Patent Publication Number: US-10780722-B2

Title: Meandering control device for web

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-067121, filed on Mar. 30, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a meandering control device configured to correct meandering of a web. 
     2. Related Art 
     There is known an inkjet printing apparatus which performs printing on a print medium being a long web by ejecting inks from inkjet heads to the web while conveying the web. 
     When meandering of the web which is deviation of the position of the web in a width direction occurs in such an inkjet printing apparatus, ink landing position deviation occurs and print image quality decreases in some cases. 
     To counter this, Japanese Patent Application Publication No. 2012-224468 proposes a meandering control device which suppresses the meandering of a web in such a way that a roller on which the web is wound is turned to tilt with respect to a width direction of the web and thereby corrects the position of the web in the width direction. 
     The meandering control device suppresses the meandering of the web by controlling the angle of the roller such that an edge of the web returns to a normal position, based on a position signal outputted by an edge sensor configured to detect the position of the edge of the web. 
     The position signal outputted by the edge sensor sometimes includes a noise component due to an effect of disturbance. In order to reduce a decrease in accuracy of meandering correction caused by the effect of disturbance, the noise component is removed from the position signal by using a filter formed of passive elements such as a resistor, a capacitor, and an inductor. 
     SUMMARY 
     When a filter formed of passive elements as described above is used, the number of parts on an electric circuit increases. Moreover, it is time-consuming to select parts for setting the time constant of the filter suitable for the apparatus. 
     The disclosure is directed to a meandering control device which can reduce the number of parts on an electric circuit and time required for selection of parts. 
     A meandering control device in accordance with some embodiments includes: a detector configured to detect a position of an edge of a web being conveyed in a width direction of the web; a corrector configured to correct a position of the web being conveyed in the width direction; and a controller including a digital filter configured to perform filtering on a position signal indicating the position of the edge detected by the detector to attenuate a noise component. The controller is configured to drive the corrector to correct the position of the web in the width direction based on the position signal after the filtering. 
     According to the aforementioned configuration, it is possible to reduce the number of parts on an electric circuit and time required for selection of parts. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration view of a print system including an inkjet printing apparatus having a meandering control device according to a first embodiment. 
         FIG. 2  is a perspective view illustrating a meandering corrector and an edge sensor of the inkjet printing apparatus. 
         FIG. 3  is a control block diagram of a print system illustrated in  FIG. 1 . 
         FIG. 4  is a block diagram illustrating a configuration of a printing apparatus controller in the inkjet printing apparatus. 
         FIG. 5  is a block diagram of a digital filter included in a meandering controller of the printing apparatus controller. 
         FIG. 6  is a view illustrating an example of signal waveforms before and after filtering by the digital filter in the first embodiment. 
         FIG. 7  is a view explaining an example of a time constant in a second embodiment. 
         FIG. 8  is a view illustrating an example of signal waveforms before and after filtering by a digital filter in the second embodiment. 
         FIG. 9  is a view explaining a web with holes. 
         FIG. 10  is a view explaining an example of a time constant in a third embodiment. 
         FIG. 11  is a view illustrating an example of signal waveforms before and after filtering by a digital filter in the third embodiment. 
     
    
    
