Patent Publication Number: US-11396436-B2

Title: Conveying device, image forming apparatus incorporating the conveying device, and method of conveying a medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos. 2019-210796, filed on Nov. 21, 2019, and 2020-167915, filed on Oct. 2, 2020, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to a conveying device, an image forming apparatus incorporating the conveying device, and a method of conveying a medium. 
     Background Art 
     Various types of serial-type inkjet image forming apparatuses, a winding mechanism is employed to wind a roll type medium after image formation. Such a roll type medium is wound with a constant torque via a torque limiter, with application of constant tension from a tension bar disposed upstream from a winding unit, or with other different method. 
     Further, a known image forming apparatus that performs the following control prevents skew (misaligned winding) due to misalignment of a winding shaft of a winder. The known image forming apparatus causes the winding unit to start taking up the roll type medium after a certain time has elapsed from the start of conveyance by a conveyance unit. By so doing, the known image forming apparatus conveys the roll type medium with a certain slack amount, in other words, while the roll type medium is loosened. 
     However, in recent years, the types and shapes of media have diversified. In addition, the behavior during conveyance of the medium differs depending on the type of the medium. Therefore, the known slack control may cause insufficient tension force or excessive tension force depending on the type of a medium. 
     SUMMARY 
     Embodiments of the present disclosure described herein provide a novel conveying device including a sheet conveyor, a sheet winder, and circuitry. The sheet conveyor is configured to convey a medium in a conveyance direction of the medium. The sheet winder includes a winding roller and a winding motor. The winding roller is disposed downstream from the sheet conveyor in the conveyance direction and is configured to wind the medium. The winding motor is configured to rotate the winding roller. The circuitry is configured to control an operation performed by the sheet conveyor and an operation performed by the sheet winder. The circuitry is configured to convey the medium intermittently while repeating a first operation to cause the sheet conveyor and the sheet winder to convey the medium by a predetermined conveyance amount while the medium is loosened between the sheet conveyor and the sheet winder, a second operation to rotate the winding motor in a normal direction by a predetermined torque according to an outer winding diameter of the medium wound around the winding roller, while the sheet conveyor is stopped, to apply a predetermined tension force to the medium between the sheet conveyor and the sheet winder, and a third operation to rotate the winding motor in a reverse direction opposite the normal direction to loosen the medium by a predetermined slack amount while the predetermined tension force is applied to the medium. The circuitry is configured to determine the predetermined tension force according to the width of the medium intersecting with the conveyance direction of the medium. 
     Further, embodiments of the present disclosure described herein provide an image forming apparatus including the above-described conveying device and an image forming unit configured to form an image on the medium between the sheet conveyor and the sheet winder. 
     Further, embodiments of the present disclosure described herein provide a conveying device including a sheet conveyor, a sheet winder, and circuitry. The sheet conveyor is configured to convey a medium in a conveyance direction of the medium. The sheet winder includes a winding roller and a winding motor. The winding roller is disposed downstream from the sheet conveyor in the conveyance direction and is configured to wind the medium. The winding motor is configured to rotate the winding roller. The circuitry is configured to control an operation performed by the sheet conveyor and an operation performed by the sheet winder. The circuitry is configured to convey the medium intermittently while repeating a first operation to cause the sheet conveyor and the sheet winder to convey the medium by a predetermined conveyance amount while the medium is loosened between the sheet conveyor and the sheet winder, a second operation to rotate the winding motor in a normal direction by a predetermined torque according to an outer winding diameter of the medium wound around the winding roller, while the sheet conveyor is stopped, to apply a predetermined tension force to the medium between the sheet conveyor and the sheet winder, and a third operation to rotate the winding motor in a reverse direction opposite the normal direction to loosen the medium by a predetermined slack amount while the predetermined tension force is applied to the medium. The circuitry is configured to determine the predetermined slack amount according to the width of the medium intersecting with the conveyance direction of the medium. 
     Further, embodiments of the present disclosure described herein provide an image forming apparatus that includes the above-described conveying device and an image forming unit configured to form an image on the medium between the sheet conveyor and the sheet winder. 
     Further, embodiments of the present disclosure described herein provides a method of conveying a medium. The method includes causing a sheet conveyor configured to convey the medium in a conveyance direction of the medium and a sheet winder including a winding roller configured to wind the medium and a winding motor configured to rotate the winding roller, to convey the medium by a predetermined conveyance amount while the medium is loosened between the sheet conveyor and the sheet winder, rotating the winding motor in a normal direction while the sheet conveyor is stopped, to apply a predetermined tension force to the medium between the sheet conveyor and the sheet winder, rotating the winding motor in a reverse direction opposite the normal direction to loosen the medium by a predetermined slack amount while the predetermined tension force is applied to the medium, and repeating the causing, the rotating the winding motor in the normal direction, and the rotating the winding motor in the reverse direction to convey the medium intermittently. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Exemplary embodiments of this disclosure will be described in detail based on the following figures, wherein: 
         FIG. 1  is a perspective view illustrating a configuration of an image forming apparatus according to the present embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view illustrating the internal configuration of the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a detailed configuration of a sheet conveyor provided to the image forming apparatus of  FIG. 1 ; 
         FIG. 4  is a plan view illustrating a detailed configuration of an image forming device provided in the image forming apparatus of  FIG. 1 ; 
         FIG. 5  is a block diagram illustrating the image forming apparatus of  FIG. 1 ; 
         FIG. 6  is a flowchart of an image forming process performed in the image forming apparatus of  FIG. 1 ; 
         FIGS. 7A, 7B, 7C, and 7D  are diagrams each illustrating the state of a continuous sheet disposed between the sheet conveyor and a sheet winder of the image forming apparatus of  FIG. 1 ; 
         FIGS. 8A and 8B  are diagrams for explaining a method of specifying a slack amount; and 
         FIG. 9  is a diagram of the T-N curve of a winding motor. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     It will be understood that if an element or layer is referred to as being “on,” “against,” “connected to” or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers referred to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements describes as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors herein interpreted accordingly. 
