Patent Publication Number: US-11046543-B2

Title: Web conveyance device with brake

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-118484, filed on Jun. 22, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a conveyance device which conveys a web. 
     2. Related Art 
     Japanese Patent Application Publication No. 2011-79651 proposes a printing apparatus which prints images on a long web being a print medium while conveying the web. 
     Moreover, there is a printing apparatus which prints images on a web by ejecting inks from inkjet heads while unwinding and conveying the web from a web roll, which is the web wound into a roll, and applying a brake to rotation of the web roll by using a brake mechanism. In this printing apparatus, appropriate tension for achieving excellent print image quality is applied to the web W by applying the brake to the rotation of the web roll. 
     In this printing apparatus, after the printing is completed and the conveyance of the web is stopped, the brake mechanism is maintained in an on state to apply the tension to the web and prevent slack in the web. However, when the brake mechanism is turned off in this state as a result of power-off of the printing apparatus, the web roll is rotated by the tension of the web to send out the web and slacken the web in some cases. When the web slackens, the web comes into contact with the inkjet heads and causes problems such as damaging of the inkjet heads in some cases. 
     SUMMARY 
     The aforementioned slack in the web can be prevented by providing a mechanism such as an external brake which prevents the web roll from rotating when the brake mechanism is turned off. 
     Providing the mechanism such as the external brake which prevents the web roll from rotating as described above leads to a complex device configuration. 
     The disclosure is directed to a conveyance device which can prevent slack in a web while avoiding a complex device configuration. 
     A conveyance device in accordance with some embodiments includes: a conveyance mechanism configured to unwind and convey a web from a web roll; a brake configured to brake rotation of the web roll; a controller configured to perform control of gradually reducing a torque of the brake and then turning off the brake in an operation of turning off the brake in a state where conveyance of the web by the conveyance mechanism is stopped and the brake is on with a tension applied to the web between the web roll and the conveyance mechanism. 
     The aforementioned configuration can prevent slack in the web while avoiding a complex device configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of a printing apparatus according to an embodiment. 
         FIG. 2  is a schematic configuration of a main portion the printing apparatus illustrated in  FIG. 1 . 
         FIG. 3  is an enlarged perspective view of a main portion of a web roll holder in the printing apparatus illustrated in 
         FIG. 1 . 
         FIG. 4  is a flowchart for explaining a power off operation including brake torque gradual reduction processing. 
         FIG. 5  is a view illustrating changes in brake torque in the brake torque gradual reduction processing. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     Description will be hereinbelow provided for an embodiment of the present invention by referring to the drawings. It should be noted that the same or similar parts and components throughout the drawings will be denoted by the same or similar reference signs, and that descriptions for such parts and components will be omitted or simplified. In addition, it should be noted that the drawings are schematic and therefore different from the actual ones. 
       FIG. 1  is a block diagram illustrating a configuration of a printing apparatus  1  including a conveyance device according to an embodiment.  FIG. 2  is a schematic configuration view of a main portion of the printing apparatus  1  illustrated in  FIG. 1 .  FIG. 3  is an enlarged perspective view of a main portion of a web roll holder  2  in the printing apparatus  1  illustrated in  FIG. 1 . Note that, in the following direction, a direction orthogonal to the sheet surface of  FIG. 2  is referred to as front-rear direction and a direction from the sheet surface toward the viewer is referred to as forward. Moreover, up, down, left, and right in the sheet surface of  FIG. 2  is referred to as directions of up, down, left, and right. In  FIGS. 2 and 3 , the directions of right, left, up, down, front, and rear are denoted by RT, LT, UP, DN, FR, and RR, respectively. 
     As illustrated in  FIG. 1 , the printing apparatus  1  includes the web roll holder  2 , a conveyor  3 , printers  4 A,  4 B, a rewinder  5 , an operation panel  6 , a power supply unit  7 , and a controller  8 . The parts of the printing apparatus  1  other than the printers  4 A,  4 B form the conveyance device. 
     The web roll holder  2  holds a web roll  9 . The web roll  9  is a web W wound into a roll and the web W is a long print medium made of film, paper, or the like. As illustrated in  FIGS. 1 to 3 , the web roll holder  2  includes a web roll support shaft  11 , a brake mechanism  12 , a brake driver  13 , an intermediate gear  14 , and a brake gear sensor  15 . 