     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 schematic configuration view of a print system including an inkjet printing apparatus having a meandering control device according to a first embodiment of the present invention.  FIG. 2  is a perspective view illustrating a meandering corrector and an edge sensor of the inkjet printing apparatus.  FIG. 3  is a control block diagram of the print system illustrated in  FIG. 1 .  FIG. 4  is a block diagram illustrating a configuration of a printing apparatus controller in the inkjet printing apparatus.  FIG. 5  is a block diagram of a digital filter included in a meandering controller of the printing apparatus controller. In the following description, a direction orthogonal to the sheet surface of  FIG. 1  is referred to as front-rear direction and a direction from the sheet surface toward the viewer is referred to as front. Moreover, up, down, left, and right in the sheet surface of  FIG. 1  are referred to as directions of up, down, left, and right. In  FIGS. 1, 2, and 9  (third embodiment), directions of right, left, up, down, front, rear, width direction, and conveyance direction are denoted by RT, LT, UP, DN, FR, RR, WD, and CD, respectively. 
     As illustrated in  FIGS. 1 and 3 , the print system  1  according to the first embodiment includes an unwinder  2 , the inkjet printing apparatus  3 , and a rewinder  4 . 
     The unwinder  2  unwinds a web W being a long print medium made of film, paper, or the like to the inkjet printing apparatus  3 . The unwinder  2  includes a web roll support shaft  11 , a guide roller  12 , a brake  13 , and an unwinder controller  14 . 
     The web roll support shaft  11  rotatably supports a web roll  16 . The web roll support shaft  11  is formed in an elongated shape extending in the front-rear direction. The web roll  16  is the web W wound into a roll. 
     The guide roller  12  guides the web W between the web roll  16  and a guide roller  31  of the inkjet printing apparatus  3  to be described later. The guide roller  12  rotates by following the web W being conveyed. 
     The brake  13  applies brake to the web roll support shaft  11 . Tension is thereby applied to the web W between the web roll  16  and a pair of conveyance rollers  42  of the inkjet printing apparatus  3  to be described later. 
     The unwinder controller  14  controls the brake  13 . The unwinder controller  14  includes a CPU, a ROM, a RAM, a hard disk drive, and the like. 
     The inkjet printing apparatus  3  prints images on the web W while conveying the web W unwound from the unwinder  2 . The inkjet printing apparatus  3  includes a conveyor  21 , a meandering corrector  22 , an edge sensor  23 , printers  24 A,  24 B, and a printing apparatus controller  25 . Note that the meandering corrector (corrector)  22 , the edge sensor (detector)  23 , and the printing apparatus controller (controller)  25  form a meandering control device. 
     The conveyor  21  unwinds and conveys the web W from the web roll  16 . The conveyor  21  includes guide rollers  31  to  40 ,  20  under-head rollers  41 , the pair of conveyance rollers  42 , and a conveyance motor  43 . 
     The guide rollers  31  to  40  guide the web W conveyed in the inkjet printing apparatus  3 . The guide rollers  31  to  40  rotate by following the web W being conveyed. The guide rollers  31  to  40 , the under-head rollers  41 , the conveyance rollers  42 , and meandering correction rollers  46 ,  47  of the meandering corrector  22  to be described later form a conveyance route of the web W in the inkjet printing apparatus  3 . 
     The guide rollers  31 ,  32  guide the web W between the unwinder  2  and the meandering corrector  22 . The guide roller  31  is arranged in a lower portion of a left end portion of the inkjet printing apparatus  3 . The guide roller  32  is arranged between the guide roller  31  and the meandering correction roller  46  of the meandering corrector  22  to be described later. 
     The guide rollers  33  to  39  guide the web W between the meandering corrector  22  and the pair of conveyance rollers  42 . The guide roller  33  is arranged on the left side of the meandering correction roller  47  of the meandering corrector  22  to be described later. The guide roller  34  is arranged above the guide roller  33 . The guide roller  35  is arranged on the right side of the guide roller  34  at the same height as the guide roller  34 . The guide roller  36  is arranged at a position below the guide roller  35  and above the guide roller  33 . The guide roller  37  is arranged at a position which is on the left side of the guide roller  36  and which is near and on the right side of the web W between the guide rollers  33 ,  34  at substantially the same height as the guide roller  36 . The guide roller  38  is arranged on the lower right side of the guide roller  37 . The guide roller  39  is arranged below and slightly on the right side of the guide roller  38 . 
     The guide roller  40  guides the web W between the pair of conveyance rollers  42  and the rewinder  4 . The guide roller  40  is arranged in a lower portion of a right end portion of the inkjet printing apparatus  3 . 
