torque of the rear driving roller  32 , and is also subsidiarily generated by the torque of the winding shaft  40 . That is, the tension for image forming Th is applied to the sheet S when the torques of the rear driving roller  32  and the winding shaft  40  resist the torques of the front driving roller  31  and the feeding shaft  20  in collaboration. As a result, it is possible to apply a large tension for image forming Th to the sheet S which is supported by the platen drum  30  without applying such a large torque to the rear driving roller  32 , and to perform good image forming. 
     In addition, in the printer  1  according to the embodiment, it is possible to release the tension of the sheet S in the winding unit  4 , in a state in which the sheet S is stopped, while applying the tension for detaching medium Tl which is smaller than the tension for image forming Th to the sheet S in the process unit  3  (roll exchanging mode). In the roll exchanging mode, releasing of the tension of the sheet S in the winding unit  4  is executed, in a state in which a tension for detaching medium Tl which is relatively small is applied to the sheet S in the process unit  3 . Accordingly, when releasing the tension of the sheet S in the winding unit  4 , a tension to be resisted by the rear driving roller  32  is a tension for detaching medium Tl which is smaller than the tension for image forming Th. For this reason, it is possible for the rear driving roller  32  to suppress a deviation of the sheet S by resisting the tension which is applied to the sheet S on the platen drum (tension for detaching medium Tl), even when the rear driving roller  32  looses the support from the winding shaft  40  along with releasing of the tension in the winding unit  4 . 
     As described above, in the printer  1  according to the embodiment, it is possible to perform good image forming by applying a large tension for image forming Th to the sheet S which is supported by the platen drum  30  when performing image forming. On the other hand, when detaching the sheet S from the rotational shafts  20  and  40 , it is possible to detach the sheet S while suppressing a deviation of the sheet S which is associated with releasing of the tension of the sheet S, by executing the roll exchanging mode. As a result, it is possible to suppress a deviation of the sheet S when detaching the sheet S from the rotational shafts  20  and  40  supporting the sheet S which is wound in a roll shape, while executing good image forming by applying the tension for image forming Th to the sheet S which is supported by the platen drum  30 . 
     In addition, according to the embodiment, during the execution of the roll exchanging mode, the door  723  or  743  is locked until the tension Ta or Tc of the sheet S in the feeding unit  2  or winding unit  4  in which the roll is exchanged is released. In addition, the door  723  or  743  is unlocked after releasing the tension Ta or Tc. With such a configuration, it is possible to prevent the worker from accessing the sheet S before the tension thereof is released. For this reason, it is possible to prevent the sheet S or the rotational shaft  20  or  40  from being damaged when the worker tries to forcibly detach the sheet S before releasing tension from the rotational shaft, for example. 
     In addition, according to the embodiment, the doors  723  and  743  are locked during the execution of the roll exchanging mode. With such a configuration, it is possible to prevent the worker from accessing the sheet S on which image forming is performed. For this reason, for example, it is possible to prevent the worker from disturbing the image forming on the sheet S by accessing the sheet S in the middle of image forming by mistake. 
     In addition, in the printer  1  according to the embodiment, the monitor  210 , or the operation unit  220  which receives an input from the worker is provided. In addition, the medium detaching mode is executed when the monitor  210 , or the operation unit  220  receives an input instructing executing of the medium detaching mode by the worker. With such a configuration, it is possible to perform releasing of the tension of the sheet S by executing the medium detaching mode at an appropriate timing which corresponds to detaching of the sheet S by the worker. For this reason, it is possible for the worker to improve workability. 
     In addition, according to the embodiment, the tension Ta or Tc is applied to the sheet S when the monitor  210 , or the operation unit  220  receives an input denoting that exchanging of the sheet S with respect to the rotational shaft  20  or  40  is finished, after executing the medium detaching mode. In this manner, for example, when the worker finishes the exchanging work of mounting a new sheet S on the rotational shaft  20  or  40  after detaching the previous sheet S from the rotational shaft  20  or  40 , it is possible to rapidly apply the tension Ta or Tc to the sheet S, and to be prepared for image forming to be executed thereafter. 
     In addition, according to the embodiment, the tension for image forming Th is applied to the sheet S in the process unit  3 , when the monitor  210 , or the operation unit  220  receives an input denoting that exchanging of the sheet S with respect to the rotational shaft  20  or  40  is completed, after executing the medium detaching mode. In this manner, for example, when the worker finishes an exchanging work of mounting a new sheet S on the rotational shaft  20  or  40  after detaching the previous sheet S from the rotational shaft  20  or  40 , it is possible to rapidly apply the tension for image forming Th to the sheet S in the process unit  3 , and to be prepared for image forming to be executed thereafter. 