     The web roll support shaft  11  rotatably supports the web roll  9 . The web roll support shaft  11  is formed in a long shape extending in a front-rear direction. The web roll support shaft  11  is formed of, for example, an air shaft. A web roll support shaft gear  11   a  meshing with the intermediate gear  14  is fixed to a rear end portion of the web roll support shaft  11 . 
     The brake mechanism (brake)  12  is a mechanism which brakes rotation of the web roll  9  by braking rotation of the web roll support shaft  11 . Specifically, the brake mechanism  12  applies brake torque (brake force) to the web roll support shaft  11 , the brake torque being torque (load torque) in the opposite direction to a rotating direction (clockwise in  FIG. 1 ) of the web W in unwinding. The brake mechanism  12  is formed of, for example, a powder brake. 
     The brake mechanism  12  includes an output shaft  12   a  which outputs the brake force. A brake gear  12   b  is attached to the output shaft  12   a . The brake torque of the brake mechanism  12  is transmitted to the web roll support shaft  11  via the brake gear  12   b , the intermediate gear  14 , and the web roll support shaft gear  11   a  and the brake is thereby applied to the web roll support shaft  11 . 
     The intermediate gear  14  transmits the brake torque of the brake mechanism  12  to the web roll support shaft gear  11   a . The intermediate gear  14  meshes with the web roll support shaft gear  11   a  and the brake gear  12   b.    
     The brake gear sensor  15  detects teeth of the brake gear  12   b.    
     The conveyor  3  conveys the web W. The conveyor  3  includes guide rollers  21  to  30 , twenty under-head rollers  31 , a meandering controller  32 , a pair of conveyance rollers (conveyance mechanism)  33 , a conveyance motor  34 , a motor driver  35 , and an encoder  36 . 
     The guide rollers  21  to  30  guide the web W conveyed in the conveyor  3 . The guide rollers  21  to  30  rotate by following the conveyed web W. The guide rollers  21  to  30  are each formed in a long shape extending in the front-rear direction. The guide rollers  21  to  30 , the under-head rollers  31 , the conveyance rollers  33 , and meandering control rollers  37 ,  38  of the meandering controller  32  to be described later forma conveyance route of the web W in the conveyor  3 . 
     The guide rollers  21 ,  22  guide the web W between the web roll  9  and the meandering controller  32 . The guide roller  21  is arranged near and on the right side of the web roll holder  2 . The guide roller  22  is arranged between the guide roller  21  and the meandering control roller  37  of the meandering controller  32  to be described later. 
     The guide rollers  23  to  29  guide the web W between the meandering controller  32  and the pair of the conveyance rollers  33 . The guide roller  23  is arranged slightly above and on the left side of the meandering control roller  38  of the meandering controller  32  to be described later. The guide roller  24  is arranged above the guide roller  23 . The guide roller  25  is arranged on the right side of the guide roller  24  at the same height as the guide roller  24 . The guide roller  26  is arranged below the guide roller  25  and above the guide roller  23 . The guide roller  27  is arranged on the left side of the guide roller  26 , near and on the right side of the web W between the guide rollers  23 ,  24 , at substantially the same height as the guide roller  26 . The guide roller  28  is arranged on the lower right side of the guide roller  27 . The guide roller  29  is arranged below and slightly on the right side of the guide roller  28 . 
     The guide roller  30  guides the web W between the pair of conveyance rollers  33  and the rewinder  5 . The guide roller  30  is arranged near and on the left side of the rewinder  5 . 
     The under-head rollers  31  support the web W below later-described head units  41  between the guide rollers  24  and  25  and between the guide rollers  26 ,  27 . The under-head rollers  31  are each formed in a long shape extending in the front-rear direction. Ten under-head rollers  31  are arranged in each of an area between the guide rollers  24 ,  25  and an area between the guide rollers  26 ,  27 . Moreover, two under-head rollers  31  are arranged directly below each head unit  41 . The under-head rollers  31  rotate by following the conveyed web W. 