     The under-head rollers  41  support the web W under head units  51  to be described later in an area between the guide rollers  34 ,  35  and an area between the guide rollers  36 ,  37 . The under-head rollers  41  rotate by following the web W being conveyed. Ten under-head rollers  41  are arranged in each of the area between the guide rollers  34 ,  35  and the area between the guide rollers  36 ,  37 . Moreover, two under-head rollers  41  are arranged just under each head unit  51 . 
     The pair of conveyance rollers  42  conveys the web W toward the rewinder  4  while nipping the web W. The pair of conveyance rollers  42  is arranged between the guide rollers  39 ,  40 . 
     The conveyance motor  43  rotationally drives the conveyance rollers  42 . 
     The meandering corrector  22  corrects meandering which is fluctuation in the position of the web W in a width direction (front-rear direction) by correcting the position of the web W in the width direction. The meandering corrector  22  is also able to correct meandering generated by skewing of the web W sent out from one or more rollers. The meandering corrector  22  is arranged upstream of the printer  24 A in the conveyance direction of the web W on the conveyance route of the web W. Specifically, the meandering corrector  22  is arranged between the guide rollers  32 ,  33 . As illustrated in  FIGS. 1 to 3 , the meandering corrector  22  includes the meandering correction rollers  46 ,  47 , a meandering correction roller holder  48 , a meandering correction motor  49 , and limit switches  50 A,  50 B. 
     The meandering correction rollers  46 ,  47  are rollers for guiding the web W and correcting the meandering of the web W. The meandering correction rollers  46 ,  47  are each formed in a long columnar shape extending in the front-rear direction. The meandering correction rollers  46 ,  47  rotate by following the web W being conveyed. The meandering correction roller  46  is arranged on the right side of the guide roller  32 . The meandering correction roller  47  is arranged above the meandering correction roller  46 . The meandering correction rollers  46 ,  47  move the web W in the width direction by being turned to tilt with respect to the width direction (front-rear direction) of the web W as viewed in the left-right direction and thereby correct the meandering. 
     The meandering correction roller holder  48  holds the meandering correction rollers  46 ,  47 . The meandering correction roller holder  48  is turnable about a turning axis  48   a  parallel to the left-right direction. The meandering correction rollers  46 ,  47  can be thereby turned to tilt with respect to the width direction (front-rear direction) of the web W as viewed in the left-right direction. 
     The meandering correction motor  49  turns the meandering correction roller holder  48  about the turning axis  48   a.    
     The limit switches  50 A,  50 B are each a switch configured to detect that the turning angle of the meandering correction rollers  46 ,  47  relative to the horizontal has reached its upper limit. The limit switch  50 A turns on when the rotation angle of the meandering correction rollers  46 ,  47  in one turning direction reaches the upper limit. The limit switch  50 B turns on when the turning angle of the meandering correction rollers  46 ,  47  in the other turning direction reaches the upper limit. 
     The edge sensor  23  detects the position of an edge of the web W in the width direction and outputs a position signal indicating the detected position of the edge. The edge sensor  23  is arranged near and downstream of the meandering corrector  22  in the conveyance direction of the web W. The edge sensor  23  includes a light projector  23   a  and a light receiver  23   b.    
     The light projector  23   a  and the light receiver  23   b  are arranged to face each other with a front end portion of the web W therebetween. The light projector  23   a  emits band-shaped parallel light toward the web W. The light projector  23   a  may include a light emitter. The light receiver  23   b  receives the parallel light emitted by the light projector  23   a.    
     The width of a portion in which the parallel light emitted by the light projector  23   a  is blocked by the web W changes depending on the position of the front edge of the web W. Accordingly, the position of the edge of the web W is detected by using the width of the parallel light received by the light receiver  23   b . The light receiver  23   b  outputs an analog signal depending on the width of the received parallel light as the position signal indicating the position of the edge of the web W. 
     Note that the edge sensor  23  may be arranged on the rear end side of the web to detect the position of a rear edge of the web W. 
     The printers  24 A,  24 B print images on a front side and a back side of the web W, respectively. The printer  24 A is arranged near and above the web W between the guide rollers  34 ,  35 . The printer  24 B is arranged near and above the web W between the guide rollers  36 ,  37 . The printers  24 A,  24 B each include five head units  51 . 
     The head units  51  have 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  24 A,  24 B, the five head units  51  eject inks of different colors, respectively. 