     As described above, according to the embodiment, the printer  1  corresponds to an example of the “image forming apparatus” of the invention, the sheet S corresponds to an example of the “recording medium” in the invention, the platen drum  30  corresponds to an example of the “support member” of the invention, the recording head  51  or  52  corresponds to an example of the “head” of the invention, the process unit  3  corresponds to an example of the “second region” of the invention, the printer control unit  200  corresponds to an example of the “control unit” of the invention, the housing member  7  corresponds to an example of the “external member” of the invention, the feeding unit door  723  or the winding unit door  743  corresponds to an example of the “opening-shutting door” of the invention, the opening portion  721 , or  741  corresponds to an example of the “opening portion” of the invention, the locking mechanism for feeding unit door  725 , or the locking mechanism for winding unit door  745  corresponds to an example of the “locking mechanism” of the invention, and the monitor  210  and the operation unit  220  function as the “input unit” of the invention in collaboration. 
     In addition, the image forming mode in step S 104  corresponds to an example of the “image forming mode” in the invention, the roll exchanging mode in steps S 106  to S 109  corresponds to an example of the “medium detaching mode” of the invention, the tension for image forming Th corresponds to an example of the “tension for image forming” of the invention, and the tension for detaching medium Tl corresponds to an example of the “tension for detaching medium” of the invention. In addition, when exchanging the feed roll R 20 , the feeding shaft  20  corresponds to an example of the “rotational shaft” of the invention, the front driving roller  31  corresponds to an example of the “driving roller” of the invention, the rear driving roller  32  and the winding shaft  40  function as the “holding unit” of the invention in collaboration, and the feeding unit  2  corresponds to the “first region” of the invention. When exchanging the windup roll R 40 , the winding shaft  40  corresponds to an example of the “rotational shaft” of the invention, the rear driving roller  32  corresponds to an example of the “driving roller” of the invention, the front driving roller  31  and the feeding shaft  20  function as the “holding unit” of the invention in collaboration, and the winding unit  4  corresponds to the “first region” of the invention. 
     In addition, the invention is not limited to the above described embodiment, and it is possible to add various modifications to the above described embodiment without departing from the scope of the invention. For example, in the embodiment, a case has been exemplified in which the invention is applied to the printer  1  in which the sheet S is transported by controlling a torque of the rear driving roller  32 , while controlling a rotating speed of the front driving roller  31 , in the image forming mode. However, it is also possible to apply the invention to the printer  1  in which the sheet S is transported by controlling a rotating speed of the rear driving roller  32 , while controlling a torque of the front driving roller  31 , in the image forming mode. 
     In addition, according to the embodiment, the process tension Tb in a standby state is set to the tension for image forming Th. However, a specific set value of the process tension Tb in the standby state is not limited to this. Therefore, the process tension Tb in the standby state may be set to the tension for detaching medium Tl, may be set to a value which is smaller than the tension for image forming Th and larger than the tension for detaching medium Tl, may be set to a value which is larger than the tension for image forming Th, or may be set to a value which is smaller than the tension for detaching medium Tl. 
     In addition, according to the embodiment, the tension Ta or Tc in the standby state is set to be the same as the tension Ta or Tc in the image forming mode. However, the tension Ta or Tc in the standby state may be set to be larger, or smaller than the tension Ta or Tc in the image forming mode. 
     In addition, the printer  1  according to the embodiment is configured so that a worker specifies a roll to be exchanged between the rolls R 20  and R 40 , and instructs the printer control unit  200  to execute the roll exchanging mode. However, it is also possible to configure the printer  1  so that a worker instructs the printer control unit  200  to execute the roll exchanging mode without specifying a roll to be exchanged. In this case, the printer may be configured so that releasing of the tension can be executed while suppressing a deviation of the sheet S even when a worker exchanges any of the roll R 20  or R 40 , by executing the roll exchanging mode in steps S 106  to S 109  in both the feeding unit  2  and winding unit  4 . 
     In addition, a timing for executing opening or shutting, locking, unlocking, or the like, of the feeding unit door  723 , or the winding unit door  743 , is not limited to the above described example, and can be appropriately changed. In addition, it is also possible to apply the invention to a simple printer  1  not including its own locking mechanism of the feeding unit door  723 , or the winding unit door  743 . 
     In addition, in the roll exchanging mode, it is not essential to cut the excitation of the feeding motor M 20  or the winding motor M 40 . Accordingly, it is also possible to finish the roll exchanging mode while exciting the feeding motor M 20  or the winding motor M 40 .