Abstract:
A simple and lightweight torque clutch apparatus capable of freely changing a torque and a printer apparatus capable of tension controlling of ink ribbon in a simplified structure are proposed. A torque clutch apparatus includes a shaft, a first gear rotatably fitted to the shaft, first and second sandwiching members fitted to the shaft so as to sandwich the first gear therebetween and to rotate integrally with the shaft, a second gear rotatably screwed on the shaft, and a spring arranged between the second gear and the first or second sandwiching member opposing the second gear. A printer apparatus includes first torque-generating means for generating a variable load torque to a feed reel of ink ribbon, second torque-generating means for generating a variable rotational torque to a winding reel, and controlling means for controlling the first and/or second torque-generating means so as to generate the load torque or rotational torque corresponding to the diameter of the roll of ink ribbon wound on the feed reel and/or winding reel.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a torque clutch apparatus and printer apparatus which are suitable when being applied to a color roll printer, for example.  
           [0003]    2. Description of the Related Art  
           [0004]    In a printer apparatus using ink ribbon, when ink ribbon slackens during printing, it may wrinkles so as to deteriorate printed images.  
           [0005]    Therefore, in such a conventional printer apparatus, by controlling a winding reel and/or feed reel of ink ribbon with torque clutches such as felt clutches, a constant tension is applied to the ink ribbon.  
           [0006]    However, according to such a method, since the diameter of the roll of ink ribbon wound on the feed reel and the diameter of the roll of ink ribbon wound on the winding reel gradually change due to printing, there is a problem that a constant tension cannot be always applied to the ink ribbon.  
           [0007]    As for techniques for solving such a problem, there may be a first method of controlling the output of a motor and a second method of using a torque clutch with a variable output-torque such as a powder clutch and a hysteresis clutch. According to the first method, there has been a problem that the travelling stability of ink ribbon is damaged due to non-uniform feed motion of a gear or belt. According to the second method, there have also been problems that cost is increased and the entire printer apparatus becomes large in size and weight, because the powder clutch and hysteresis clutch are very expensive and large in size and weight.  
         SUMMARY OF THE INVENTION  
         [0008]    In view of the problems described above, the present invention has been made and it is an object of the present invention to propose a simple and lightweight torque clutch apparatus capable of freely changing a torque, and a printer apparatus capable of stably controlling tension of ink ribbon by a simple structure.  
           [0009]    In order to solve these problems, according to the present invention, a torque clutch apparatus comprises a shaft which is rotatably pivotably mounted and which has a screw thread formed on the circumferential surface thereof, a first gear rotatably fitted to the shaft, first and second sandwiching members fitted to the shaft so as to sandwich the first gear therebetween and to rotate integrally with the shaft, a second gear rotatably screwed on the shaft, and a spring arranged between the second gear and one of the first and second sandwiching members opposing the second gear. Therefore, the torque clutch apparatus is simple in construction and the output torque can be freely changed by rotating the second gear.  
           [0010]    Also, according to the present invention, a printer apparatus comprises first torque-generating means for generating a variable load torque applied to a feed reel, which is rotatably supported, of ink ribbon; second torque-generating means for variably generating a rotational torque to a winding reel, which is rotatably supported, of ink ribbon; and controlling means for controlling the first and/or second torque-generating means so as to generate one of the load torque and rotational torque in accordance with the diameter of the roll of ink ribbon wound on the feed reel and/or the winding reel. As a result, the printer apparatus can maintain the tension of ink ribbon constant at all times. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a perspective view of the structure of a color roll printer according to an embodiment;  
         [0012]    [0012]FIG. 2 is a perspective view of the structure of the color roll printer according to the embodiment;  
         [0013]    [0013]FIG. 3 is a schematic representation for showing a running path of a roll sheet;  
         [0014]    [0014]FIG. 4 is a schematic representation for showing a method of setting the roll sheet and ink ribbon;  
         [0015]    [0015]FIG. 5 is a schematic representation for showing the method of setting the roll sheet and ink ribbon;  
         [0016]    [0016]FIG. 6 is a schematic representation for showing the method of setting the roll sheet and ink ribbon;  
         [0017]    [0017]FIG. 7 is a perspective view of the internal structure of the color roll printer;  
         [0018]    [0018]FIG. 8 is a schematic representation for showing an initial operation of the color roll printer;  
         [0019]    [0019]FIG. 9 is a schematic representation for showing an initial state during image printing;  
         [0020]    [0020]FIG. 10 is a schematic representation for showing a state of print-starting;  
         [0021]    [0021]FIG. 11 is a schematic representation for showing a cutting operation;  
         [0022]    [0022]FIG. 12 is a schematic plan view showing the structure of a winding-reel driving section;  
         [0023]    [0023]FIG. 13 is a schematic side view showing the structure of the winding-reel driving section;  
         [0024]    [0024]FIG. 14 is a schematic plan view showing the structure of a feed-reel driving section;  
         [0025]    [0025]FIG. 15 is a schematic side view showing the structure of the feed-reel driving section; and  
         [0026]    [0026]FIG. 16 is a simplified block diagram of the structure of a control section.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    An embodiment according to the present invention will be described in detail below with reference to the drawings.  