     The printing apparatus controller  25  controls operations of units in the inkjet printing apparatus  3 . As illustrated in  FIG. 4 , the printing apparatus controller  25  includes a main controller  61  and a meandering controller  62 . 
     The main controller  61  is responsible for control of the entire inkjet printing apparatus  3 . 
     The meandering controller  62  includes a later-described digital filter  87  configured to perform filtering on the position signal detected by the edge sensor  23  and indicating the position of the web W to attenuate a noise component generated by disturbance, during the conveyance of the web W in the printing operation. The meandering controller  62  performs control such that the meandering corrector  22  corrects the position of the web W in the width direction based on the signal after the filtering by the digital filter  87 . The configuration of the meandering controller  62  is described in detail later. 
     The rewinder  4  rewinds the web W subjected to printing in the inkjet printing apparatus  3 . The rewinder  4  includes a buffer unit  71 , a brake roller  72 , a brake  73 , a rewinding shaft  74 , a rewinding motor  75 , and a rewinder controller  76 . 
     The buffer unit  71  maintains slack of the web W between the guider roller  40  of the inkjet printing apparatus  3  and the brake roller  72 . The buffer unit  71  includes supporting rollers  77 ,  78  and a dancer roller  79 . 
     The supporting rollers  77 ,  78  support the web W between the guide roller  40  and the brake roller  72 . The supporting rollers  77 ,  78  are arranged away from each other in the left-right direction at the same height. The supporting rollers  77 ,  78  rotate by following the web W being conveyed. 
     The dancer roller  79  pushes down the web W with its own weight between the supporting rollers  77 ,  78 . The buffer unit  71  thereby absorbs the slack of the web W between the guide roller  40  and the brake roller  72 . The dancer roller  79  moves up and down depending on fluctuation in the slack amount of the web W between the guide roller  40  and the brake roller  72 . 
     The brake roller  72  is a roller for applying brake to the web W rewound on the rewinding shaft  74 . The brake roller  72  rotates by following the web W rewound on the rewinding shaft  74 . 
     The brake  73  applies tension to the web W rewound on the rewinding shaft  74  by applying brake to the web W via the brake roller  72 . This prevents formation of wrinkles and the like in the web W rewound on the rewinding shaft  74 . 
     The rewinding shaft  74  rewinds and holds the web W. 
     The rewinding motor  75  rotates the rewinding shaft  74  clockwise in  FIG. 1 . Rotation of the rewinding shaft  74  causes the web W to be rewound on the rewinding shaft  74 . 
     The rewinder controller  76  controls the brake  73  and the rewinding motor  75 . The rewinder controller  76  includes a CPU, a ROM, a RAM, a hard disk drive, and the like. 
     Next, the configuration of the meandering controller  62  is described. 
     As illustrated in  FIG. 4 , the meandering controller  62  includes a drive controller  81  and a motor driver  82 . 
     The drive controller  81  controls the motor driver  82 . The drive controller  81  is formed of a micro-computer or the like including a CPU, a ROM, a RAM, and the like. The drive controller  81  includes an A/D convertor  86 , the digital filter  87 , an edge position convertor  88 , a subtractor  89 , a control switching part  90 , a PI controller  91 , an operation amount limiter  92 , an on-off controller  93 , a limit switch detector  94 , a limit switch limiter  95 , an operation mode switching part  96 , and an output controller  97 . The aforementioned parts of the drive controller  81  are implemented by causing the CPU to execute a program. 
     The A/D convertor  86  converts the position signal which is an analog signal received from the edge sensor  23  to a digital signal. 
     The digital filter  87  is a low-pass filter configured to perform filtering on the position signal converted to the digital signal to attenuate the noise component generated by disturbance. As illustrated in  FIG. 5 , the digital filter  87  includes delayers  101 ,  102 , adders  103 ,  104 , and multipliers  105 ,  106 . 
     A digital value of the position signal is inputted into the digital filter  87  at a sampling cycle Ts in the AD conversion performed by the A/D convertor  86 . The delayer  101  delays the input value by one cycle. The adder  103  adds the input value delayed by one cycle in the delayer  101  to the input value. The multiplier  105  multiplies the calculation result of the adder  103  by a reciprocal 1/Kg 0  of a filter coefficient Kg 0 . The delayer  102  delays a calculation result of the adder  104  by one cycle. The multiplier  106  multiplies the calculation result of the adder  104  delayed by one cycle in the delayer  102  by a filter coefficient Kg 1 . The adder  104  adds the calculation result of the multiplier  106  to the calculation result of the multiplier  105  and outputs the result of the addition. 
     The following formula (1) is the aforementioned digital filter  87  expressed in a formula.
 