     The terminology used herein is for describing particular embodiments and examples and is not intended to be limiting of exemplary embodiments of this disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings for explaining the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below. 
     EMBODIMENT OF THE PRESENT DISCLOSURE 
     Hereinafter, the image forming apparatus  1  according to an embodiment of the present disclosure will be described with reference to  FIGS. 1 to 5 . 
       FIG. 1  is a perspective view illustrating a configuration of an image forming apparatus  1  according to the present embodiment of the present disclosure. 
       FIG. 2  is a cross-sectional view illustrating the internal configuration of the image forming apparatus  1  of  FIG. 1 . 
       FIG. 3  is a diagram illustrating a detailed configuration of a sheet conveyor  20  provided to the image forming apparatus  1  of  FIG. 1 . 
       FIG. 4  is a plan view illustrating a detailed configuration of an image forming device provided in the image forming apparatus  1  of  FIG. 1 . 
       FIG. 5  is a block diagram of the image forming apparatus  1  of  FIG. 1 . 
     The image forming apparatus  1  according to the present embodiment is an inkjet image forming apparatus that forms an image on the continuous sheet P that is a strip-shaped medium by discharging ink onto the continuous sheet P. However, the image forming method performed in the image forming apparatus  1  is not limited to the inkjet method and may be an electrophotographic method. The image forming apparatus  1  mainly includes a sheet feeder  10 , a sheet conveyor  20 , an image forming device  30 , a sheet winder  40 , and a controller  50  that functions as circuitry. 
     The sheet feeder  10  applies a predetermined tension force to the continuous sheet P between the sheet feeder  10  and the sheet conveyor  20 . The sheet feeder  10  mainly includes a sheet feeding roller  11 , a sheet feed motor  12 , a torque limiter  13 , a sheet remaining encoder sheet  14 , a sheet remaining encoder sensor  15 , a sheet motor encoder sheet  16 , and a sheet feed motor encoder sensor  17 . 
     The continuous sheet P before bearing an image is wound around the sheet feeding roller  11 . The sheet feed motor  12  rotates the sheet feeding roller  11  by application of drive voltage caused by the controller  50 . The torque limiter  13  manages the upper limit of the torque to be transmitted from the sheet feed motor  12  to the sheet feeding roller  11 . 
     The sheet remaining encoder sheet  14  rotates together with the sheet feeding roller  11  as a single unit. The sheet remaining encoder sensor  15  reads the number of rotations of the sheet remaining encoder sheet  14  and outputs the pulse signal indicating the read number of rotations of the sheet remaining encoder sheet  14 , to the controller  50 . The sheet motor encoder sheet  16  rotates together with the output shaft of the sheet feed motor  12  as a single unit. The sheet feed motor encoder sensor  17  reads the number of rotations of the sheet motor encoder sheet  16  and outputs the pulse signal indicating the read number of rotations of the sheet motor encoder sheet  16 , to the controller  50 . 
     The sheet feeder  10  rotates the sheet feeding roller  11  in a sheet winding direction in which the continuous sheet P held between a sheet conveyance roller  21  and a pressure roller  22  is wound. As a result, the image forming apparatus  1  applies a predetermined tension force that corresponds to the upper limit value of the torque set in the torque limiter  13 , to the continuous sheet P between the sheet feeder  10  and the sheet conveyor  20 . 
     The sheet conveyor  20  conveys the continuous sheet P fed from the sheet feeder  10  to the sheet winder  40  via a position facing the image forming device  30 . The sheet conveyor mainly includes the sheet conveyance roller  21 , the pressure roller  22 , a sheet conveyance motor  23 , a sheet conveyance encoder sheet  24 , and a sheet conveyance encoder sensor  25 . 
     The sheet conveyance roller  21  and the pressure roller  22  rotate while holding the continuous sheet P from both sides in a thickness direction of the continuous sheet P. Further, as illustrated in  FIG. 3 , the sheet conveyance roller  21  is provided with flanges  26  at both ends. Each flange  26  contacts a widthwise end of the continuous sheet P, in other words, an end in the width direction of the continuous sheet P. The flange  26  on the left end of the sheet conveyance roller  21  is illustrated in  FIG. 3 . Note that the flange  26  on the right end of the sheet conveyance roller  21  is identical to the flange  26  on the left end of the sheet conveyance roller  21  in the configuration and function. As the sheet conveyance motor  23  transmits the driving force via the torque limiter  13 , the sheet conveyance roller  21  and the flanges  26  receive the driving force and rotate. The pressure roller  22  is pressed by the sheet conveyance roller  21  by application of a predetermined pressure and is rotated with rotation of the sheet conveyance roller  21 . 