         [0028]    (1) The Entire Structure of a Color Roll Printer According to the Embodiment  
         [0029]    Referring to FIGS. 1 and 2, a color roll printer  1  according to the present invention as a whole comprises a case  2 ; a door  3 , a power switch  4 , and a sheet-discharge tray  5 , these which are arranged on the front face of the case  2 ; and a connector connection (not shown) comprising plural connectors for external connections arranged on the rear face of the case  2 .  
         [0030]    The door  3  is constructed of a door chassis  6  arranged rotatably about the lower end of the case  3  and a door panel  7  attached on the front face of the door chassis  6 . The door panel  7  is respectively provided with an operating panel  8  having various kinds of switches and a liquid crystal panel  9  for displaying various kinds of messages on the front face of the door panel  7 , and has a sheet-discharge opening  7 A formed at the lower end of the door panel  7 .  
         [0031]    Therefore, in the color roll printer  1 , operations of the various switches, confirmations of the various messages, and taking out of printed outputs can be performed on the front face of the printer.  
         [0032]    On the front face of the door chassis  6  covered with the door panel  7 , a cutter mechanism  10  for cutting a roll sheet is arranged by corresponding to the sheet-discharge opening  7 A on the door panel  7 . Inside the door chassis  6 , a line-type thermal head  12  held by a head-supporting mechanism  11  is arranged.  
         [0033]    On the other hand, in the front inside of the case  2  shrouded with the door  3 , a platen  13  is rotatably arranged in parallel to the arrow y direction by corresponding to the thermal head  12 , while in the vicinity of the platen  13 , a grip roller  15  and pinch roller  16  for driving and running a roll sheet  14  are arranged.  
         [0034]    Also, in the front inside of the case  2 , a first winding-reel retainer  17 A and a first feed-reel retainer  18 A are rotatably arranged at an upper step position and middle step position on the left internal face, respectively, while a second winding-reel retainer  17 B and a second feed-reel retainer  18 B are respectively arranged at an upper step position and middle step position on the right internal face by corresponding to these retainers  17 A and  18 A.  
         [0035]    Therefore, in the color roll printer  1 , a winding reel  20  and a feed reel  21  for ink ribbon  19  are respectively and rotatably supported with the first and second winding-reel retainers  17 A and  17 B and the first and second feed-reel retainers  18 A and  18 B in parallel with the platen  13  by sandwiching the platen  13  between them in the height direction.  
         [0036]    Furthermore, at the lower position of the platen  13 , an internal door  22  being rotatable about the upper end thereof is arranged, and a paper holder  23  is mounted in the inner part of the internal door  22  and has curved portions, each having a predetermined curvature, respectively arranged in the upper and lower parts, as shown in FIG. 3.  
         [0037]    On both internal side-faces of the paper holder  23  in the vicinity of the bottom surface thereof, pairs of supporting rollers  24  are rotatably mounted, respectively, and as shown in FIG. 3, a roll of the roll sheet  14  (referred to as a sheet roll  14 A below) is to be rotatably supported with these supporting rollers  24 .  
         [0038]    As is apparent from FIG. 3, a lower fixed guide  26 , a movable guide  27 , and an upper fixed guide  28  are respectively arranged in the external periphery of the paper holder  23  via clearances so as to cover a region ranging from the lower front end to the lower rear end of the paper holder  23 , the rear of the paper holder  23 , and a region from the upper rear to the upper front end of the paper holder  23 , while a sheet-discharge guide  32  is arranged on the internal door  22  via a clearance so as to cover a region ranging from the vicinity of the upper end to the lower end vicinity of the internal door  22 .  
         [0039]    Therefore, in the color roll printer  1 , a roll sheet  14  drawn from a sheet roll  14 A is guided by the paper holder  23 , the lower fixed guide  26 , the movable guide  27 , and the upper fixed guide  28 , and passing through the rear part of the sheet roll  14 A, toward an image printing position, which is sandwiched between the platen  13  and the thermal head  12  via the grip roller  15  and the pinch roller  16 , while the roll sheet  14  fed from the image printing position is further guided to run toward the sheet-discharge opening  7 A (FIG. 1) of the door panel  7  (FIG. 1) via the cutter mechanism  10 .  
         [0040]    Therefore, at the lower front end inside the paper holder  23 , a guide roller  30  is rotatably mounted for guiding the roll sheet  14  drawn from the sheet roll  14 A to the opening front end (referred to as a roll sheet insertion-hole  29  below) of the clearance between the paper holder  23  and the lower fixed guide  26 . On the external peripheral face of the paper holder  23  and opposing faces to the paper holder  23  in the lower fixed guide  26  and the upper fixed guide  28 , plural guide rollers  31  and  33 , each having a rotational shaft parallel to the arrow y direction, are rotatably arranged, and on the opposing face of the sheet-discharge guide  32  to the internal door  22 , plural guide rollers  33  having rotational shafts are also rotatably arranged, so that the roll sheet  14  is prevented beforehand by the guide rollers  30 ,  31  and  33  from generating an unwanted load in running the sheet or from damaging the printing surface of the roll sheet  14  due to the friction between the end of the roll sheet insertion-hole  29  or the external peripheral face of the paper holder  23  and the roll sheet  14 .  
         [0041]    Accordingly, in the color roll printer  1 , as shown in FIG. 4, the door  3  is opened and then the internal door  22  is opened so as to place the sheet roll  14 A within the paper holder  23  inside the case  2 . Then, as shown in FIG. 5, the roll sheet  14  drawn from the sheet roll  14 A can be set by pushing it into the roll sheet insertion-hole  29  until the tip end thereof abuts the internal door  22 .  
         [0042]    Also, as shown in FIG. 6, the internal door  22  is then closed, so that the ink ribbon  19  can be set by respectively engaging the winding reel  20  and the feed reel  21  of the ink ribbon  19  with the first and second retainers  17 A and  17 B for the winding reel and the first and second retainers  18 A and  18 B for the feed reel in predetermined states. In such manners, in the color roll printer  1 , the roll sheet  14  and the ink ribbon  19  can be loaded.  
         [0043]    In addition, the color roll printer  1  is configured to print images while the roll sheet  14  is pulled back, as will be described later.  
         [0044]    The movable guide  27  is therefore rotatable about a shaft  34  arranged at the lower end thereof in parallel to the arrow direction y in the color roll printer  1 . Thus, when printing, the movable guide  27  is rotated about the shaft  34  in a direction separating from the paper holder  23  as shown in FIG. 8, for example, so as to form a space  35  (FIG. 8) between the paper holder  23  and the movable guide  27 , enabling the roll sheet  14  to be pulled back during the printing to be slackened within the space  35 .  
         [0045]    The lower end part of the movable guide  27  and the lower end part of the door chassis  6  are connected via a link mechanism  36 , so that the movable guide  27  can be returned from the falling-down state as shown in FIG. 8 (referred to as an open state below) to the state opposing the paper holder  23  via a clearance as shown in FIG. 3 (referred to as a closed state below) by linking with the opening operation of the door  3  (FIG. 1).  
         [0046]    Thereby, when setting the roll sheet  14  in the color roll printer  1 , the opposing surface of the movable guide  27  to the paper holder  23  can serve as a guide for the roll sheet  14  in the running direction.  
         [0047]    On the other hand, as shown in FIG. 7, inside the case  2 , in addition to the paper holder  23  and together with the cutter mechanism  10 , the head-supporting mechanism  11 , and the link mechanism  36 , these which are mentioned above, there is accommodated a mechanical mechanism  37  comprising a door-operating buffer mechanism  38  for the door  3 , a movable-guide locking mechanism  39  for locking the movable guide  27  in the closed state, a grip-roller driving mechanism  40  for rotationally driving the grip roller  15 , a pinch-roller moving mechanism  41  for moving the pinch roller  16  toward and from the grip roller  15 , a winding-reel driving mechanism  42  and a feed-reel retaining mechanism  43  for winding the ink ribbon  19  while maintaining a constant tension, and a skewing-correction mechanism  45  (not shown) having first and second guide plates  44 A and  44 B arranged so as to sandwich the roll sheet  14  from both sides in the width direction, each being arranged at a predetermined position of a main chassis  47 . Also, in the lower rear side inside the case  2 , a control section  130  (FIG. 16) for controlling the entire operation of the color roll printer  1  is mounted.  
         [0048]    The control section  130  controls various operations of the entire color roll printer  1  by controlling each mechanism of the mechanical mechanism  37  based on a predetermined control program.  
         [0049]    In practice, as mentioned above referring to FIGS. 4 and 5, the control section  130  controls that after setting the tip of the sheet roll  14 A to abut the internal door  22 , when the door  3  is closed, the pinch roller  16  is pressed onto the grip roller  15  via the roll sheet  14  by firstly driving the pinch-roller moving mechanism  41  and then, the movable guide  27  is switched to the open state by driving the movable-guide locking mechanism  39  to release the locking of the movable guide  27  (FIG. 3).  
         [0050]    In addition, the reason why that the movable guide  27  is switched to the open state after pressing the pinch roller  16  onto the grip roller  15  is to prevent a possibility that the tip end of the roll sheet  14  may hang down by its own weight within the space  35  between the paper holder  23  and the movable guide  27  in the open state, if the movable guide  27  is switched the open state before clamping the roll sheet  14  with the pinch roller  16  and the grip roller  15 .  
         [0051]    Then, the control section  130  controls to discharge the tip end part of the roll sheet  14  by a predetermined length outside via the sheet-discharge opening  7 A, as shown in FIG. 8, by driving the grip-roller driving mechanism  40  to run the roll sheet  14  in the discharging direction, and then, by driving the cutter mechanism  10 , the roll sheet  14  discharged from the sheet-discharge opening  7 A is cut off, as shown in FIG. 9.  
         [0052]    In such a manner, the control section  130  discards the tip end part of the roll sheet  14 , which may get stained when setting the roll sheet  14 . In the color roll printer  1 , the state immediately after the tip end part of the roll sheet  14  is cut off by the cutter mechanism  10  shown in FIG. 9 is the initial state of the printing operation.  
         [0053]    On the other hand, thereafter, when a printing start button arranged on the operation panel  8  (FIG. 1) on the front face of the door  3  is pushed, the control section  130  moves the first and second guide plates  44 A and  44 B so as to sandwich the roll sheet  14  from the width direction by firstly driving the skewing-correction mechanism  45 , while pressing the pinch roller  16  onto the grip roller  15  via the roll sheet  14  by driving the pinch-roller moving mechanism  41 .  
         [0054]    Then, the control section  130  pulls back the roll sheet  14  until the tip end thereof becomes-the state of being fed from the printing position by a length corresponding to a picture size, as shown in FIG. 10, by driving the grip-roller driving mechanism  40 , while winding the ink ribbon  19  onto the winding reel  20  so as to rewind the ink ribbon  19  by driving the winding-reel driving mechanism  42 .  
         [0055]    The control section  130  successively presses the thermal head  12  onto the platen  13  via the ink ribbon  19  and the roll sheet  14  in order by driving the head-supporting mechanism  11 , and then, prints images based on supplied image data by one line one after another by the thermal head  12  while synchronizing the pulling back of the roll sheet  14  with the winding of the ink ribbon  19 .  
         [0056]    Upon completion of the hot printing process for one color in such a manner, the control section  130  separates the thermal head  12  from the platen  13  by driving the head-supporting mechanism  11 , while the ink ribbon  19  is rewound for the next by driving the winding-reel driving mechanism  42 .  
         [0057]    Furthermore, after pulling back the roll sheet  14  to the printing position again by driving the grip-roller driving mechanism  40 , the control section  130  presses the thermal head  12  onto the platen  13  via the ink ribbon  19  and the roll sheet  14  in order by driving the head-supporting mechanism  11  so as to operate hot printing processes for residual colors and lamination in the same manner as described above.  
         [0058]    Upon completion of the printing process of color images for one page in such a manner, the control section  130  feeds the roll sheet  14  by driving the grip-roller driving mechanism  40  after separating the thermal head  12  from the platen  13  by driving the head-supporting mechanism  11 . Thereafter, the end part of the roll sheet  14  having images formed thereon is cut off by controlling the grip-roller driving mechanism  40  and the cutter mechanism  10  with predetermined timing, as shown in FIG. 11.  
         [0059]    Then, the control section  130  repeats the processes described above until completion of sequential forming of the entire images on the roll sheet  14  based on the image data supplied thereafter.  
         [0060]    In such a manner, the color roll printer  1  sequentially forms images on the roll sheet  14  based on the supplied image data.  
         [0061]    (2) Detailed Structure of Ink Ribbon Driving System and Control Process for Ink Ribbon Driving by the Control Section  130   
         [0062]    Next, the detailed structure of an ink ribbon driving system (the winding-reel driving mechanism  42  and the feed-reel retaining mechanism  43 ) and the control process for driving the ink ribbon by the control section  130  in the color roll printer  1  will be described below.  
         [0063]    (2-1) The Structure of the Winding-reel Driving Mechanism  42   
         [0064]    As shown in FIGS. 12 and 13, the winding-reel driving mechanism  42  comprises a ribbon driving motor  51  fixed to a holder  50 , and a worm gear  52  is attached to the output shaft of the ribbon driving motor  51  so as to mesh with a torque gear  60  of a torque limiter  55  via a gear  53  and a gear  54  formed integrally with the gear  53  in order.  
         [0065]    The torque limiter  55  comprises the torque gear  60  having pieces of felt  61  and  62  bonded on both sides thereof, first and second sandwiching members  67  and  68  which sandwich the torque gear  60  therebetween by interposing a first insertion plate  63  and a second insertion plate  66  having pieces of felt  64  and  65  bonded on both sides thereof in one side of the torque gear  60 , and a shaft  69  having a screw thread formed on the circumferential surface thereof, to which the first and second sandwiching members  67  and  68  are fitted to rotate integrally with the shaft  69 , wherein the torque gear  60  and the first and second insertion plates  63  and  66  are fitted to one end of the shaft  69  so as to be rotatable relative to the shaft  69 , and furthermore, the other end of the shaft  69  is screwed with a spring compression gear  71 , and a compression coil spring  70  is interposed between the second sandwiching member  68  and the compression gear  71 .  
         [0066]    Also, in the torque limiter  55 , the shaft  69  is pivotably and rotatably mounted on first and second bearings  72  and  73  attached to the holder  50 , so that the entire structure is rotatable about the shaft  69 . At one end of the shaft  69 , the second winding-reel retainer  17 B mentioned above is fixed.  
         [0067]    In the winding-reel driving mechanism  42 , the torque limiter  55  and the second winding-reel retainer  17 B, which is integral with the torque limiter  55 , can thereby be rotationally driven based on the rotational output of the ribbon driving motor  51  so as to rotate the winding reel  20  retained to the second winding-reel retainer  17 B.  
         [0068]    In the torque limiter  55  configured in such a manner, the rotational torque produced in the second winding-reel retainer  17 B is determined by the pressure sandwiching the torque gear  60  with the first and second sandwiching members  67  and  68  therebetween (referred to as the sandwiching pressure below). The sandwiching pressure is determined by the urging force applied to the second sandwiching member  68  by the compression coil spring  70 , and the urging force is determined by the spring length of the compression coil spring  70 .  
         [0069]    Therefore, in the torque limiter  55 , by rotating the spring compression gear  71  so as to move the spring compression gear  71  along the shaft  69  in the arrow y direction or in the opposite to this direction, the rotational torque to be produced in the second winding-reel retainer  17 B can be changed.  
         [0070]    Therefore, in the winding-reel driving mechanism  42 , a spring-compression-gear rotational driving system  80  is arranged as rotating means for moving the spring compression gear  71  along the shaft  69  in the arrow y direction or in the opposite to this direction by engaging with the spring compression gear  71  only when adjusting the torque to be produced in the second winding-reel retainer  17 B, and the rotating means does not interfere the rotation of the spring compression gear  71  which rotates integrally with the shaft  69  when the winding reel  20  is rotationally driven.  
         [0071]    In practice, the spring-compression-gear rotational driving system  80  comprises a torque adjusting motor  81  fixed to the holder  50 , a worm gear  82  attached to the output shaft of the torque adjusting motor  81 , first and second pendulum gears  85 A and  85 B which are meshed with the worm gear  82  via a gear  83  and a gear  84  in order, and an encoder gear  87  which is meshed with the spring compression gear  71  and has substantially the same thickness as the movable range of the spring compression gear  71  in the shaft  69 .  
         [0072]    In the situation, the first and second pendulum gears  85 A and  85 B are rotatably mounted on a pendulum plate  88  arranged rotatably about the same rotational shaft as that for the gears  83  and  84 . The pendulum plate  88  is also restricted in rotation under normal conditions by the fitting between a notch  88 A formed at a predetermined position of the pendulum plate  88  and a claw  91 A at one end of a lock arm  91  arranged rotatably about a shaft  90  as shown in FIG. 13, thereby enabling the first and second pendulum gears  85 A and  85 B not to be meshed with the encoder gear  87 .  
         [0073]    Furthermore, in the other end side of the lock arm  91 , the output shaft of a plunger  92  fixed to the holder  50  is fixed so as to release the rotational restriction of the pendulum plate  88  by driving the plunger  92  so as to rotate the lock arm  91  in the arrow c direction.  
         [0074]    In the released state of the rotational restriction of the pendulum plate  88  in such a manner, when the torque adjusting motor  81  is driven in the normal direction, the gear  84  rotates in the arrow d direction and the first and second pendulum gears  85 A and  85 B rotate integrally therewith while the pendulum plate  88  is rotated in the arrow d direction by the rotational momentum of the gear  84  so that the first pendulum gear  85 A is meshed with the encoder gear  87  so as to apply a rotational force for proceeding in the opposite to the arrow y direction to the spring compression gear  71  via the encoder gear  87 .  