output=[(1+ Z   −1 )]/[( Kg 0×(1+ Kg 1× Z   −1 )]×input  (1)
 
     The filter coefficients Kg 0 , Kg 1  in this formula are coefficients expressed by the following formulae (2) and (3), respectively.
 
 Kg 0=(2 ×τ/Ts )+1  (2)
 
 Kg 1=[(2 ×τ/Ts )−1]/ Kg 0  (3)
 
     Note that τ is the time constant of the digital filter  87 . A value set as a value at which the noise component included in the position signal can be removed is used as the time constant τ. 
     The edge position convertor  88  converts the value (digital value) of the signal after the filtering by the digital filter  87  to the value of the edge position of the web W. 
     The subtractor  89  subtracts the value of the edge position calculated by the edge position convertor  88  from a target value indicating a target position of the edge position to calculate an edge position error. 
     The control switching part  90  switches a control method of the meandering correction motor  49  between PI control and on-off control. In this example, the on-off control is a method of controlling the meandering correction motor  49  based only on the direction in which the edge of the web W deviates from the target position, regardless of the amount of the edge position error. Switching of the control method between the PI control and the on-off control is performed according to, for example, a command from the main controller  61  given in response to an instruction of a user. 
     The PI controller  91  performs PI calculation to calculate an operation amount of the meandering correction motor  49  for moving the position of the edge of the web W to the target position based on the edge position error. In this example, the operation amount is a value indicating the rotation speed and the rotation direction of the meandering correction motor  49 . 
     When the operation amount calculated by the PI controller  91  is a value outside a preset range from a lower limit value to an upper limit value of the operation amount, the operation amount limiter  92  limits the operation amount such that the operation amount falls within this range. 
     The on-off controller  93  outputs an operation amount of the meandering correction motor  49  for returning the edge of the web W in the direction toward the target position based on the edge position error. This operation amount is a value set in advance depending on the direction of deviation of the web W from the target position. 
     The limit switch detector  94  detects turn-on of the limit switches  50 A,  50 B and outputs a signal of this detection to the limit switch limiter  95 . 
     When any one of the limit switches  50 A,  50 B turns on, the limit switch limiter  95  limits the operation amount such that the rotation angle of the meandering correction rollers  46 ,  47  does not further increase from then on. 
     The operation mode switching part  96  switches an operation mode of the meandering control between an automatic mode and a manual mode. The automatic mode is a mode in which the meandering control of correcting the position of the web W in the width direction is performed automatically based on the position signal from the edge sensor  23 . In the printing operation, the operation mode is generally set to the automatic mode. In the automatic mode, the operation mode switching part  96  outputs the operation amount from the limit switch limiter  95  to the output controller  97 . 
     The manual mode is a mode in which the web W is moved to a specified position in the width direction according to a command from the main controller  61  given in response to an instruction of the user. In the manual mode, the operation mode switching part  96  outputs the operation amount for moving the web W which corresponds to the specified position to the output controller  97 . 
     The output controller  97  converts the operation amount received from the operation mode switching part  96  to a duty ratio of a pulse width modulation (PWM) signal for controlling the drive of the meandering correction motor  49 . Moreover, the output controller  97  determines the rotation direction of the meandering correction motor  49  based on the operation amount. Then, the output controller  97  outputs the PWM signal with the calculated duty ratio and a signal for controlling the rotation direction of the meandering correction motor  49  to the motor driver  82 . 
     The motor driver  82  drives the meandering correction motor  49  based on the PWM signal and the signal for controlling the rotation direction of the meandering correction motor  49  received from the output controller  97 . 
     Next, operations of the print system  1  are described. 
     In printing, when a print job is inputted, the main controller  61  of the printing apparatus controller  25  starts the drive of the conveyance motor  43 , the unwinder controller  14  starts the drive of the brake  13 , and the rewinder controller  76  starts the drive of the brake  73  and the rewinding motor  75 . The conveyance rollers  42  thereby start to convey the web W while unwinding the web W from the web roll  16  by using tension and the rewinding shaft  74  starts the rewinding of the web W. Thus, the conveyance of the web W is started. 