     The sheet conveyance encoder sheet  24  rotates together with the sheet conveyance roller  21  and the flanges  26  as a single unit. The sheet conveyance encoder sensor  25  reads the number of rotations of the sheet conveyance encoder sheet  24  and outputs the pulse signal indicating the read number of rotations of the sheet conveyance encoder sheet  24 , to the controller  50 . 
     The image forming device  30  is disposed downstream from the sheet conveyor  20  in the sheet conveyance direction of the continuous sheet P. The image forming device  30  discharges ink to the continuous sheet P that is conveyed by the sheet conveyor  20  in a sub-scanning direction B, so that an image is formed on the continuous sheet P. As illustrated in  FIGS. 1, 2, and 4 , the image forming device  30  mainly includes a carriage  31 , a main scanning motor  32 , a drive force transmission mechanism  33 , a platen  34 , a main scanning encoder sheet  35 , a main scanning encoder sensor  36 , and a media sensor  37 . 
     As illustrated in  FIGS. 1 and 4 , the carriage  31  reciprocally moves along a guide rod  38   a  and a sub-guide rail  38   b , both extending in a main scanning direction A (i.e., the width direction of the continuous sheet P) perpendicular to the sub-scanning direction B (i.e., the sheet conveyance direction of the continuous sheet P). In other words, the width direction of the continuous sheet P intersects with the sheet conveyance direction of the continuous sheet P. Further, recording heads  31   k ,  31   c ,  31   m , and  31   y  that discharge inks of respective colors (that is, black, cyan, magenta, and yellow) are mounted on the carriage  31 . The recording heads  31   k ,  31   c ,  31   m , and  31   y  discharge the inks supplied from ink cartridges  39   k ,  39   c ,  39   m , and  39   y , respectively, toward the continuous sheet P that is supported by the platen  34 . Note that the main scanning direction A corresponds to the width direction of the continuous sheet P is perpendicular to (intersects with) the sub-scanning direction B that corresponds to the sheet conveyance direction of the continuous sheet P. 
     As the drive force transmission mechanism  33  transmits the driving force of the main scanning motor  32  to the carriage  31 , the carriage  31  moves in the main scanning direction A. To be more specific, the drive force transmission mechanism  33  includes a drive pulley  33   a , a pressure pulley  33   b , and a timing belt  33   c . The drive pulley  33   a  and the pressure pulley  33   b  are disposed spaced apart in the main scanning direction A. The timing belt  33   c  is an endless loop wound around the drive pulley  33   a  and the pressure pulley  33   b.    
     As the main scanning motor  32  transmits the driving force to the drive pulley  33   a , the drive pulley  33   a  rotates. Along with rotation of the drive pulley  33   a , the timing belt  33   c  is rotated to reciprocally move the carriage  31  mounted on the timing belt  33   c  in the main scanning direction A. Further, the pressure pulley  33   b  applies a predetermined tension to the timing belt  33   c.    
     The platen  34  is disposed facing the carriage  31  in the vertical direction. Then, the platen  34  supports the continuous sheet P conveyed by the sheet conveyor  20 . Further, the platen  34  has a color having the reflectance of light lower than the reflectance of light of the continuous sheet P. For example, the continuous sheet P is white while the platen  34  is black. 
     The main scanning encoder sheet  35  is extended in the main scanning direction A at the position facing the carriage  31 . The main scanning encoder sensor  36  is mounted on the carriage  31 . Further, the main scanning encoder sensor  36  reads the main scanning encoder sheet  35  and outputs the pulse signal indicating the read number of rotations of the main scanning encoder sheet  35 , to the controller  50 . 
     The media sensor  37  emits light toward the platen  34  or the continuous sheet P that is supported by the platen  34  and receives the reflection light reflected on the platen  34  or the continuous sheet P. Then, the media sensor  37  outputs a signal of intensity indicating the intensity of the received reflection light, to the controller  50 . The media sensor  37  is used, for example, to detect the widthwise end of the continuous sheet P, in other words, the end in the width direction of the continuous sheet P. 
     The sheet winder  40  is disposed downstream from the sheet conveyor  20  and the image forming device  30  in the sheet conveyance direction of the continuous sheet P. The sheet winder  40  winds the continuous sheet P on which an image is formed by the image forming device  30 . The sheet winder  40  includes a sheet winding roller  41 , a sheet winding motor  42 , a torque limiter  43 , a winding amount encoder sheet  44 , a winding amount encoder sensor  45 , a winding motor encoder sheet  46 , and a winding motor encoder sensor  47 . 