     The meandering controller  32  corrects meandering which is fluctuation in the position of the web Win the width direction thereof (front-rear direction). The meandering controller  32  includes the meandering control rollers  37 ,  38 . 
     The meandering control rollers  37 ,  38  are rollers for guiding the web W and correcting the meandering of the web W. The meandering control rollers  37 ,  38  are each formed in a long shape extending in the front-rear direction. The meandering control rollers  37 ,  38  rotate by following the conveyed web W. The meandering control rollers  37 ,  38  are turned by a not-illustrated motor to be tilted with respect to the width direction of the web W as viewed in the left-right direction and thereby move the web W in the width direction to correct the meandering. The meandering control roller  37  is arranged on the right side of the guide roller  22 . The meandering control roller  38  is arranged above the meandering control roller  37 . 
     The pair of conveyance rollers  33  conveys the web W toward the rewinder  5  while nipping the web W. Conveyance of the web W by the conveyance rollers  33  causes the web roll  9  to rotate together with the web roll support shaft  11  and the web W is unwound from the web roll  9 . One of the paired conveyance rollers  33  is rotationally driven by the conveyance motor  34  and the other conveyance roller  33  rotates by following the one conveyance roller  33 . The pair of conveyance rollers  33  is arranged between the guide rollers  29 ,  30 . 
     The conveyance motor  34  rotationally drives one of the paired conveyance rollers  33 . 
     The motor driver  35  drives the conveyance motor  34 . 
     The encoder  36  outputs a pulse signal every time an output shaft of the conveyance motor  34  rotates by a predetermined angle. 
     The printers  4 A,  4 B print images respectively on a front side and a back side of the web W. The printer  4 A is arranged near and above the web W between the guide rollers  24 ,  25 . The printer  4 B is arranged near and above the web W between the guide rollers  26 ,  27 . Each of the printers  4 A,  4 B includes five head units  41 . 
     The head units  41  include inkjet heads (not illustrated) and print images by ejecting inks from nozzles of the inkjet heads to the web W. In each of the printers  4 A,  4 B, the five head units  41  eject inks of different colors, respectively. 
     The rewinder  5  rewinds the web W subjected to printing in the printers  4 A,  4 B. The rewinder  5  includes a rewinding shaft  46 , a rewinding motor  47 , and a motor driver  48 . 
     The rewinding shaft  46  rewinds and holds the web W. The rewinding shaft  46  is formed in a long shape extending in the front-rear direction. 
     The rewinding motor  47  rotates the rewinding shaft  46  clockwise in  FIG. 1 . Rotation of the rewinding shaft  46  causes the web W to be rewound on the rewinding shaft  46 . 
     The motor driver  48  drives the rewinding motor  47 . 
     The operation panel  6  displays various input screens and the like and receives input operations made by a user. The operation panel  6  includes a display unit (not illustrated) having a liquid crystal display panel and the like and an input portion (not illustrated) having various operation keys, a touch panel, and the like. 
     The power supply unit  7  supplies electric power to parts of the printing apparatus  1 . 
     The controller  8  controls operations of the entire printing apparatus  1 . The controller  8  includes a CPU, a RAM, a ROM, a hard disk drive, and the like. 
     When in the operation of printing, the controller  8  performs control such that the conveyor  3  unwinds and conveys the web W from the web roll  9  and the rewinder  5  rewinds the web W while the brake is applied to the rotation of the web roll  9  by using the torque of the brake mechanism  12  to apply tension to the web W. Then, the controller  8  prints images by causing the inks to be ejected from the head units  41  of the printers  4 A,  4 B to the conveyed web W. 
     Moreover, in an operation of turning off the brake mechanism  12  in a state where the conveyance of the web W by the conveyance rollers  33  is stopped and the brake mechanism  12  is on to apply tension to the web W between the web roll  9  and the pair of conveyance rollers  33 , the controller  8  performs control of turning off the brake mechanism  12  by performing brake torque gradual reduction processing. The brake torque gradual reduction processing is processing of gradually reducing the torque of the brake mechanism  12  and turning off the brake mechanism  12 . Specifically, for example, in the operation of powering off the printing apparatus  1  in a standby state where the conveyance of the web W is stopped and the brake mechanism  12  is on to apply tension to the web W between the web roll  9  and the pair of conveyance rollers  33 , the controller  8  turns off the brake mechanism  12  by performing the brake torque gradual reduction processing and then turns off the power. 
