Patent Publication Number: US-9403653-B2

Title: Conveying apparatus, printing apparatus, control method, and sheet feeding method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a conveying apparatus, printing apparatus, control method, and sheet feeding method. 
     2. Description of the Related Art 
     A printing apparatus which prints images on a roll-like sheet such as roll paper has been proposed. A printing apparatus like this includes a conveying mechanism which pulls out a sheet from a roll and conveys the sheet. The conveying mechanism includes, for example, a pair of conveyance rollers which clamp and convey the sheet. The sheet conveyance accuracy of the conveying mechanism has influence on the quality of a printed image. The tension of the sheet between the roll and conveyance roller pair has influence on the conveyance accuracy. When the roll is exchanged or the sheet is consumed, the roll diameter changes, and this fluctuates the tension of the sheet. Japanese Patent Laid-Open No. 2009-208921 has disclosed an apparatus which adjusts the tension of the sheet by a spindle motor for driving a spindle which supports the roll. This apparatus disclosed in Japanese Patent Laid-Open No. 2009-208921 executes a process of estimating the roll diameter whenever a printing execution instruction is issued or when a user&#39;s instruction is input, and controls the spindle motor based on the estimated roll diameter, thereby suppressing the fluctuation in tension of the printing medium. 
     The sheet tension can also fluctuate due to a cause other than the roll exchange or sheet consumption. For example, an error of the mechanism of the apparatus or the eccentricity of the rotation center of the roll can also be a cause. It is difficult for the apparatus disclosed in Japanese Patent Laid-Open No. 2009-208921 to cope with the fluctuations in tension of the printing medium resulting from these causes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique of improving the conveyance accuracy of a roll-like sheet. 
     According to an aspect of the present invention, there is provided a conveying apparatus comprising: a supporting unit configured to rotatably support a roll in which a sheet is wound; a conveying unit configured to convey the sheet pulled out from the roll supported by the supporting unit; a detection unit configured to detect a rotation amount of the roll supported by the supporting unit; a driving unit configured to give a rotational driving force to the roll supported by the supporting unit; and a control unit configured to control the driving unit based on a detection result of the detection unit, wherein the control unit sets a control amount for the driving unit in accordance with a rotation phase of the roll, in order to adjust a tension of the sheet between the roll and the conveying unit. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a printing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a view for explaining mechanisms and a control unit of the printing apparatus shown in  FIG. 1 ; 
         FIG. 3A  is a view for explaining a calculation example of the radius of a roll, and  FIG. 3B  is a view for explaining a rotation phase area; 
         FIGS. 4A and 4B  are flowcharts showing processing examples to be executed by the control unit; 
         FIGS. 5A and 5B  are views for explaining setting examples of a control amount taking account of the eccentricity of the roll; 
         FIG. 6  is a view for explaining an example of the correction amount of a conveyance amount; and 
         FIGS. 7A and 7B  are flowcharts showing processing examples to be executed by the control unit. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     &lt;Arrangement of Apparatus&gt; 
       FIG. 1  is a schematic view of a printing apparatus  1  according to an embodiment of the present invention.  FIG. 2  is a view for explaining mechanisms and a control unit  4  of the printing apparatus  1 . In this embodiment, a case in which the present invention is applied to an inkjet printing apparatus will be explained. However, the present invention is also applicable to other types of printing apparatuses. Also, in this embodiment, a case in which the present invention is applied to a serial type inkjet printing apparatus will be explained. However, the present invention is also applicable to a line type inkjet printing apparatus. 
     Note that “printing” includes not only the formation of significant information such characters and figures, but also the formation of significant or insignificant information such as images, designs, and patterns on printing media, or the processing of media. That is, whether information is so actualized as to be visually perceivable by humans does not matter. Also, it is assumed that “a printing medium” is a paper sheet M in this embodiment, but it is also possible to use a sheet of another material such as cloth or a plastic film. 
     Referring to  FIGS. 1 and 2 , the printing apparatus  1  includes a conveying apparatus  2 , a printing mechanism  3 , and the control unit  4 . In  FIGS. 1 and 2 , an arrow X indicates a main scan direction, and an arrow Y indicates a sub scan direction perpendicular to the main scan direction. 
     The printing apparatus  1  is an apparatus which pulls out a band-like sheet M from a roll (roll paper) RL on which the sheet M is wound, and prints an image on the sheet M. The roll RL is a cylindrical member formed by winding the sheet M on a cylindrical core (for example, a paper tube). 
     The conveying apparatus  2  includes a supporting unit  21 , conveying mechanism  22 , and tension generating mechanism  23  (a roll RL driving mechanism). The supporting unit  21  rotatably supports the roll RL. In this embodiment, the supporting unit  21  includes a pair of supporting members  21   a  and  21   b  and a rotating shaft  21   c . The pair of supporting members  21   a  and  21   b  rotatably support the rotating shaft  21   c , and also support other members of the printing apparatus  1 . The rotating shaft  21   c  has two end portions detachably supported by the pair of supporting members  21   a  and  21   b , and extends parallel to the X direction. The rotating shaft  21   c  is fitted in the center of the roll RL, and the rotating shaft  21   c  and roll RL rotate together. The roll RL is attached to the rotating shaft  21   c  by using a known attaching/detaching mechanism, so that the roll RL can be exchanged. 