     The sheet winding roller  41  winds the continuous sheet P after image formation, in other words, the continuous sheet P on which an image is formed. The sheet winding motor  42  rotates the sheet winding roller  41  by applying a drive voltage from the controller  50 . The torque limiter  43  manages the upper limit of the torque transmitted from the sheet winding motor  42  to the sheet winding roller  41 . 
     The winding amount encoder sheet  44  rotates together with the sheet winding roller  41 . The winding amount encoder sensor  45  reads the number of rotations of the winding amount encoder sheet  44  and outputs the pulse signal indicating the read number of rotations of the winding amount encoder sheet  44 , to the controller  50 . The winding motor encoder sheet  46  rotates together with the output shaft of the sheet winding motor  42  as a single unit. The winding motor encoder sensor  47  reads the number of rotations of the winding motor encoder sheet  46  and outputs the pulse signal indicating the read number of rotations of the winding motor encoder sheet  46 , to the controller  50 . 
     The controller  50  controls the operation of the image forming apparatus  1 . To be more specific, the controller  50  controls the operations of the sheet feeder  10 , the sheet conveyor  20 , the image forming device  30 , the sheet winder  40 , and a control panel (operation unit)  55 . By so doing, an image is formed on the continuous sheet P. 
     As illustrated in  FIG. 5 , the controller  50  mainly includes a field-programmable gate array (FPGA)  51 , a central processing unit (CPU)  52 , a memory  53 , and a motor driver  54 . The CPU  52  reads and executes the program stored in the memory  53 . Such processing configures a software controller including various functional modules of the image forming apparatus  1 . The software controller thus configured cooperates with hardware resources of the image forming apparatus  1  construct functional blocks to implement functions, illustrated as functional blocks, of the image forming apparatus  1 . In addition, the image forming apparatus  1  may use the FPGA  51  to implement the function customized for each of separate image forming apparatuses  1 . 
     The controller  50  rotates each of the sheet feed motor  12 , the sheet conveyance motor  23 , the main scanning motor  32 , and the sheet winding motor  42  by applying a drive voltage via the motor driver  54 . Further, the controller  50  outputs a discharge signal to each of the recording heads  31   k ,  31   c ,  31   m , and  31   y , so as to cause the recording heads  31   k ,  31   c ,  31   m , and  31   y  to discharge ink. 
     Further, the controller  50  acquires pulse signals from the sheet remaining encoder sensor  15 , the sheet feed motor encoder sensor  17 , the sheet conveyance encoder sensor  25 , the main scanning encoder sensor  36 , the winding amount encoder sensor  45 , and the winding motor encoder sensor  47 . Further, the controller  50  counts the pulse signals acquired from the sheet remaining encoder sensor  15 , the sheet feed motor encoder sensor  17 , the sheet conveyance encoder sensor  25 , the main scanning encoder sensor  36 , the winding amount encoder sensor  45 , and the winding motor encoder sensor  47 . Hereinafter, the number of pulse signals counted by the controller  50  is referred to as an “encoder value”. Then, the controller  50  determines the number of rotations of each motor and the amount of movement of the carriage  31 , based on the encoder value. 
     Further, in the course of movement of the carriage  31  in the main scanning direction A, the controller  50  detects the position of the end of the continuous sheet P in the main scanning direction A (i.e., the width direction), based on the change of the signal of intensity that is output from the media sensor  37 . To be more specific, in the course of movement of the carriage  31  from the left to the right in  FIG. 4 , the controller  50  detects the position at which the signal of intensity has changed from a point less than the threshold to a point equal to or greater than the threshold, as the left end of the continuous sheet P. Further, in the course of movement of the carriage  31  from the left to the right in  FIG. 4 , the controller  50  detects the position at which the signal of intensity has changed from a point equal to or greater than the threshold to a point less than the threshold, as the right end of the continuous sheet P. 
     The control panel  55  includes, for example, a display for displaying an image, a touch panel for detecting an input operation by an operator who presses buttons displayed on the display, and a push button pressed by the operator. The controller  50  displays an image on the display. Further, the controller  50  acquires an operation signal corresponding to the input operation by the operator pressing the buttons on the touch panel or the push button, through the control panel  55 . 
     Next, a description is given of the image forming process, with reference to  FIGS. 6 to 9 . 
       FIG. 6  is a flowchart of the image forming process performed in the image forming apparatus  1  of  FIG. 1 . 
       FIGS. 7A, 7B, 7C, and 7D  are diagrams each illustrating the state of the continuous sheet P disposed between the sheet conveyor  20  and the sheet winder  40  of the image forming apparatus  1  of  FIG. 1 , at each step of image forming process in the flowchart of  FIG. 6 . 
       FIGS. 8A and 8B  are diagrams for explaining a method of specifying the slack amount when the continuous sheet P is loosened by the sheet winder  40 . 
       FIG. 9  is a diagram of the T-N curve of the sheet winding motor  42 . The T-N curve is stored in the memory  53 . 
     The T-N curve illustrated in  FIG. 9  indicates the characteristics of the sheet winding motor  42  that is actually measured in the assembly process of the image forming apparatus  1 . 