     The controller  8  stores a brake torque reduction rate table (not illustrated). The brake torque reduction rate table is a table in which motor torque, the type of the web W, the width of the web W, and a reduction rate of the brake torque are associated with one another. The brake torque reduction rate table is used to set (determine) the reduction rate of the brake torque in the brake torque gradual reduction processing. 
     The motor torque is torque of the conveyance motor  34 . The motor torque corresponds to the tension of the web W at the position of the conveyance rollers  33 . The types of the web W include a thick paper, a thin paper, and the like. The reduction rate of the brake torque is a reduction amount of the brake torque per unit time. 
     In the brake torque reduction rate table, the reduction rate of the brake torque depending on the type and width of the web W is set in advance for each range of a value of the motor torque at the start of the brake torque gradual reduction processing. If the type and width of the web W are the same, the higher the motor torque at the start of the brake torque gradual reduction processing is, the lower the reduction rate of the brake torque is set. Moreover, if the motor torque at the start of the brake torque gradual reduction processing and the width of the web W are the same, the more stretchable the type of the web W is, the lower the reduction rate of the brake torque is set. Furthermore, if the motor torque at the start of the brake torque gradual reduction processing and the type of the web W are the same, the greater the width of the web W is, the lower the reduction rate of the brake torque is set. 
     The higher the motor torque is, the higher the tension of the web W is and the greater is the risk that, if the brake torque is rapidly reduced, the tension in the web W is still present when the brake torque reaches zero and causes the web roll  9  to rotate, thereby slackening the web W. Accordingly, the higher the motor torque at the start of the brake torque gradual reduction processing is, the lower the reduction rate of the brake torque is set to slowly reduce the brake torque. 
     Moreover, the more stretchable the web W, the greater the risk that, if the brake torque is rapidly reduced, the greatly-stretched web W is rapidly unstretched and the rapid unstretching vigorously rotates the web roll  9 , thereby slackening the web W. Accordingly, the more stretchable the type of the web W is, the lower the reduction rate of the brake torque is set to slowly reduce the brake torque. 
     Furthermore, if the type of the web and the length of the web W remaining in the web roll  9  are the same, the larger the width of the web W is, the heavier the web roll  9  is. The heavier the web roll  9  is, the less likely the rotation of the web roll  9  is to stop once it starts to rotate due to large inertia and the more likely the web W is to slacken. In other words, the greater the width of the web W is, the more likely the web W is to slacken, if the other factors are the same. Accordingly, the greater the width of the web W is, the lower the reduction rate of the brake torque is set to slowly reduce the brake torque. 
     Next, operations in printing of the printing apparatus  1  are described. 
     When a print job is inputted, the controller  8  starts drive of the conveyance motor  34  and the rewinding motor  47  to start the conveyance of the web W. 
     In this example, the web roll  9  is assumed to be a roll continuously used after the completion the previous print job. Moreover, the power of the printing apparatus  1  is assumed to be left on after the completion the previous print job. In this case, the brake mechanism  12  is left on after the completion the previous print job and a value of brake torque T at the completion the previous print job is set. 
     When the conveyance of the web W is started, the controller  8  increases speed of conveyance of the web W by the conveyance rollers  33  at a predetermined acceleration, from the start of conveyance. 
     Then, when the speed of conveyance of the web W by the conveyance rollers  33  reaches a predetermined print conveyance speed Vg, the controller  8  controls the conveyance motor  34  and the rewinding motor  47  such that the conveyance transitions to constant speed conveyance of the web W at the print conveyance speed Vg from this moment. 
     After the start of the constant speed conveyance of the web W at the print conveyance speed Vg, the controller  8  starts printing based on the print job by using the printers  4 A,  4 B. 
     After the start of the constant speed conveyance of the web W at the print conveyance speed Vg, the controller  8  controls the conveyance motor  34  such that the conveyance speed of the web W is maintained to be constant at the print conveyance speed Vg, based on the output pulse signal of the encoder  36 . The conveyance speed of the web W is maintained to be constant to prevent deviation of landing positions of the inks ejected from the head units  41  to the web W and achieve excellent print image quality. 
     Specifically, the controller  8  calculates the number of revolutions of the conveyance motor  34  based on the output pulse signal of the encoder  36  and calculates the conveyance speed of the web W corresponding to the calculated number of revolutions. Then, the controller  8  controls the conveyance motor  34  such that the difference between the calculated conveyance speed and the print conveyance speed Vg is eliminated. The speed of conveyance of the web W by the conveyance motor  34  is thereby controlled to match the print conveyance speed Vg. 
     Moreover, during the conveyance of the web W, the controller  8  controls the brake torque T of the brake mechanism  12  depending on the outer diameter D of the web roll  9  such that the tension of the web W is maintained at set tension F. 
     At the start of the constant speed conveyance of the web W at the print conveyance speed Vg, a decrease amount of the outer diameter D of the web roll  9  from the start of conveyance of the web W is small. In other words, the outer diameter D of the web roll is substantially constant until the start of the constant speed conveyance of the web W at the print conveyance speed Vg. Accordingly, the brake torque T of the brake mechanism  12  is set to the value at the completion the previous print job which is the value at the start of conveyance of the web W, until the start of the constant speed conveyance of the web W at the print conveyance speed Vg. 
     However, the outer diameter D of the web roll  9  gradually decreases with lapse of time from the start of conveyance of web W. Accordingly, the controller  8  calculates the outer diameter D of the web roll  9  in real time and sets the brake torque of the brake mechanism  12  to the brake torque T depending on the calculation result in real time, in the period of the constant speed conveyance of the web W at the print conveyance speed Vg. 
     Specifically, the controller  8  calculates the number of revolutions of the brake mechanism  12  (number of revolution of the brake gear  12   b ) in real time based on an output signal of the brake gear sensor  15  and converts the calculated number of revolutions into the number of revolution N of the web roll  9 . Then, the controller  8  calculates the outer diameter D of the web roll  9  based on the following formula (1) by using the number of revolution N of the web roll  9  and the print conveyance speed Vg.
 
 D=Vg /( N ×π)  (1)
 
     Then, the controller  8  calculates the brake torque T depending on the outer diameter D of the web roll  9  by using the following formula (2) and sets the brake torque of the brake mechanism  12  to the calculated brake torque T.
 
 T=F×D /(2× Gb )  (2)
 
     In this formula, Gb is a reduction ratio (gear ratio) between the brake mechanism  12  and the web roll support shaft  11 . 
     As described above, the brake torque T is changed in real time depending on the decrease of the outer diameter D of the web roll  9  to maintain the tension of the web W at the set tension F. 
     When the printing based on the print job is completed, the controller  8  stops the conveyance motor  34  and the rewinding motor  47  and terminates the conveyance of the web W. The series of operations in printing is thereby completed. 
     The brake mechanism  12  is maintained in the on state after the completion of the printing operation. In this case, the brake torque T is maintained at the value at the completion of the conveyance of the web W. Maintaining the on state of the brake mechanism  12  after the completion of the printing operation prevents the web W from slackening due to free rotation of the web roll  9  and coming into contact with the inkjet heads of the head units  41 . 
     In the case where the web roll  9  is continuously used without being removed after the print operation is completed, the brake torque at the start of conveyance of the web W to perform a print operation based on the next print job is set to the same value as the brake torque at the completion of the previous print operation. This is because the outer diameter D of the web roll  9  at the start of the print operation based on the next print job is the same as the outer diameter D of the web roll  9  at the completion of the previous print operation. 
     When the web roll  9  is replaced and the print operation based on the print job is to be performed by using a new web roll  9 , conveyance of the web W for roll diameter calculation (calculation of the outer diameter D of the web roll  9 ) is performed before the start of the print operation. 
     In the conveyance of the web W for roll diameter calculation, the controller  8  causes the web W to be conveyed at roll diameter calculation conveyance speed Vc slower than the print conveyance speed Vg. In this case, the controller  8  sets the brake torque to a predetermined initial value. 
     During the conveyance of the web W at the roll diameter calculation conveyance speed Vc, the controller  8  calculates the number of revolutions N of the web roll  9  based on the output signal of the brake gear sensor  15 . Then, the controller  8  calculates the outer diameter D of the web roll  9  by using the following formula (3).
 
 D=Vc /( N ×π)  (3)
 
     After calculating the outer diameter D of the web roll  9 , the controller  8  calculates the brake torque T depending on the calculated outer diameter D by using the aforementioned formula (2). Then, the controller  8  sets the brake torque of the brake mechanism  12  to the calculated brake torque T and terminates the conveyance of the web W for roll diameter calculation. Thereafter, the controller  8  executes the aforementioned print operation. 
     Next, a power off operation including the brake torque gradual reduction processing is described with reference to the flowchart of  FIG. 4 . 
     The power off operation of the printing apparatus  1  is performed in response to a power off instruction made by the user when the printing apparatus  1  is in the standby state. In the printing apparatus  1 , the standby state is a state where the conveyance of the web W by the conveyance rollers  33  is stopped and the brake mechanism  12  is on and tension is applied to the web W between the web roll  9  and the pair of conveyance rollers  33 . 
     In step S 1  of  FIG. 4 , the controller  8  determines whether the power off instruction is made or not. The power off instruction is made by the user operating the operation panel  6 . When the controller  8  determines that the power off instruction is not made (step S 1 : NO), the controller  8  repeats step S 1 . 
     When the controller  8  determines that the power off instruction is made (step S 1 : YES), in step S 2 , the controller  8  determines the reduction rate of the brake torque in the brake torque gradual reduction processing. 
     Specifically, the controller  8  obtains the value of the current motor torque from the motor driver  35 . Then, the controller  8  refers to the brake torque reduction rate table and determines the reduction rate of the brake torque based on the motor torque, the type of the web W, and the width of the web W. The type and width of the web W are obtained from job data of the print job in the latest print operation. 
     Next, in step S 3 , the controller  8  starts reducing the brake torque. Specifically, the controller  8  starts the brake torque gradual reduction processing. Then, the controller  8  gradually reduces the brake torque as illustrated in  FIG. 5 . In this case, the controller  8  reduces the brake torque at the reduction rate determined in step S 2 . 
       FIG. 5  illustrates changes in the brake torque in the cases where the motor torque at the start of the brake torque gradual reduction processing is “high” and “low” in each of the cases where the outer diameter D of the web roll  9  at the start of the brake torque gradual reduction processing is “large” and “small.” Note that, in the example of  FIG. 5 , the type and width of the web W are assumed to be the same in all cases. 
     As apparent from the aforementioned formula (2), the brake torque T is proportional to the outer diameter D of the web roll  9 . Accordingly, in  FIG. 5 , the cases where the brake torque at the start of the brake torque gradual reduction processing is T 1  correspond to the cases where the outer diameter D of the web roll  9  is “large” and the cases where the brake torque is T 2  correspond to the cases where the outer diameter D of the web roll  9  is “small.” 
     As illustrated in  FIG. 5  and as described above, the higher the motor torque at the start of the brake torque gradual reduction processing is, the lower the reduction rate of the brake torque is set. 
     As described above, the motor torque corresponds to the tension of the web W at the position of the conveyance rollers  33 . As described above, the tension of the web W is controlled to be the set tension F by setting the brake torque T depending on the outer diameter D of the web roll  9 . However, since the degree of stretching of the web W and the like vary depending on the type of the web W and the like, the actual tension of the web W at the position of the conveyance rollers  33  is sometimes different from the set tension F. Accordingly, in the embodiment, the reduction rate of the brake torque in the brake torque gradual reduction processing is adjusted by using the motor torque which corresponds to the tension of the web W at the position of the conveyance rollers  33  at the start of the brake torque gradual reduction processing as described above. 
     Returning to  FIG. 4 , in step S 4 , the controller  8  determines whether the brake torque has reached zero. When the controller  8  determines the brake torque has not reached zero yet (step S 4 : NO), the controller  8  repeats step S 4 . 