     When the conveyance of the web W starts and then the conveyance speed of the web W reaches predetermined print conveyance speed Vg, the main controller  61  controls the conveyance motor  43  such that the print conveyance speed Vg is maintained. 
     After the start of constant speed conveyance of the web W at the print conveyance speed Vg, the main controller  61  controls the printers  24 A,  24 B based on the print job to cause them to print images on the web W. 
     Moreover, after the start of constant speed conveyance of the web W at the print conveyance speed Vg, the main controller instructs the meandering controller  62  to start the meandering correction control of the web W by the meandering corrector  22  before the printing by the printers  24 A,  24 B is started. 
     The meandering controller  62  starts the meandering correction control of the web W in response to the instruction of the main controller  61 . In this example, it is assumed that, in the meandering controller  62 , the control switching part  90  has selected the PI control. Moreover, the automatic mode is assumed to be set as the operation mode of the meandering control. 
     In the meandering correction control, the A/D convertor  86  of the meandering controller  62  converts the analog position signal received from the edge sensor  23  to the digital signal. The digital filter  87  performs filtering on the digitized position signal to attenuate the noise component generated by disturbance. 
     The edge position convertor  88  converts the digital value obtained after the filtering by the digital filter  87  to the value indicating the edge position of the web W. The subtractor  89  calculates the edge position error from the target value and the value indicating the edge position which is calculated by the edge position convertor  88 . 
     The PI controller  91  performs the PI calculation to calculate the operation amount of the meandering correction motor  49  based on the edge position error calculated by the subtractor  89 . The operation amount limiter  92  provides upper and lower limits to limit the operation amount calculated by the PI controller  91 . The limit switch limiter  95  limits the operation amount when any one of the limit switches  50 A,  50 B turns on. 
     In this case, since the automatic mode is set as the operation mode of the meandering control, the operation mode switching part  96  outputs the operation amount received from the limit switch limiter  95  to the output controller  97 . 
     The output controller  97  outputs the PWM signal with the duty ratio corresponding to the received operation amount and the signal for controlling the rotation direction of the meandering correction motor  49  to the motor driver  82 . Then, the motor driver  82  drives the meandering correction motor  49  based on the PWM signal and the signal for controlling the rotation direction of the meandering correction motor  49  received from the output controller  97 . 
     The meandering correction motor  49  thus turns the meandering correction rollers  46 ,  47  to reduce the deviation of the web W from the target position and the meandering of the web W is thereby reduced. 
     An example of signal waveforms before and after the filtering by the digital filter  87  is illustrated in  FIG. 6 . As illustrated in  FIG. 6 , since the noise component is removed by the filtering, a decrease in accuracy of the meandering correction caused by an effect of disturbance is reduced. 
     When the printing based on the print job is completed, the unwinder controller  14  stops the brake  13 , the main controller  61  of the printing apparatus controller  25  stops the conveyance motor  43 , and the rewinder controller  76  stops the brake  73  and the rewinding motor  75 . The conveyance of the web W is thereby terminated and the series of operations is completed. 
     As described above, in the inkjet printing apparatus  3 , the digital filter  87  performs the filtering on the position signal to attenuate the noise component. Accordingly, there is no need to form the filter by using passive elements such as a resistor and the number of parts on an electric circuit and time required to select the parts can be reduced. 
     Next, description is given of a second embodiment partially changed from the first embodiment. 
     In the second embodiment, the time constant τ of the digital filter  87  is set such that the digital filter  87  passes the meandering component of the web W corresponding to the rotation cycle of the web roll  16  while removing the noise component in the position signal. 
     The web roll  16  sometimes swings in the front-rear direction while rotating depending on the holding state by the web roll support shaft  11 . In such a case, the meandering of the web W with a meandering cycle equal to the rotation cycle of the web roll  16  occurs. 
     In the inkjet printing apparatus  3 , the web W is conveyed at the constant speed of the print conveyance speed Vg [m/s] in the printing operation as described above. Accordingly, the rotation cycle (meandering cycle) Tr [s] of the web roll  16  in the printing operation is expressed by the following formula (4).
 