     Note that the controller  50  stores the integrated value of the encoder value of the sheet conveyance encoder sensor  25  since the continuous sheet P is set on the sheet feeding roller  11  and the sheet winding roller  41 , in the memory  53  that stores the T-N curve. Hereinafter, this integrated value is referred to as the “integrated conveyance amount”. That is, the controller  50  adds the encoder value of the sheet conveyance encoder sensor  25  through steps S 13  to S 18 , to the integrated conveyance amount, to reset the integrated conveyance amount at the timing to replace the continuous sheet P. 
     Further, as illustrated in  FIG. 7A , the continuous sheet P is loosened by the predetermined slack amount, between the sheet conveyor  20  and the sheet winder  40 , the start of the image forming process. Further, the controller  50  monitors the encoder value of the winding amount encoder sensor  45  until step S 15  is executed, and causes the sheet winding motor  42  to rotate in a direction to correct the change of the slack amount of the continuous sheet P (I.e., a positioning stop control). 
     First, the controller  50  measures the length of the continuous sheet P in the main scanning direction A (that is, the width “w” of the continuous sheet P), based on the encoder value of the main scanning encoder sensor  36  and the signal of intensity of the media sensor  37  (step S 11 ). Note that, in a case in which the width “w” of the continuous sheet P has already been measured, the controller  50  may skip the process of step S 11 . 
     Specifically, the controller  50  drives the main scanning motor  32  to move the carriage  31  in the main scanning direction A, so as to detect the left end and the right end of the continuous sheet P. Then, the controller  50  specifies the width “w” of the continuous sheet P based on the encoder value of main scanning encoder sensor  36  from detection of the left end of the continuous sheet P to detection of the right end of the continuous sheet P. That is, the controller  50  specifies the width “w” of the continuous sheet P by multiplying the distance of movement of the carriage  31  at the interval of the pulse signals output from the main scanning encoder sensor  36 , by the above-described encoder value. 
     Next, in order to synchronize the operation of the sheet conveyor  20  and the operation of the sheet winder  40  for conveying the continuous sheet P, the controller  50  determines the sheet winding speed “v” according to the outer diameter of the continuous sheet P wound around the sheet winding roller  41 . The outer diameter of the continuous sheet P is hereinafter referred to as the “outer winding diameter D” (step S 12 ). The sheet winding speed “v” is the rotational speed of the sheet winding roller  41  that rotates in synchrony with the rotation of the sheet conveyance roller  21 . 
     To be more specific, the controller  50  determines the current outer winding diameter D [mm] based on the integrated conveyance amount stored in the memory  53 , the thickness of the continuous sheet P, and the outer diameter of the sheet winding roller  41 . Then, the controller  50  determines the sheet winding speed “v” [rpm] using the following Equation 1. The constant “k” is a value previously determined and set based on the conveying speed of the continuous sheet P by the sheet conveyor  20  and is previously stored in the memory  53 .
 
 v=k/D   Equation 1.
 
     Next, the controller  50  starts driving the sheet conveyance motor  23  at the predetermined speed in a state in which the sheet winding motor  42  is stopped (step S 13 ). Then, the controller  50  determines whether the predetermined standby time has elapsed (step S 14 ). When the predetermined standby time has not elapsed (NO in step S 14 ), the controller  50  continues (repeats) this state until a predetermined standby time elapses. Accordingly, as illustrated in  FIG. 7B , the slack amount of the continuous sheet P between the sheet conveyor and the sheet winder  40  increases as the time elapses. Further, the standby time is assumed to be previously set within the range, for example, between 100 ms and 500 ms (typically, 200 ms). 
     Next, when the predetermined standby time has elapsed from the start of driving of the sheet conveyance motor  23  (YES in step S 14 ), the controller  50  starts the sheet winding motor  42  to rotate in the normal direction to rotate the sheet winding roller  41  at the sheet winding speed “v” determined in step S 12  (step S 15 ). Here, a direction of rotation of the sheet winding motor  42  to rotate the sheet winding roller  41  in the direction to wind the continuous sheet P is defined as a “normal rotation” and a direction of rotation of the sheet winding motor  42  to rotate the sheet winding roller  41  in the direction opposite the direction to wind the continuous sheet P is defined as a “reverse rotation.” 
     Here, the sheet winding speed “v” determined in step S 12  is a value that matches the conveyance amount of the continuous sheet P by the sheet conveyor  20  and the wound amount of the continuous sheet P by the sheet winder  40 . In the actual operation, however, the conveyance amount of the continuous sheet P by the sheet conveyor  20  and the wound amount of the continuous sheet P by the sheet winder  40  do not match exactly, for example, susceptible to an error due to eccentricity of the shaft of the sheet winding roller  41  and an error in the size of the outer winding diameter D. 
     Therefore, the controller  50  determines, in step S 17 , whether the conveyance amount of the continuous sheet P by the sheet conveyor  20  has reached the predetermined conveyance amount. When the conveyance amount of the continuous sheet P by the sheet conveyor  20  has not reached the predetermined conveyance amount (NO in step S 17 ), the process goes back to step S 16  and the controller  50  performs the feedback control of the rotational speed of the sheet winding motor  42  based on the encoder value of the winding amount encoder sensor in step S 16 . Accordingly, as illustrated in  FIG. 7C , the sheet winding roller  41  winds the continuous sheet P while the slack amount of the continuous sheet P between the sheet conveyor  20  and the sheet winder  40  is maintained. 