     In this embodiment, the conveying mechanism  22  includes a conveyance roller  221  and pinch roller  222  forming a roller pair. The conveyance roller  221  and pinch roller  222  extend in the X direction. The conveyance roller  221  includes, for example, a shaft, and a cylindrical rubber member covering the surface of this shaft. The pinch roller  222  is urged against the conveyance roller  221  by an elastic member such as a spring (not shown), and rotates in synchronism with the rotation of the conveyance roller  221 . 
     When the end portion of the sheet M is clamped between nip portions of the conveyance roller  221  and pinch roller  222  and the conveyance roller  221  is rotated, the sheet M is pulled out from the roller RL, clamped in the Y direction by the nip portions, and conveyed. 
     The conveying mechanism  22  includes a driving mechanism for rotating the conveyance roller  221 . The driving mechanism includes a motor  223  as a driving source, and a transfer mechanism which transfers the output from the motor  223  to the conveyance roller  221 . In this embodiment, the transfer mechanism is a decelerating mechanism using gears, that is, includes a gear  224   a  fixed to the output shaft of the motor  223 , and a gear  224   b  which is fixed to the end portion of the conveyance roller  221  and meshes with the gear  224   a . The conveyance roller  221  can be rotated by driving the motor  223 , and the rotation direction of the conveyance roller  221  can be switched by the rotation direction of the motor  223 . The transfer mechanism may also be another type of mechanism such as a belt transfer mechanism. 
     The tension generating mechanism  23  is a rotational driving mechanism (driving unit) for the roll RL, that is, includes a motor  231  as a driving source, and a transfer mechanism  232  for transferring the output from the motor  231  to the roll RL, and gives a rotational driving force to the supported roll RL. In this embodiment, the transfer mechanism  232  is a decelerating mechanism using gears, that is, includes a gear  232   a  fixed to the output shaft of the motor  231 , and a gear  232   b  which is fixed to the end portion of the rotating shaft  21   c  and meshes with the gear  232   a . The roll RL can be rotated by driving the motor  231 , and the rotation direction of the roll RL can be switched by the rotation direction of the motor  231 . Feeding the sheet M in the Y direction by the rotation of the roll RL will sometimes be called “forward feed”, and the rotation direction of the roller RL at that time will sometimes be called “a forward direction”. Also, rewinding the sheet M by the rotation of the roller RL will sometimes be called “back feed”, and the rotation direction of the roll RL at that time will sometimes be called “a backward direction”. 
     When conveying the sheet M in this embodiment, the sheet M is conveyed as it is fed by rotationally driving the roll RL in addition to the rotational driving of the conveyance roller  221 . In addition, it is possible to generate a tension on the sheet M between the roll RL supported by the supporting unit  21  and the conveyance roller  221  by controlling the motors  223  and  231  in synchronism with each other. Note that the transfer mechanism  232  may also be another type of mechanism such as a belt transfer mechanism. 
     The conveying apparatus  2  also includes sensors  24  and  25 . The sensor  24  senses the rotation amount of the roll RL supported by supporting unit  21 . In this embodiment, the sensor  24  is a rotary encoder and includes a slit disc  241  and photosensor  242 . The slit disc  241  is a disc having a plurality of slits formed in the peripheral portion. In this embodiment, the slit disc  241  is coaxially fixed to the output shaft of the motor  231 . The photosensor  242  is a transmitting type photosensor including a light-receiving element and light-emitting element opposing each other, and is so arranged as to sense the presence/absence of the slits of the slit disc  241 . The rotation amount of the motor  241  is sensed by counting the slits sensed by the photosensor  242 . The rotation amount of the roll RL can be sensed from the rotation amount of the motor  241  and the deceleration ratio of the transfer mechanism  23 . Note that it is also possible to adopt an arrangement in which the slit disc  241  is coaxially fixed to the rotating shaft  232   b . Note also that the sensor  24  may be another type of sensor as long as the sensor can sense the rotation amount of the roll RL. 
     The sensor  25  senses the rotation amount of the conveyance roller  221 . In this embodiment, the sensor  25  is a rotary encoder including a slit disc  251  and photosensor  252 , like the sensor  24 . The slit disc  251  is coaxially fixed to the conveyance roller  221 , and the rotation amount of the conveyance roller  221  is sensed by counting the slits sensed by the photosensor  252 . Note that it is also possible to adopt an arrangement in which the slit disc  251  is coaxially fixed to the motor  223 . Note also that the sensor  25  may be another type of sensor as long as the sensor can sense the rotation amount of the conveyance roller  221 . 