 tr=φ×π/Vg   (4)
 
     In this formula, φ is a roll diameter [m] which is the outer diameter of the web roll  16 . 
     Moreover, the time constant τ is expressed by the following formula (5).
 
τ=− T /(log  e (1− Am/ 100))  (5)
 
     In this formula, Am is a signal amplitude percentage [%] and T is a signal cycle [s]. 
     Assuming that T=Tr, the following formula (6) is obtained from formulae (4) and (5).
 
τ=−(φ×π/ Vg )/(log  e (1− Am/ 100))  (6)
 
     The closer the signal amplitude percentage is to 100%, the smaller the degree of the attenuation of the signal is. Accordingly, it is possible to obtain a time constant τ which hardly attenuates the meandering component of the web W corresponding to the rotation cycle of the web roll  16  by setting a value which is almost 100% as the signal amplitude percentage Am in formula (6). 
     Note that, in the printing operation, the roll diameter φ becomes smaller and the rotation cycle Tr becomes shorter as the printing proceeds. The state where the signal amplitude percentage Am is almost 100% can be maintained throughout a period in which the roll diameter φ is reduced to the smallest diameter from the greatest roll diameter φ by setting the time constant τ to a value at which the signal amplitude percentage Am is almost 100% when the roll diameter φ is smallest. Here, the state where the roll diameter φ is greatest is the state where the web roll  16  is fresh and the state where the roll diameter φ is smallest is the state where the web W of the web roll  16  is completely used. 
     Accordingly, in the second embodiment, the time constant of the digital filter  87  is set to a time constant τ calculated from formula (6) with the signal amplitude percentage Am set to a predetermined signal amplitude percentage Amp which is almost 100% (for example, a value equal to or higher than 99.99%) and with the roll diameter φ set to the smallest value. In this case, the smallest value of the roll diameter φ corresponds to the outer diameter of a core (paper tube) of the web roll  16 . 
     For example, assume that Vg=0.7 m/s, the smallest value of the roll diameter φ is 0.12 m, and Am=Amp=99.99%. In this case, the time constant τ in the case where the roll diameter φ takes the smallest value which is calculated from formula (6) is 0.058 s and this value is set for the digital filter  87 . 
     A relationship between the signal cycle and the signal amplitude percentage Am in the case where time constant τ=0.058 s is illustrated in  FIG. 7 . 
     When the roll diameter φ is 0.12 m which is the smallest value, the rotation cycle Tr of the web roll  16  is 0.54 s according to formula (4). In this case, as illustrated in  FIG. 7 , the signal amplitude percentage Am of almost 100% (99.99%) is obtained at the rotation cycle Tr of 0.54 s which corresponds to the roll diameter φ of 0.12 m. 
     Moreover, the signal amplitude percentage Am of almost 100% (99.99% or more) is obtained also when the rotation cycle Tr is greater than 0.54 s, that is when the roll diameter φ is greater than 0.12 m which is the smallest value. 
     Accordingly, in this case, the digital filter  87  can pass the meandering component of the web W while removing the noise component in the position signal throughout a period until the entire web W in the fresh web roll  16  is completely used. 
     An example of signal waveforms before and after the filtering by the digital filter  87  in the second embodiment as described above is illustrated in  FIG. 8 . As illustrated in  FIG. 8 , the filtering by the digital filter  87  removes the noise component while leaving a large wave which is the meandering component of the web W corresponding to the rotation cycle of the web roll  16 . 
     As described above, in the second embodiment, the printing apparatus controller  25  sets the time constant τ of the digital filter  87  such that the digital filter  87  passes the meandering component of the web W corresponding to the rotation cycle of the web roll  16  while removing the noise component in the position signal. This can improve extraction accuracy of the meandering component in the position signal. Accordingly, the decrease in the accuracy of the meandering correction of the web W can be reduced. 
     Note that the time constant τ of the digital filter  87  for passing the meandering component of the web W corresponding to the rotation cycle of the web roll  16  may be set to change depending on the change in the rotation cycle of the web roll  16  caused by the change in the roll diameter φ. In the digital filter  87 , the time constant τ can be changed also during the printing operation by a program. Moreover, the roll diameter φ can be calculated based on the print conveyance speed Vg and the number of revolutions of the web roll  16  which can be detected by installing an encoder on the web roll support shaft  11  or by a similar method. 
     Next, description is given of a third embodiment partially changed from the first and second embodiments. 
     In the third embodiment, as illustrated in  FIG. 9 , a continuous sheet in which multiple holes  111  which are so-called sprocket holes are formed is used as the web W. 
     The holes  111  are formed to be aligned in the conveyance direction of the web W at a predetermined pitch d in one side end portion and the other side end portion of the web W in the width direction (front-rear direction). 
     When the web W with holes as illustrated in  FIG. 9  is used, light of the light projector  23   a  in the edge sensor  23  passes through the holes  111  and a signal component corresponding to detection of the holes  111  is thereby included in the position signal outputted by the edge sensor  23  in some cases. The signal component corresponding to the holes  111  leads to a decrease in the accuracy of the meandering correction of the web W performed by the meandering corrector  22 . 
     To counter this, in the third embodiment, the printing apparatus controller  25  sets the time constant τ of the digital filter  87  such that the digital filter  87  attenuates the signal component corresponding to the holes  111  of the web W. 
     A hole cycle Th being a cycle at which the holes  111  passes the position of the edge sensor  23  in the printing operation is expressed by the following formula (7).
 
 Th=d/Vg   (7)
 
     From formulae (5) and (7), the time constant τh for attenuating the signal component corresponding to the holes  111  is expressed by the following formula (8).
 
τ h =−( d/Vg )/(log  e (1− Am/ 100))  (8)
 
     Moreover, from formula (8), the signal amplitude percentage Am of the signal component corresponding to the holes  111  is expressed by the following formula (9).
 
 Am =(1−exp(− d /( Vg×τh )))×100  (9)
 