     To be more specific, the controller  50  may increase or decrease the drive voltage applied to the sheet winding motor  42  so as to approach the sheet winding speed “v” determined based on the encoder value of the winding amount encoder sensor  45 . Further, the controller  50  stores the drive voltage V 1  [V] converged by the feedback control, in the memory  53 . 
     On the other hand, when the sheet conveyor  20  has conveyed the continuous sheet P by the predetermined conveyance amount, in other words, when the conveyance amount of the continuous sheet P by the sheet conveyor  20  has reached the predetermined conveyance amount (YES in step S 17 ), the controller  50  stops the sheet conveyance motor  23  and the sheet winding motor  42  (step S 18 ). At this time, the slack amount of the continuous sheet P between the sheet conveyor  20  and the sheet winder  40  is equal to the slack amount of the continuous sheet P at the start of step S 15 . Next, the controller  50  determines the predetermined tension force F 1  [N], the predetermined torque T 3  [mmN], the error torque ΔT [mmN], the total torque T 4  [mmN], and the predetermined slack amount [mm] (step S 19 ). 
     The predetermined tension force F 1  refers to the tension force applied to the continuous sheet P between the sheet conveyor  20  and the sheet winder  40  in steps S 20  and S 21  described below. The controller  50  determines a predetermined tension force F 1  by using, for example, the following Equation 2. Note that the above-described “determination” includes, for example, calculation of the tension force based on a predetermined calculation equation and determination of the tension force by referring to the “medium width-tension force table” previously stored in the memory. The same conditions are applied to the “determination” of other values. 
     The reference width w 0  [mm] refers to the width of the reference continuous sheet P in the main scanning direction A. The reference tension force F 0  [N] refers to the predetermined tension force applied to the continuous sheet P having the reference width w 0 . That is, the controller  50  determines the predetermined tension force F 1  according to the width of the continuous sheet P. To be more specific, the controller  50  increases the predetermined tension force F 1  as the width of the continuous sheet P increases.
 
 F   1   =F   0 ×( w/w   0 )  Equation 2.
 
     The predetermined torque T 3  is a theoretical value of the torque to be generated by the sheet winding motor  42  in order to apply the predetermined tension force F 1  to the continuous sheet P. The controller  50  determines the predetermined torque T 3  by using the following Equation 3. That is, the controller  50  determines the predetermined torque T 3  according to the current outer winding diameter D. To be more specific, the controller  50  determines the predetermined torque T 3  according to the predetermined tension force F 1  and the current outer winding diameter D. More specifically, the controller  50  increases the predetermined torque T 3  as the predetermined tension force F 1  increases and increases the predetermined torque T 3  as the outer winding diameter D increases.
 
 T   3   =F   1 ×( D/ 2)  Equation 3.
 
     The error torque ΔT is the difference of the theoretical value of the torque to be generated by the sheet winding motor  42  (that is, a theoretical torque T 1 ) to achieve the winding speed “v” and the actual value (that is, the actual torque T 2 ) of the torque that is actually generated by the sheet winding motor  42  in step S 16 . The theoretical torque T 1  [mmN] is a predetermined theoretical torque that is previously stored in the memory  53 . On the other hand, the controller  50  uses Equation 4, for example, to determine the actual torque T 2  [mmN], and uses Equation 5 to determine the error torque ΔT. 
     As described in Equation 4, the actual torque T 2  is a value corresponding to the drive voltage V 1  that has actually been applied to the sheet winding motor  42  in order to synchronize and operate the sheet conveyor  20  and the sheet winder  40 . Note that, in Equation 4, the restraint torque “a” [mmN] corresponds to the restraint torque on the T-N curve stored in the memory  53 , that is, the value on the horizontal axis of the graph illustrated in  FIG. 9 . Further, in Equation 4, the number of unloaded rotations “b” [rpm] corresponds to the number of unloaded rotations on the T-N curve stored in the memory  53 , that is, the value on the vertical axis of the graph illustrated in  FIG. 9 . The number “24” is a constant [V] represents the drive voltage at the restraint torque “a” and the number of unloaded rotations “b”.
 
 T   1 ={( b/ 24)× V   1   −v}×a/b   Equation 4.
 
Δ T=T   2   −T   1   Equation 5.
 
     The total torque T 4  is a value obtained by adding the predetermined torque T 3  and the error torque ΔT. That is, the total torque T 4  is a predetermined torque T 3  corrected by the error torque ΔT. In other words, the total torque T 4  is the actual value of the torque that should be generated by the sheet winding motor  42  (that is, a value considering variation in each image forming apparatus  1 ) to apply the predetermined tension force F 1  to the continuous sheet P. The controller  50  determines the total torque T 4  using Equation 6.
 
 T   4   =T   3   +ΔT   Equation 6.
 
     The predetermined slack amount is the amount of loosening the continuous sheet P between the sheet conveyor  20  and the sheet winder  40  in step S 22 . The controller  50  determines the predetermined slack amount according to the width of the continuous sheet P. To be more specific, the controller  50  increases the predetermined slack amount as the width of the continuous sheet P decreases. 