     The conveying apparatus  2  further includes sensors  26  to  29 . The sensor  26  is arranged upstream of the conveyance roller  221 , and senses the sheet M. For example, the sensor  26  is a photosensor. The sensor  27  senses the output torque of the motor  231 , and is, for example, a current sensor which senses an electric current to be supplied to the motor  231 . The sensor  28  senses the output torque of the motor  223 , and is, for example, a current sensor which senses an electric current to be supplied to the motor  223 . The sensor  29  senses a sensing target piece  21   c ′ formed on the rotating shaft  21   c , and is a photosensor or the like. When the sensor  29  senses the sensing target piece, the rotation position of the rotating shaft  21   c  can be set as an initial position. 
     Next, the printing mechanism  3  will be explained. The printing mechanism  3  includes a carriage  31  and printing unit  32 . The carriage  31  is guided by a guide shaft  33  extending in the X direction, and movably supported in the X direction. The carriage  31  is moved back and forth in the X direction by a driving mechanism (including, for example, a belt transfer mechanism) using a carriage motor CM as a driving source. 
     The printing unit  32  is mounted on the carriage  31 . The printing unit  32  includes a printhead. The printhead prints an image by discharging ink supplied from an ink tank onto the sheet M. The ink tank can form the printing unit  32  together with the printhead, or can be mounted on the carriage  31  independently of the printhead. A platen PL is arranged below the moving path of the carriage  31 . The platen PL includes, for example, a mechanism which holds the sheet M by suction. 
     Sensors  34  and  35  are mounted on the carriage  31 . The sensor  34  senses the position of the carriage  31 , and is an encoder sensor or the like. The encoder sensor can read an encoder scale set in the X direction, and can sense the position of the carriage  31  in the X direction from the read result. The sensor  35  senses the sheet M, and is a photosensor or the like. The width of the sheet M can be calculated based on the position of the carriage  31  when the sensing result of the sensor  35  has changed. 
     The control unit  4  will now be explained. The control unit  4  includes a processing unit  41 , storage unit  42 , and interface unit  43 . The processing unit  41  is a CPU or the like, and controls the whole printing apparatus  1 . The storage unit  42  includes one or a plurality of storage devices. For example, the storage device is a ROM, RAM, or hard disk. The storage unit  42  stores programs to be executed by the processing unit  41 , print data received from a host computer (not shown), and the like. The storage unit  42  also temporarily stores various kinds of data generated when the processing unit  41  executes the programs. 
     The storage unit  42  further stores various kinds of information. The various kinds of information include sheet information, motor information, and tension setting information  42   a . The sheet information is, for example, information of each type of sheet, and includes the sheet density, the sheet thickness, and the diameter or radius of the paper tube. The motor information includes the torque coefficient of the motor. The tension setting information  42   a  is information about a control amount for the motor  231  of the tension generating mechanism  23 . 
     The interface unit  43  includes an I/O interface for input data from the above-described various sensors and an operation unit  5  and outputting data to the above-described various motors and the printhead, and a communication interface for communicating with a host computer (not shown). Note that the control unit  4  may also include a signal processing circuit for processing signals from the above-described sensors, and a driving circuit for driving the motor. The operation unit  5  is an input device for accepting user&#39;s instructions, and is a touch panel type operation panel or the like. 
     Next, the printing operation of the printing apparatus  1  will be explained. The conveying mechanism  22  pulls out the sheet M from the roll RL. The carriage  31  moves above (scans) the sheet M. During this movement, the printing unit  32  discharges ink droplets and prints an image. While the image is printed, the sheet M is not conveyed, and the platen PL holds the sheet M by suction, thereby improving the planarity. When the printing unit  32  has printed one line, the sheet M is conveyed by a predetermined amount in the Y direction and stopped. Subsequently, the next one line is printed. Thus, the printing apparatus  1  performs printing line by line by repeating the intermittent conveyance of the sheet M. 
     &lt;Control of Tension&gt; 
     When conveying the sheet M, the processing unit  41  of the control unit  4  controls not only the driving of the motor  223  for rotating the conveyance roller  221 , but also the driving of the motor  231  for rotating the roll RL. The driving of the motor  231  is so controlled as to generate a predetermined tension F [N] between the roll RL and conveyance roller  221 . The tension F functions as a back tension to the conveyance roller  221 . A slip amount or the like between the conveyance roller  221  and sheet M is held constant by controlling the motor  231  so as to hold the tension F constant when viewed from the conveyance roller  221 . Accordingly, the conveyance accuracy of the sheet M can be increased. An example of the cause of fluctuating the tension F and a method of suppressing the fluctuation will be explained below. 
     As shown in  FIG. 3A , a position where the sheet M is pulled out from the roll RL is a point A. The distance from the rotation center of the roll RL to the point A is the radius of the roll RL. As the sheet M is pulled out from the roll RL, the radius of the roll RL decreases. When the roll RL is exchanged, the radius is different from that before the exchange. If the output from the motor  231  is constant, the radius of the roll RL fluctuates the tension F. Therefore, control corresponding to the radius of the roll RL is necessary. This makes it necessary to calculate the radius of the roll RL. 
       FIG. 3A  shows an example of a method of calculating the radius of the roll RL. Under conditions by which the sheet M is practically not slackened or slipped, the sheet M is conveyed by a predetermined amount by the conveyance roller  221 . Let θ0 [rad] be the rotation angle of the motor  231  sensed by the sensor  24  in this state, n1 be the number of teeth of the gear  232   b , n2 be the number of teeth of the gear  232   a , and θ1 [rad] be the rotation angle of the conveyance roller  221  sensed by the sensor  25 . 
     Letting R1 [m] be the radius of the conveyance roller  221 , a radius R0 [m] of the roll RL is calculated by:
 
 R 0= R 1×θ1/θ0/ n 1× n 2  (1)
 
Note that in the arrangement of this embodiment, the calculation result of the radius R0 contains transfer errors of the gears  232   a  and  232   b.  
 
     The fluctuation in radius R0 occurs in cases other than the case in which the sheet M is pulled out from the roll RL or the roll RL is exchanged. For example, this fluctuation occurs due to the roundness of the roll RL itself or axial misalignment between the rotating shaft  21   c  and roll RL. That is, the tension F may fluctuate because the radius R0 increases or decreases during one rotation of the roll RL. 
     There are causes, other than the fluctuation in radius of the roll RL, which fluctuate the tension F. An example is a transfer error of the driving force from the motor  231  to the rotating shaft  21   c . Practical examples are axial misalignment between the rotating shaft  21   c  and gear  232   c , and axial misalignment between the output shaft of the motor  231  and the gear  232   a . If there is a transfer error like this, the tension F may fluctuate even when the motor  231  is driven at a constant velocity. The tension F may fluctuate during one rotation of the roll RL in this case as well. 
     In this embodiment, therefore, the control amount of the motor  231  is set in accordance with the rotation phase during one rotation of the roll RL. An example of the control amount of the motor  231  is the torque control amount (for example, the driving duty ratio). This makes it possible to reduce the fluctuation in tension F during one rotation of the roll RL, thereby increasing the conveyance accuracy of the sheet M. 
     When setting the control amount of the motor  231  in accordance with the rotation phase during one rotation of the roll RL, the processing sometimes becomes complicated if the control amount is continuously set in accordance with the rotation angle of the roll RL. In this embodiment, therefore, the control amount is set for each rotation phase area obtained by dividing the angle range (that is, 360°) of one rotation of the roll RL into a plurality of regions. This can prevent the processing from becoming complicated. 
     When the deceleration ratio (=n1/n2) of the transfer mechanism  232  is an integer N, the number of divided rotation phase areas can be N. For example, when the deceleration ratio is 8, the rotation phase area is equally divided into eight areas area1 to area8, as shown in  FIG. 3B . That is, one rotation phase area has an angle range of 45°. The basis of each rotation phase area can be the rotation position of the rotating shaft  21   c  when the sensor  29  senses the sensing target piece  21   c ′. For example, rotation phase area area1 can be an angle range until the rotating shaft  21   c  rotates 45° after the sensor  29  has sensed the sensing target piece  21   c′.    
     When equally dividing the rotation phase area into eight rotation phase areas area1 to area8, the motor  231  rotates N times while the roll RL rotates once. Accordingly, the fluctuation in tension F caused by a transfer error of the driving force repeats whenever the roll RL rotates once (that is, repeats for each period). Therefore, the fluctuation in tension F can be reduced more effectively by setting the control amount of the motor  231  for each of rotation phase areas area1 to area8. 
     A transfer error of the driving force depends on the design of the transfer mechanism  232 . Accordingly, it is possible to derive the reference control amount of the motor  231  for each of rotation phase areas area1 to area8, which cancels the target fluctuation in tension F caused by a transfer error of the driving force, by an experiment using a prototype of the printing apparatus  1  or by simulation. This experiment or simulation need only be performed on each identically designed machine type. To derive a more accurate control amount, however, the experiment may also be conducted on each product before it is shipped. 
     The reference control amount can be stored as the above-described tension setting information  42   a  in the storage unit  42 , and used when conveying the sheet M. As shown in  FIG. 2 , the tension setting information  42   a  contains information of the reference control amounts associated with rotation phase areas area1 to area8. 
     The above-mentioned experiment or simulation is performed by using or assuming the roll RL having a specific diameter (called a reference diameter), so the reference control amount can be used as a control amount when using the roll RL having the reference diameter. In this case, it is also possible to set a plurality of reference diameters, and set the reference control amount for each reference diameter. Furthermore, the reference diameter may also be the diameter of an unused roll RL (that is, a maximum diameter). In the following explanation, it is assumed that the reference diameter is the diameter of an unused roll RL. 
     The above-mentioned experiment or simulation can be performed for, for example, each type of the sheet M or each width of the sheet M, and the reference control amount can be set for each type of the sheet M or each width of the sheet M. Consequently, it is possible to process a plurality of types of the sheets M or the sheets M having a plurality of widths. As for the width of the sheet M, it is also possible to obtain the reference control amount for only a possible maximum width. When applying the reference control amount to the sheet M having a different width, the reference control amount need only be corrected in accordance with the ratio to the maximum width. For example, when applying the reference control amount to the sheet M having a width which is ½ of the maximum width, the reference control amount need only be corrected such that the output torque becomes ½. 
     As already explained above, the fluctuation in radius of the roll RL is sometimes caused by, for example, the roundness of the roll RL itself. This makes it difficult to grasp this fluctuation in advance, unlike a transfer error of the driving force. When conveying the sheet M, therefore, the motor  231  is controlled while the radius of the roll RL is calculated by equation (1). In this case, it is possible to calculate the radius of the roll RL in accordance with equation (1) for each of rotation phase areas area1 to area8, and correct the reference control amount based on the ratio of the value twice the calculated radius to the reference diameter, thereby obtaining a final control amount. This makes it difficult to suppress the fluctuation in tension F in accordance with both a transfer error of the driving force and the fluctuation in radius of the roll RL. 
     Processing Examples 
     Examples of processing executed by the processing unit  41  of the control unit  4  will be explained with reference to  FIGS. 4A and 4B .  FIG. 4A  is a flowchart showing a processing example when a new unused roll RL is attached. In the following explanation, tension control is performed on each of rotation phase areas area1 to area8 as shown in  FIG. 3B . 
     In step S 1 , a preparation process corresponding to a roll RL attaching work by the user is performed. The user sets the roll RL in the supporting unit  21 , and abuts the distal end portion of the sheet M against the nip portions of the conveyance roller  221  and pinch roller  222 . Since the sensor  26  senses the sheet M, the control unit  4  rotates the conveyance roller  221  and conveys the sheet M by a predetermined amount in the Y direction. Also, the carriage  31  is moved in the X direction and positioned above the platen PL. 
     In step S 2 , a user&#39; selection input is accepted via the operation unit  5 . For example, selection of the type of the sheet M after exchange is accepted. 
     In step S 3 , the conveyance roller  221  and rotating shaft  21   c  are rotated by driving the motors  223  and  231 , thereby starting conveyance of the sheet M. In this step, the sheet M is pulled out and conveyed by a predetermined amount. Pulling out the sheet M from the roll RL is also called “forward feed”. In this step, the roll RL rotates once or more after the sensor  29  has sensed the sensing target piece  21   c ′. When the sensor  29  senses the sensing target piece  21   c ′, it is possible to discriminate between rotation phase areas area1 to area8. 
     In step S 3 , the tension setting information  42   a  corresponding to the sheet M of the type selected by the user in step S 2  is also read out. In step S 3 , the reference control amount set in the readout tension setting information  42   a  is used as the control amount for the motor  231 , and the control amount is switched in accordance with each of rotation phase areas area1 to area8 passing through the point A (see  FIGS. 3A and 3B ). 
     In parallel to the processing in step S 3 , in step S 4 , the radius of the roll RL is calculated for each of rotation phase areas area1 to area8 from above-mentioned equation (1) based on the sensing results of the sensors  24  and  25 . More specifically, when rotation phase areas area1 to area8 which pass through the point A are switched, an area which passes through the point A after the switching is specified, and the radius of the roll RL is calculated for the specified rotation phase area. 
     The calculation result is saved in the storage unit  42 . When forward feed is complete, the carriage  31  is moved back and forth in the X direction, and the width of the sheet M is calculated from the sensing results of the sensors  34  and  35 . The calculation result is saved in the storage unit  42 . 
     In step S 5 , the reference control amount read out in step S 3  is corrected based on the calculation result in step S 4 . An example of the correction method has already been described above. The corrected control amount is set as the control amount for the motor  231  for each of rotation phase areas area1 to area8 at the first rotation of the roll RL, and saved in the storage unit  42  in step S 6 . 
     In step S 7 , the conveyance roller  221  and rotating shaft  21   c  are rotated backward by driving the motors  223  and  231 , thereby rewinding the sheet M to the roll RL until the sheet M is positioned in a predetermined waiting position. Rewinding the sheet M to the roll RL is also called “back feed”. 
     Next, a process of updating the control amount for the motor  231  during a printing operation will be explained with reference to  FIG. 4B . In the printing operation, the control amount is changed in accordance with rotation phase areas area1 to area8 passing through the point A (see  FIGS. 3A and 3B ). In this case, the control amount is updated in accordance with the fluctuation in radius of the roll RL.  FIG. 4B  shows a processing example pertaining to the update of the control amount. 
     In step S 11 , the conveyance of the sheet M in the printing operation is started. In this step, the conveyance roller  221  and rotating shaft  21   c  are rotated by driving the motors  223  and  231 , thereby performing the forward feed of the sheet M. 
     In step S 12 , when rotation phase areas area1 to area8 which pass through the point A (see  FIGS. 3A and 3B ) are switched, an area which passes through the point A after the switching is specified. The specified area will be called a target area. Note that in the example shown in  FIGS. 3A and 3B , it is assumed that the point A is positioned at the apex of the roll RL. Although processing may also be performed by fixing the position of the point A, the position of the point A sometimes changes due to the change in radius of the roll RL. Accordingly, it is also possible to successively calculate the position of the point A from the radius and diameter of the roll RL, the radius and diameter of the conveyance roller  221 , and their central positions. 
     In step S 13 , the sensing results of the sensors  24  and  25  are acquired, and the radius of the roll RL is calculated from above-mentioned equation (1) for the target area specified in step S 12 . 
     In step S 14 , the reliability of the radius of the roll RL calculated in step S 13  is determined. If it is determined that there is reliability, the process advances to step S 15 . If it is determined that there is no reliability, the processing of one unit is terminated. The reliability is determined in order to prevent false sensing of the radius of the roll RL due to the action of an unexpected external force, for example, to prevent false sensing when the user pulls out the sheet M from the roll RL. An example of the determination method is to compare the calculation result of the radius of the roll RL saved by the last processing for the target area specified in step S 12  with the radius of the roll RL calculated this time in step S 13 . If the difference between the two radii is not less than twice the thickness of the sheet M, it is determined that there is no reliability. If it is determined that there is no reliability, processes in steps S 15  and S 16  to be described below are not performed, that is, the radius information and control amount information are not updated. 
     In step S 15 , the control amount of the target area is updated based on the radius of the roll RL calculated in step S 13 . The update method may be correction of the reference control amount, and may also be correction of the control amount set by the last processing. In step S 16 , the radius information and control amount information of the target area stored in the storage unit  42  are updated by the radius calculated in step S 13  and the control amount updated in step S 15 . While the target area is passing through the point A, the control amount for the motor  231  is the updated control amount. Thus, the processing of one unit is complete. During the printing operation, the process shown in  FIG. 4B  is repetitively executed, and the control amount for each rotation phase area is updated one after another. 
     In this embodiment as explained above, the control amount for the motor  231  is set in accordance with the rotation phase during one rotation of the roll RL, so the tension fluctuation of the sheet M during one rotation of the roll RL can be reduced. This makes it possible to increase the conveyance accuracy of the sheet M. Consequently, a conveyance shift decreases, and the quality of an image formed by the printing mechanism  3  can also be improved. 
     Note that in this embodiment, rotation phase areas area1 to area8 are set based on the position where the sensor  29  senses the sensing target piece  21   c ′, in order to reduce the tension fluctuation caused by a transfer error of the driving force from the motor  231  to the rotating shaft  21   c . However, if a transfer error of the driving force is negligible, it is unnecessary to align the mechanism with rotation phase areas area1 to area8. 
     In this case, therefore, the sensor  29  and sensing target piece  21   c ′ are unnecessary. Even when the sensor  29  and sensing target piece  21   c ′ are not used, rotation phase areas area1 to area8 can be set as needed when the roll RL is exchanged. By changing the control amount for the motor  231  for each rotation phase area, it is possible to reduce the tension fluctuation of the sheet M caused by, for example, the roundness of the roll RL itself. In this case, it is also possible to set the number of divided rotation phase areas regardless of the mechanism. 
     Second Embodiment 
     When the radius of the roll RL fluctuates during one rotation, the rotation center of the roll RL is shifted from the center of gravity of the roll RL. This shift causes the tension fluctuation of the sheet M. Therefore, a method of reducing the tension fluctuation of the sheet M caused by this shift will be explained below.  FIGS. 5A and 5B  are views for explaining the method. 
     Referring to  FIGS. 5A and 5B , it is assumed that the number of divided rotation phase areas of the roll RL is 8, as in the first embodiment. R1 to R8 represent the radii of the roll RL in rotation phase areas area1 to area8, and the radii R1 to R8 each indicate the radius in the position where the rotation phase areas are switched. That is, the rotation phase area is switched to area1 in a position where the distal end of the arrow R1 overlaps a point A, and switched to area1 in a position where the distal end of the arrow R2 overlaps the point A. The meaning of the point A is the same as that in the first embodiment. 
     The position of a rotation center P of the roll RL is calculated from the radii R1 to R8. The position of a center of gravity G is calculated from Rave as the average value of the radii R1 to R8. A shift amount L indicating the eccentricity between the rotation center P and center of gravity G is the horizontal distance between the rotation center P and center of gravity G. The shift amount L is negative (−) when the center of gravity G exists on the left side of the rotation center P, and positive (+) when the center of gravity G exists on the right side of the rotation center P. 
     Referring to  FIG. 5A , the center of gravity G is shifted to the left side from the rotation center P. Letting W be the weight of the roll RL, the weight W is applied on the center of gravity G. Accordingly, the roll RL generates a self-rotating force Fg which is counterclockwise in  FIG. 5A  as the forward feed direction. 
     The weight W can be calculated from the average value Rave of the radius, the density and thickness of the sheet M, and the paper tube diameter. Information of the sheet M necessary to calculate the weight W can be stored as sheet information in a storage unit  42 . 
     In the example shown in  FIG. 5B , the center of gravity G is shifted to the right from the rotation center P. Accordingly, the roll RL generates a self-rotating force Fg which is clockwise in  FIG. 5B  as the back feed direction. 
     That is, when the rotation phase of the roll RL is in the state shown in  FIG. 5A , the self-rotating force Fg acts in the direction of reducing the back tension of the roll RL. When the rotation phase of the roll RL is in the state shown in  FIG. 5B , the self-rotating force Fg acts in the direction of increasing the back tension. 
     The self-rotating force Fg can simply be represented by:
 
 Fg=L/R ave× W   (2)
 
     When calculating the control amount of a motor  231  in step S 15  of  FIG. 4B , it is possible to suppress the tension fluctuation of the sheet M caused by the shift between the rotation center of the roll RL and the center of gravity of the roll RL by adding a value equivalent to the self-rotating force Fg. 
     Third Embodiment 
     In the first and second embodiments, the tension fluctuation of the sheet M is suppressed by changing the back tension by the control amount for the motor  231 . However, it is also possible to increase the conveyance accuracy by the conveyance control amount for the motor  223 . That is, the real conveyance amount is held constant by changing the conveyance amount with respect to the tension fluctuation of the sheet M in control. The control amount of the motor  223  is set in accordance with the rotation phase during one rotation of the roll RL in this embodiment as well. This makes it possible to suppress the fluctuation in conveyance amount caused by the fluctuation in radius of the roll RL. In the following explanation, the number of divided rotation phase areas is 8 as in the first and second embodiments. 
     In this embodiment, the control amount for the motor  231  can be constant. By contrast, the control amount of the motor  231  may also be set in accordance with the rotation phase during one rotation of the roll RL as in the first embodiment. In this case, the control amount for each of rotation phase areas area1 to area8 can be kept at the reference control amount, in order to reduce, by the motor  231 , the tension fluctuation of the sheet M caused by a transfer error of the driving force from the motor  231  to the rotating shaft  21   c . Alternatively, the reference control amount may also be corrected based on the value equivalent to the self-rotating force Fg explained in the second embodiment. 
     The contents of control of this embodiment will now be explained. In this embodiment, the control amount for the motor  223  is set by correcting a unit control amount based on a correction table. The unit control amount is related to the rotation amount of the motor  223  required to convey the sheet M by one distance unit. When the motor  223  is a stepping motor, the unit control amount is defined by the number of driving pulses. 
       FIG. 6  shows an example of the correction table. In this correction table shown in  FIG. 6 , the correction amount is plotted on the ordinate, and the radius of the roll RL is plotted on the abscissa. The correction amount is 0 when the radius of the roll RL is maximum, and increases as the radius of the roll RL decreases. The correction amount is constant when the radius of the roll RL is smaller than r. 
     In this embodiment, the unit control amount is corrected by determining a correction amount from the calculation result of the radius of the roll RL and the correction table, and adding the determined correction amount to the unit control amount. Accordingly, as the radius of the roll RL decreases, the control amount for the motor  223  increases (the rotation amount of the motor  223  increases). This means that as the radius of the roll RL decreases, the slip amount of the sheet M with respect to the conveyance roller  221  increases. 
     The correction table can be formed by an experiment using a prototype of the printing apparatus  1  or by simulation. This experiment or simulation need only be performed on each identically designed machine type. To derive a more accurate control amount, however, the experiment may also be conducted on each product before it is shipped. 
     The correction table can be stored in the storage unit  42 , and used when conveying the sheet M. The above-mentioned experiment or simulation can be performed for, for example, each type of the sheet M or each width of the sheet M, and the reference control amount can be set for each type of the sheet M or each width of the sheet M. Consequently, it is possible to process a plurality of types of the sheets M or the sheets M having a plurality of widths. 
     Processing Examples 
     Examples of processing to be executed by the processing unit  41  of the control unit  4  will be explained with reference to  FIGS. 7A and 7B .  FIG. 7A  is a flowchart showing a processing example when a new unused roll RL is attached. In the following explanation, a process of setting a correction amount based on the correction table is performed for each of rotation phase areas area1 to area8 as shown in  FIG. 3B . 
     In step S 21 , a preparation process corresponding to a roll RL attaching work by the user is performed. The user sets the roll RL in the supporting unit  21 , and abuts the distal end portion of the sheet M against the nip portions of the conveyance roller  221  and pinch roller  222 . Since the sensor  26  senses the sheet M, the control unit  4  rotates the conveyance roller  221  and conveys the sheet M by a predetermined amount in the Y direction. Also, the carriage  31  is moved in the X direction and positioned above the platen PL. 
     In step S 22 , a user&#39; selection input is accepted via the operation unit  5 . For example, selection of the type of the sheet M after exchange is accepted. 
     In step S 23 , the conveyance roller  221  and rotating shaft  21   c  are rotated by driving the motors  223  and  231 , thereby starting forward feed of the sheet M. In this step, the roll RL rotates once or more, and rotation phase areas area1 to area8 are allocated. 
     Note that when reducing, by the motor  231 , the tension fluctuation of the sheet M caused by a transfer error of the driving force from the motor  231  to the rotating shaft  21   c , the rotation phase areas are discriminated based on the position where the sensor  29  has sensed the sensing target piece  21   c ′, as in the first embodiment. 
     In parallel to the processing in step S 23 , in step S 24 , the radius of the roll RL is calculated for each of rotation phase areas area1 to area8 from above-mentioned equation (1) based on the sensing results of the sensors  24  and  25 . More specifically, when rotation phase areas area1 to area8 which pass through the point A are switched, an area which passes through the point A after the switching is specified, and the radius of the roll RL is calculated for the specified rotation phase area. 
     The calculation result is saved in the storage unit  42 . When forward feed is complete, the carriage  31  is moved back and forth in the X direction, and the width of the sheet M is calculated from the sensing results of the sensors  34  and  35 . The calculation result is saved in the storage unit  42 . 
     In step S 25 , the correction table corresponding to the sheet M is read out, and a correction amount for each of rotation phase areas area1 to area8 is determined based on the calculation result in step S 24 , and saved in the storage unit  42  in step S 26 . 
     In step S 27 , the conveyance roller  221  and rotating shaft  21   c  are rotated backward by driving the motors  223  and  231 , thereby rewinding the sheet M to the roll RL until the sheet M is positioned in a predetermined waiting position. 
     Next, a process of updating the correction amount for the motor  231  during a printing operation will be explained with reference to  FIG. 7B . In the printing operation, the correction amount is changed in accordance with rotation phase areas area1 to area8 passing through the point A (see  FIGS. 3A and 3B ). In this case, the correction amount is updated in accordance with the fluctuation in radius of the roll RL.  FIG. 7B  shows a processing example pertaining to the update of the correction amount. 
     In step S 31 , the conveyance of the sheet M in the printing operation is started. In this step, the conveyance roller  221  and rotating shaft  21   c  are rotated by driving the motors  223  and  231 , thereby performing the forward feed of the sheet M. 
     In step S 32 , when rotation phase areas area1 to area8 which pass through the point A (see  FIGS. 3A and 3B ) are switched, an area which passes through the point A after the switching is specified. The specified area will be called a target area. Note that processing may also be performed by fixing the position of the point A, but it is also possible to successively calculate the position of the point A in accordance with the change in radius of the roll RL, as described in the first embodiment. 
     In step S 33 , the sensing results of the sensors  24  and  25  are acquired, and the radius of the roll RL is calculated from above-mentioned equation (1) for the target area specified in step S 32 . 
     In step S 34 , the reliability of the radius of the roll RL calculated in step S 33  is determined. If it is determined that there is reliability, the process advances to step S 35 . If it is determined that there is no reliability, the processing of one unit is terminated. The reliability is determined for the same reason as that described in the first embodiment. If it is determined that there is no reliability, processes in steps S 35  and S 36  to be described below are not performed, that is, the radius information and correction amount information are not updated. 
     In step S 35 , the correction amount of the target area is updated based on the radius of the roll RL calculated in step S 33 . In this step, the correction table corresponding to the sheet M is read out, and a correction amount is determined from the radius of the roll RL calculated in step S 33 . In step S 36 , the radius information and correction amount information of the target area stored in the storage unit  42  are updated by the radius calculated in step S 33  and the correction amount updated in step S 35 . While the target area is passing through the point A, the control amount for the motor  223  is corrected by the updated correction amount for one distance unit. Thus, the processing of one unit is complete. During the printing operation, the process shown in  FIG. 7B  is repetitively executed, and the correction amount for each rotation phase area is updated one after another. 
     In this embodiment as explained above, the control amount for the motor  223  is set in accordance with the rotation phase during one rotation of the roll RL. Even when the tension fluctuation of the sheet M occurs during one rotation of the roll RL, therefore, the actual conveyance amount of the sheet M per one distance unit can be held constant. This makes it possible to increase the conveyance accuracy of the sheet M. Consequently, a conveyance shift decreases, and the quality of an image formed by the printing mechanism  3  can also be improved. 
     Other Embodiments 
     The present invention is applied to a printing apparatus in each of the above-mentioned embodiments, but the application field of the present invention is not limited to this, and the present invention is applicable to various conveying apparatuses which pull out a sheet from a roll in which the sheet is wound and convey the sheet, or to various kinds of sheet feed. 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefits of Japanese Patent Application No. 2014-131800, filed Jun. 26, 2014, which is hereby incorporated by reference herein in its entirety.