     In this case, when the brake torque reaches zero, the brake mechanism  12  is turned off and the brake torque gradual reduction processing is terminated. When the controller  8  determines that the brake torque has reached zero (step S 4 : YES), in step S 5 , the controller  8  turns the power off. Specifically, the controller  8  causes the power supply unit  7  to stop supply of power to the parts of the printing apparatus  1 . The power off operation including the brake torque gradual reduction processing is thus completed. 
     Note that, in the printing apparatus  1 , the controller  8  turns off the brake mechanism  12  by performing the brake torque gradual reduction processing not only when the power is turned off as described above but also when, for example, the conveyance of the web W is stopped and the brake mechanism is turned off because abnormality is expected to occur. 
     Specifically, the controller  8  sometimes stops the conveyance of the web W by the conveyance rollers  33  and turns off the brake mechanism  12  because conveyance abnormality caused by abnormality in the conveyance motor  34  or abnormality of the web W being unwound from the web roll  9  to the limit is expected to occur. In such cases, the controller  8  turns off the brake mechanism  12  by performing the brake torque gradual reduction processing as described above. When the expected abnormality is avoided, the controller  8  sets the brake torque of the brake mechanism  12  to the brake torque at the stop of the conveyance and restart the conveyance of the web W. 
     As described above, in the printing apparatus  1 , in the operation of turning off the brake mechanism  12  in the state where the conveyance of the web W by the conveyance rollers  33  is stopped and the brake mechanism  12  is on to apply tension to the web W between the web roll  9  and the pair of conveyance rollers  33 , the controller  8  turns off the brake mechanism  12  by performing the brake torque gradual reduction processing to gradually reduce the torque of the brake mechanism  12 . The brake mechanism  12  is thereby turned off with the tension of the web W gradually released. This prevents the web W from slackening due to rotation of the web roll  9 . 
     Moreover, since there is no need to provide a mechanism such as an external brake to prevent the rotation of the web roll  9 , it is possible to avoid a complex device configuration and an increase in cost. 
     Accordingly, the printing apparatus  1  can prevent slack in the web W while avoiding a complex device configuration. 
     Moreover, the controller  8  sets the reduction rate of the brake torque in the brake torque gradual reduction processing depending on the motor torque at the start of the brake torque gradual reduction processing, the type of the web W, and the width of the web W. The reduction rate of the brake torque in the brake torque gradual reduction processing can be thereby appropriate set depending on the tension of the web W and the like. Accordingly, slack in the web W can be further prevented. 
     Note that, although three factors which are the motor torque at the start of the brake torque gradual reduction processing, the type of the web W, and the width of the web W are used to determine the reduction rate of the brake torque in the brake torque gradual reduction processing in the aforementioned embodiment, one or two of these three factors may be omitted. Moreover, it is possible to use a predetermined reduction rate set in advance without using any of these factors. 
     Moreover, although the motor torque at the start of the brake torque gradual reduction processing is used as the torque corresponding to the tension of the web W to determine the reduction rate of the brake torque in the brake torque gradual reduction processing in the aforementioned embodiment, the set tension F may be used. Moreover, both of the set tension F and the tension of the web W calculated from the motor torque at the start of the brake torque gradual reduction processing may be used. For example, an average value of the set tension F and the calculated tension may be used. 
     The embodiments of the disclosure have, for example, the following configurations. 
     A conveyance device includes: a conveyance mechanism configured to unwind and convey a web from a web roll; a brake configured to brake rotation of the web roll; a controller configured to perform control of gradually reducing a torque of the brake and then turning off the brake in an operation of turning off the brake in a state where conveyance of the web by the conveyance mechanism is stopped and the brake is on with a tension applied to the web between the web roll and the conveyance mechanism. 
     The controller may be configured to determine a reduction rate of the torque of the brake depending on at least one of a tension of the web at start of reduction of the torque of the brake, a type of the web, and a width of the web. 
     The conveyance mechanism may include a pair of rollers. 
     Embodiments of the present invention have been described above. However, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     Moreover, the effects described in the embodiments of the present invention are only a list of optimum effects achieved by the present invention. Hence, the effects of the present invention are not limited to those described in the embodiment of the present invention.