     Next, the controller  50  causes the sheet winding motor  42  to rotate in the normal direction, in other words, perform the normal rotation, at the total torque T 4  that is determined in step S 19  while the sheet conveyance motor  23  is stopped, in step S 20 . Then, based on the encoder value of the winding amount encoder sensor  45 , the controller  50  continues the normal rotation of the sheet winding motor  42  until the sheet winding roller  41  stops rotating (NO in step S 21 ). The drive voltage V 2  [V] that is applied to the sheet winding motor  42  in step S 20  is calculated using Equation 7, for example.
 
 V   2 =(24/ a )× T   4   Equation 7.
 
     As a result, the slack of the continuous sheet P between the sheet conveyor  20  and the sheet winder  40  gradually decreases. Then, as illustrated in  FIG. 7D , when the slack amount of the continuous sheet P comes to zero (0), the sheet conveyor  20  and the sheet winder  40  pull the continuous sheet P taut. Further, when the predetermined tension force F 1  is applied to the continuous sheet P between the sheet conveyor  20  and the sheet winder  40 , the sheet winding motor  42  is locked to stop rotation of the sheet winding roller  41 . For example, the controller  50  may determine that the sheet winding roller  41  has stopped because the pulse signal is not continuously output from the winding amount encoder sensor  45  for a predetermined period of time. 
     Next, the controller  50  stops the normal rotation of the sheet winding motor  42  when the rotation of the sheet winding roller  41  is stopped (YES in step S 21 ). Then, the controller  50  causes the sheet winding motor  42  to rotate in reverse, so as to loosen the continuous sheet P to which the predetermined tension force F 1  is applied, by the predetermined slack amount (step S 22 ). Accordingly, as illustrated in  FIG. 7A , the continuous sheet P is loosened by the predetermined slack amount, between the sheet conveyor  20  and the sheet winder  40 . 
     As illustrated in  FIG. 8 , the slack amount of the continuous sheet P corresponds to the number of rotations of the sheet winding roller  41  in the direction in which the continuous sheet P is unwound. Therefore, based on the current outer winding diameter D and the encoder value of the winding amount encoder sensor  45 , the controller  50  cause the sheet winding motor  42  to rotate in the reverse direction until the sheet winding roller  41  rotates by the number of rotations corresponding to the predetermined slack amount. Then, after the continuous sheet P is loosened by the predetermined slack amount, the controller  50  executes the positioning stop control until the controller  50  starts the process in step S 15 . 
     Next, the controller  50  forms an image in the area on the continuous sheet P that faces the image forming device  30  (step S 23 ). To be more specific, the controller  50  drives the main scanning motor  32  to move the carriage  31  in the main scanning direction A and outputs a discharge signal to the recording heads  31   k ,  31   c ,  31   m , and  31   y  at the predetermined timings. The output timing of the discharge signal changes depending on the image recorded on the continuous sheet P. 
     Next, the controller  50  determines whether the whole image formation to the continuous sheet P is finished (step S 24 ). When the controller  50  determined that the image formation has not yet been finished (NO in step S 24 ), the controller  50  executes the processes in and after step S 11 . That is, the controller  50  executes the processes of steps S 11  to S 22  repeatedly to convey the continuous sheet P intermittently by the predetermined conveyance amount of the continuous sheet P. Then, when the controller  50  determined that the image formation has been finished (YES in step S 24 ), the controller  50  completes the image forming process. 
     According to the above-described embodiment, the following operational effects, for example, are achieved. 
     In step S 13 , the controller  50  according to the above-described embodiment causes the sheet conveyor  20  to start conveyance of the continuous sheet P while the continuous sheet P is loosened (step S 22 ). Accordingly, the image forming apparatus  1  prevents occurrence of shock applied at the start of conveyance of the continuous sheet P as well as skew caused by the difference in tension force in the width direction of the continuous sheet P. As a result, the image forming apparatus  1  enhances the stable conveyance quality. Further, since the controller  50  applies the predetermined tension force F 1  to the continuous sheet P, and then loosens the continuous sheet P (steps S 20  and S 21  (YES) to step S 22 ). Therefore, the appropriate slack amount is set to the continuous sheet P. 
     Further, the controller  50  according to the above-described embodiment determines the predetermined torque T 3  applied to the sheet winding motor  42  in step S 20 , according to the outer winding diameter D. Accordingly, a constant tension force is applied to the continuous sheet P regardless of the wound amount of the sheet winding roller  41 . 
     Note that, as the width “w” of the continuous sheet P decreases, the tension force per unit width increases. Therefore, as in the above-described embodiment, the controller  50  adjusts the predetermined tension force F 1  according to the width “w” of the continuous sheet P, the constant tension force per unit width is maintained regardless of the width “w” of the continuous sheet P. 
     Further, when the width “w” of the continuous sheet P is relatively small, when compared with a case in which the width “w” of the continuous sheet P is relatively large, the tension force remains even if the continuous sheet P is loosened. Therefore, as in the above-described embodiment, the controller  50  adjusts the predetermined slack amount according to the width “w” of the continuous sheet P, the sheet conveyor  20  starts conveyance of the continuous sheet P while no tension force remains in the continuous sheet P. 