     In the third embodiment, a time constant τ at which the signal amplitude percentage Am calculated by using formula (9) takes a value equal to or less than an upper limit value Amq (for example 30%) is set as the time constant τ of the digital filter  87 , the upper limit value Amq being a signal amplitude percentage Am at which the signal component corresponding to the holes  111  is considered to be sufficiently attenuated. 
     Moreover, the time constant τ set for the digital filter  87  as described above is set to a value at which the digital filter  87  passes the meandering component of the web W corresponding to the rotation cycle of the web roll  16  as in the second embodiment. 
     Specifically, the time constant τ obtained from formula (6) with the signal amplitude percentage Am set to the signal amplitude percentage Amp which is almost 100% (for example, a value equal to or higher than 99.99%) and with the roll diameter φ set to the smallest value is calculated as an upper limit value τ max of the time constant τ set for the digital filter  87 . Then, the time constant τh which is equal to or less than the calculated upper limit value τ max of the time constant τ and at which the signal amplitude percentage Am calculated by using formula (9) is equal to or less than its upper limit value Amq is set as the time constant τ of the digital filter  87 . 
     For example, assume that Vg=0.7 m/s, the smallest value of the roll diameter φ is 0.12 m, and Am=Amp=99.99%. Moreover, assume that the pitch d of the holes  111 =0.0127 m and the upper limit value Amq of the signal amplitude percentage Am of the signal component corresponding to the holes  111 =30%. 
     In this case, the time constant τ in the case where the roll diameter φ takes the smallest value which is calculated from formula (6) is 0.058 s and this value is set as the upper limit value τ max. When 0.058 s is plugged into τh=τ max and 0.0127 m is plugged into d in formula (9), Am is 26.7% and satisfies the condition that Am is equal to or less than the upper limit value Amq=30%. Accordingly, in this case, τh=0.058 s can be set as the time constant τ of the digital filter  87 . 
     Also in this example, the signal amplitude percentage Am of almost 100% (99.99% or more) is obtained at a rotation cycle Tr equal to or more than 0.54 s which corresponds to the roll diameter φ of 0.12 m being the smallest value as in the example described in the second embodiment. Accordingly, the digital filter  87  can pass the meandering component of the web W throughout the period until the entire web W in the fresh web roll  16  is completely used. 
     Moreover, as illustrated in  FIG. 10 , the signal amplitude Am for the hole cycle Th of 0.018 s which corresponds to the pitch d of the holes  111  of 0.0127 m is 26.7% which is the value calculated from the aforementioned formula (9). In other words, in the case where the time constant τ=0.058 s, the signal component corresponding to the holes  111  can be attenuated to 26.7% of the original amplitude. 
     An example of waveforms before and after the filtering by the digital filter  87  in the third embodiment as described above is illustrated in  FIG. 11 . As illustrated in  FIG. 11 , the filtering by the digital filter  87  attenuates the signal component corresponding to the holes  111  which is a small wave while leaving a large wave which is the meandering component of the web W corresponding to the rotation cycle of the web roll  16 . 
     As described above, in the third embodiment, the printing apparatus controller  25  sets the time constant τ of the digital filter  87  such that the digital filter  87  attenuates the signal component corresponding to the holes  111  of the web W. This can reduce the decrease in the extraction accuracy of the meandering component in the position signal also when the web W with holes is used for the printing. Accordingly, the decrease in the accuracy of the meandering correction of the web W can be reduced. 
     Note that the time constant τ of the digital filter  87  for passing the meandering component of the web W corresponding to the rotation cycle of the web roll  16  may be set to attenuate the signal component corresponding to the holes  111  of the web W without the upper limit value max of the timing constant τ being provided. Also in this case, it is possible to obtain the effect of reducing the decrease in the extraction accuracy of the meandering component in the position signal by attenuating the signal component corresponding to the holes  111 . 
     Although the edge sensor is the optical sensor including the light projector and the light receiver in the first to third embodiments, the edge sensor may be a different type of sensor such as an ultrasonic sensor. 
     Moreover, although the configuration in which the unwinder and the rewinder are connected to the inkjet printing apparatus as separate apparatuses is described in the first to third embodiments, the configuration may be such that the unwinder and the rewinder are incorporated in the inkjet printing apparatus. 
     The embodiments of the disclosure have, for example, the following configurations. 
     A meandering control device includes: a detector configured to detect a position of an edge of a web being conveyed in a width direction of the web; a corrector configured to correct a position of the web being conveyed in the width direction; and a controller including a digital filter configured to perform filtering on a position signal indicating the position of the edge detected by the detector to attenuate a noise component. The controller is configured to drive the corrector to correct the position of the web in the width direction based on the position signal after the filtering. 
     The web may be unwound and conveyed from a web roll. The controller may be configured to set a time constant of the digital filter such that the digital filter passes a meandering component of the web in the position signal corresponding to a rotation cycle of the web roll. 
     The web may have holes aligned in a conveyance direction of the web. The controller may be configured to set a time constant of the digital filter such that the digital filter attenuates a signal component in the position signal corresponding to the holes. 
     The corrector may include a roller configured to rotate by following the web being conveyed. 
     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.