         [0075]    On the other hand, in the released state of the rotational restriction of the pendulum plate  88 , when the torque adjusting motor  81  is driven in the reverse direction, the gear  84  rotates in the opposite to the arrow d direction and the first and second pendulum gears  85 A and  85 B rotate integrally therewith while the pendulum plate  88  is rotated in the opposite to the arrow d direction by the rotational momentum of the gear  84  so that the second pendulum gear  85 B is meshed with the encoder gear  87  so as to apply a rotational force for proceeding in the arrow y direction to the spring compression gear  71  via the encoder gear  87 .  
         [0076]    By this structure in the winding-reel driving mechanism  42 , in the restricted sate in rotation of the torque gear  60  by stopping driving the ribbon driving motor  51 , driving the torque adjusting motor  81  in the normal direction while driving the plunger  92  increases the rotational torque to be produced in the second winding-reel retainer  17 B. On the other hand, under the same conditions, driving the torque adjusting motor  81  in the reverse direction decreases the rotational torque to be produced in the second winding-reel retainer  17 B.  
         [0077]    In addition, in the winding-reel driving mechanism  42 , since the lock arm  91  is urged in the opposite to the arrow c direction by a torsion spring (not shown), after adjusting the rotational-torque produced in the second winding-reel retainer  17 B by driving the torque adjusting motor  81 , when the torque adjusting motor  81  is driven next in the direction opposite to this, the first and second pendulum gears  85 A and  85 B can be held not to be meshed with the encoder gear  87  by the pendulum plate  88 .  
         [0078]    Also, in the winding-reel driving mechanism  42 , in one end part of the circumferential surface of the encoder gear  87  in the thickness direction, a thin encoder part  87 A having slits  87 AX formed at predetermined intervals along the whole circumference is mounted while an interrupter-type rotation detecting sensor  93  (FIG. 13) is attached to the holder  50  at a position corresponding to that of the encoder part  87 A. In the holder  50 , a position detecting sensor  94  (FIG. 13) for detecting the spring compression gear  71  positioned at a home position arranged at the end of the shaft  69  in the arrow y direction is disposed.  
         [0079]    By this structure in the winding-reel driving mechanism  42 , the position of the spring compression gear  71  in the shaft  69  can be detected on the basis of outputs of the rotation detecting sensor  93  and the position detecting sensor  94 .  
         [0080]    (2-2) Structure of the Feed-reel Retaining Mechanism  43   
         [0081]    On the other hand, as shown in FIGS. 14 and 15, the feed-reel retaining mechanism  43  comprises a torque limiter  107  configured in the same structure as that of the torque limiter  55  (FIGS. 11 and 12) of the winding-reel driving mechanism  42  (FIGS. 11 and 12) except that a torque gear  102  having pieces of felt  100  and  101  bonded on both sides thereof is directly sandwiched between first and second sandwiching members  103  and  104 , and a spring-compression-gear rotational driving system  108  configured in the same structure as that of the spring-compression-gear rotational driving system  80  (FIGS. 11 and 12) of the winding-reel driving mechanism  42 . At one end of a shaft  105  of the torque limiter  107 , the second feed-reel retainer  18 B mentioned above is fixed.  
         [0082]    The feed-reel retaining mechanism  43 , as shown in FIG.  15 , is provided with a lock arm  112  attached to a holder  111  rotatably about a shaft  110 , a plunger  113  having an output shaft attached to one end of the lock arm  112 , and an idler gear  114  attached so as to meshed the torque gear  102  of the torque limiter  107 .  
         [0083]    In this lock arm  112 , there are provided first and second gears  112 A and  112 B corresponding to the torque gear  102  and the idler gear  114 , respectively, while the lock arm  112  is urged in the arrow e direction by a torsion spring (not shown).  
         [0084]    By this structure, in the feed-reel retaining mechanism  43 , under normal conditions, the first and second gears  112 A and  112 B of the lock arm  112  are respectively meshed with the torque gear  102  and the idler gear  114  so as to lock the torque gear  102  so that it does not rotate and a first load torque set up by the spring-compression-gear rotational driving system  108  (FIG. 14) can be produced in the second feed-reel retainer  18 B. Furthermore, in this state, by driving a torque adjusting motor  115  in the spring-compression-gear rotational driving system  108 , the load torque to be produced in the second feed-reel retainer  18 B can be changed.  
         [0085]    Also, the idler gear  114  is rotatably attached to the end part of a shaft  120  mounted to the holder  111  in parallel to the arrow y direction, and has a predetermined rotational load due to the urging force in the arrow y direction applied by a compression coil spring  122  arranged between a spring receiver  121  fitted to the shaft  120  and the idler gear  114 .  
         [0086]    By this structure of the feed-reel retaining mechanism  43 , when the locking of the torque gear  102  by the lock arm  112  is released by driving the plunger  113  on demand, a second load torque can be produced in the second feed-reel retainer  18 B, which is smaller than the first load torque and corresponding to the urging force applied to the idler gear  114  by the compression coil spring  122 .  
         [0087]    In such a manner, in the feed-reel retaining mechanism  43 , the load torque to be produced in the second feed-reel retainer  18 B can be switched according to two-stage.  
         [0088]    (2-3) Control Process by the Control Section  130  for Ink-ribbon Driving  
         [0089]    The control section  130 , as shown in FIG. 16, comprises a CPU (Central Processing Unit)  131 , an ROM (Read Only Memory)  132 , and an RAM (Random Access Memory)  133 , and the CPU  131  controls and drives corresponding actuators such as motors and plungers in the mechanical mechanism  37  via a mechanical mechanism control section  134  on the basis of various control programs stored in the ROM  132  and various sensor outputs fed from various sensors in the mechanical mechanism  37 , so that various operations described above can be performed.  
         [0090]    In practice, during image printing, for example, the CPU  131  feeds the ink ribbon  19  by one line one after another, while synchronizing with the roll sheet  14 , by driving the ribbon driving motor  51  in the winding-reel driving mechanism  42 , as described above, and also it rewinds the ink ribbon  19 .  
         [0091]    The CPU  131 , during the rewinding the ink ribbon  19 , reduces the load torque of the feed reel  21  enabling high-speed winding of the ink ribbon  19  by driving the plunger  113  in the feed-reel retaining mechanism  43  so as to switch the load torque to be produced in the second feed-reel retainer  18 B to the second load torque smaller than the first load torque during the image printing.  
         [0092]    In addition to such control processes, the CPU  131 , during the image printing, estimates the diameter of the roll of ink ribbon  19  wound onto the winding reel  20  and the diameter of the roll of ink ribbon  19  wound onto the feed reel  21 , and it drives the torque adjusting motor  81  of the winding-reel driving mechanism  42  and the torque adjusting motor  115  of the feed-reel retaining mechanism  43  so as to maintain the tension of the ink ribbon  19  at all times based on estimated results.  
         [0093]    That is, during the image printing, first and second FG (Frequency Generator) pulses, which rise every rotation at a predetermined angle of the torque limiters  55  and  107 , are applied to the CPU  131  from a rotation detecting sensor (not shown) for detecting the rotation of the torque limiter  55  disposed in the winding-reel driving mechanism  42  and a rotation detecting sensor (not shown) for detecting the rotation of the torque limiter  107  disposed in the feed-reel retaining mechanism  43 .  
         [0094]    Then, the CPU  131  firstly computes the diameter of the roll of ink ribbon  19  wound onto the winding reel  20  and the diameter of the roll of ink ribbon  19  wound onto the feed reel  21  on the basis of the first and second FG pulses.  
         [0095]    In practice, when the length of the ink ribbon  19  for one color is L; the number of the FG pulses obtained at this time is FG; the roll diameter of the ink ribbon  19  wound onto the winding reel  20  or the feed reel  21  at this time is R; the tension of the ink ribbon  19  at this time is F; and the number of the FG pulses for one round of the winding reel  20  or the feed reel  21  is FG 1 , the following equation holds:  
       Numerical Formula 1  
         L =( FG /92.6316)*2 πR   (1) 
         [0096]    Thereby, the computation can be performed by the following equation:  
       Numerical Formula 2  
         R =(½π)·( FG   1 / FG )· L   (2) 
         [0097]    Then, in order to maintain a constant tension of the ink ribbon  19 , the CPU  131  computes the number of revolutions of the spring compression gear  71  of the torque limiter  55  in the winding-reel driving mechanism  42  or a spring compression gear  140  of the torque limiter  107  in the feed-reel retaining mechanism  43 , that is, the CPU  131  computes how much drive the torque adjusting motors  81  and  115  until how many numbers of FG pulses are applied from the above-mentioned rotation detecting sensor  93  (FIG. 13) in the winding-reel driving mechanism  42  or the feed-reel retaining mechanism  43 .  
         [0098]    In practice, when the rotational torque or load torque of the second winding-reel retainer  17 B or the second feed-reel retainer  18 B is T, the following equation holds:  
       Numerical Formula 3  
         F=T/R=T /((½π)·( FG   1 / FG )· L )=(2 π/FG   1 )×( FG×T/L )  (3) 
         [0099]    Thereby, in order to maintain the constant tension F of the ink ribbon  19 , the rotational torque or load torque T of the second winding-reel retainer  17 B or the second feed-reel retainer  18 B may be produced so as to satisfy the following equation:  
       Numerical Formula 4  
         T =( FG ½π)·( F×L/FG )  (4) 
         [0100]    Also, the relationship between the rotational torque or load torque T of the second winding-reel retainer  17 B or the second feed-reel retainer  18 B and the number of the FG pulses applied from the rotation detecting sensor  93  (FIG. 13) in the winding-reel driving mechanism  42  or the feed-reel retaining mechanism  43  can be detected by measurements in advance. For example, according to the embodiment, when the number of FG pulses is tFG, the relationship in the winding side is obtained from the following equation:  
       Numerical Formula 5  
         T=tFG ×1.55+500( gf·cm ) (Experimental Result)∴ tFG =( T −500)/1.55  (5) 
         [0101]    Also, the relationship in the feed side is obtained from the following equation:  
       Numerical Formula 6  
         T=tFG ×0.24+150( gf·cm )∴ tFG =( T −150)/0.24 (Experimental Result)  (6) 
         [0102]    By performing these computations, the CPU  131  computes each of tFGs in the winding and feed sides. On the basis of computed results, every time when the printing for one color or one picture, for example, is finished, the plunger  92  (FIG. 13) in the winding-reel driving mechanism  42  or the feed-reel retaining mechanism  43  is driven in a state that the ribbon driving motor  51  in the winding-reel driving mechanism  42  is stopped driving while the torque adjusting motors  81  and  115  (FIGS. 12 and 14) in the winding-reel driving mechanism  42  or the feed-reel retaining mechanism  43  are driven. In such a manner, the CPU  131  maintains the tension of the ink ribbon  19  at all times.  
         [0103]    In addition, upon switching-on, the CPU  131  performs control by setting temporary rotational torque and load torque from the residual sheets of the ink ribbon  19 , and then, it controls so as to maintain the tension of the ink ribbon  19  at all times by the same processing based on the FG pulses obtained by the printing operation thereafter from the rotation detecting sensor in the winding-reel driving mechanism  42  and the rotation detecting sensor in the feed-reel driving mechanism  43 .  
         [0104]    (3) Operations and Advantages of the Embodiment  
         [0105]    In the structure described above, the CPU  131  estimates the diameter of the roll of ink ribbon  19  wound onto the winding reel  20  and the diameter of the roll of ink ribbon  19  wound onto the feed reel  21 , and based on the estimated results, it controls the winding-reel driving mechanism  42  and the feed-reel retaining mechanism  43  so as to maintain the tension of the ink ribbon  19  at all times.  
         [0106]    Therefore, in the color roll printer  1 , the tension of the ink ribbon  19  can be maintained constant at all times regardless of the diameter of the roll of ink ribbon  19  wound onto the winding reel  20  and the diameter of the roll of ink ribbon  19  wound onto the feed reel  21 .  
         [0107]    Also, in the color roll printer  1 , the torque limiters  55  and  107  in the winding-reel driving mechanism  42  and the feed-reel retaining mechanism  43  are configured as shown in FIGS.  12  to  15 , so that the torque limiter capable of torque-adjusting at will can be inexpensively constructed in a simple and light-weight structure, resulting in advances in the simplification and reduction in weight and cost of the entire color roll printer  1 .  
         [0108]    By the structure described above, the diameter of the roll of ink ribbon  19  wound onto the winding reel  20  and the diameter of the roll of ink ribbon  19  wound onto the feed reel  21  are estimated, and based on the estimated results, the winding-reel driving mechanism  42  and the feed-reel retaining mechanism  43  are controlled so as to maintain the tension of the ink ribbon  19  constant. Thereby, the tension of the ink ribbon  19  can be maintained constant at all times, resulting in achievement of a color roll printer capable of constant tension-controlling in a simple structure.  
         [0109]    (4) Other Embodiments  
         [0110]    In addition, in the embodiment described above, the present invention is described by applying it to the color roll printer  1  shown in FIG. 1; however, the present invention is not limited to this, and it can be applied to other printer apparatuses with various structures.  
         [0111]    In the embodiment described above, as the first torque generating means for variably generating the load torque to the feed reel  21  of the ink ribbon  19 , the feed-reel driving mechanism  43  configured as shown in FIGS. 14 and 15 is described; however, the present invention is not limited to this and other various structures may be widely applied.  
         [0112]    Furthermore, in the embodiment described above, as the second torque generating means for variably generating the load torque to the winding reel  20  of the ink ribbon  19 , the winding-reel driving mechanism  42  configured as shown in FIGS. 12 and 13 is described; however, the present invention is not limited to this and other various structures may be widely applied.  
         [0113]    Furthermore, in the embodiment described above, as the controlling means for controlling the winding-reel driving mechanism  42  and the feed-reel retaining mechanism  43  so as to produce the load torque or rotational torque corresponding to the roll diameter of the ink ribbon  19  wound onto the feed reel  21  and/or the winding reel  20 , the control section  130  configured as shown in FIG. 16 is described; however, the present invention is not limited to this and other various structures may be widely applied.