     Further, the controller  50  according to the above-described embodiment executes the positioning stop control after the continuous sheet P is loosened by the predetermined slack amount. Accordingly, the constant slack amount of the continuous sheet P is provided when the sheet conveyor  20  starts conveyance of the continuous sheet P. As a result, the positional deviation of the continuous sheet P on the platen  34  is prevented, and therefore the image forming apparatus  1  forms an image at the appropriate position in step S 23 . 
     Further, due to individual differences in the image forming apparatus  1 , for example, the eccentricity of the shaft of the sheet winding roller  41 , even if the sheet winding motor  42  rotates at the ideal value of the predetermined torque T 3 , the tension force applied to the continuous sheet P varies for each image forming apparatus  1 . Therefore, as in the above embodiment, the controller  50  corrects the predetermined torque T 3  with the error torque ΔT, which is the difference between the theoretical torque T 1  and the actual torque T 2 . Accordingly, individual differences in the image forming apparatus  1  are absorbed, and a constant tension force is applied to the continuous sheet P. 
     Further, as in the above-described embodiment, the controller  50  executes various processes in step S 19  based on the T-N curve created in the manufacturing process of the image forming apparatus  1 . Accordingly, the controller  50  absorbs the individual difference of the image forming apparatus  1  to calculate a correct value. 
     Note that the above-described embodiment has described an example in which the controller  50  adjusts both the predetermined tension force F 1  and the predetermined slack amount. However, the controller  50  may adjust at least one of the predetermined tension force F 1  and the predetermined slack amount. As an example, the controller  50  may provide the predetermined slack amount as a fixed value in the above-described embodiment. As another example, the controller  50  may adjust the predetermined slack amount alone when the constant tension force is applied to the continuous sheet P via the torque limiter  43 . 
     Further, the above-described embodiment has described an example in which the controller  50  determines the predetermined tension force F 1  and the predetermined slack amount according to the width “w” of the continuous sheet P. However, the controller  50  may determine the predetermined tension force F 1  and the predetermined slack amount based on a parameter other than the width of the continuous sheet P. As another example, the controller  50  may determine the predetermined tension force F 1  according to the rigidity of the continuous sheet P. That is, as the rigidity of the continuous sheet P increases, the controller  50  may increase the predetermined tension force F 1  and the predetermined slack amount. The width “w” of the continuous sheet P and the rigidity of the continuous sheet P are examples of the “types of a medium”. 
     Further, the above-described embodiment has described an example in which the controller  50  determines the predetermined tension force F 1  based on the controller  50 . However, an operator of the image forming apparatus  1  may adjust the predetermined tension force. To be more specific, the control panel  55  may receive an input by an operator to increase or decrease the tension force to be applied to the continuous sheet P. Then, the controller  50  may increase or decrease (in other words, adjust) the predetermined tension force F 1  determined in step S 19  according to the operation on the control panel  55  by the operator. 
     According to the above-described variation, when the whole image formation on the continuous sheet P is completed and the continuous sheet P is conveyed to the subsequent process, the winding amount of the continuous sheet P is set appropriately. 
     Further, in the above-described embodiments and variation, the present disclosure is applied to the image forming apparatus  1 . However, the present disclosure may be widely applied to a conveying device that conveys the continuous sheet P. The conveying device includes the conveying device  60  including the sheet feeder  10 , the sheet conveyor  20 , the sheet winder  40 , the controller  50 , and the control panel  55 , as described above. Further, the strip-shaped medium is not limited to the continuous sheet P. For example, as long as the medium is strip-shaped, a cloth or a resin film may be applied. 
     Note that the present disclosure is not limited to specific embodiments described above, and numerous additional modifications and variations are possible in light of the teachings within the technical scope of the appended claims. It is therefore to be understood that, the disclosure of this patent specification may be practiced otherwise by those skilled in the art than as specifically described herein, and such, modifications, alternatives are within the technical scope of the appended claims. Such embodiments and variations thereof are included in the scope and gist of the embodiments of the present disclosure and are included in the embodiments described in claims and the equivalent scope thereof. 
     The present disclosure is not limited to specific embodiments described above, and numerous additional modifications and variations are possible in light of the teachings within the technical scope of the appended claims. It is therefore to be understood that, the disclosure of this patent specification may be practiced otherwise by those skilled in the art than as specifically described herein, and such, modifications, alternatives are within the technical scope of the appended claims. Such embodiments and variations thereof are included in the scope and gist of the embodiments of the present disclosure and are included in the embodiments described in claims and the equivalent scope thereof. 
     The effects described in the embodiments of this disclosure are listed as the examples of preferable effects derived from this disclosure, and therefore are not intended to limit to the embodiments of this disclosure. 
     The embodiments described above are presented as an example to implement this disclosure. The embodiments described above are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, or changes can be made without departing from the gist of the invention. These embodiments and their variations are included in the scope and gist of the present disclosure and are included in the scope of the invention recited in the claims and its equivalent. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), digital signal processor (DSP), field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions.