Abstract:
A printer comprising a printer main body ( 11 ), a cover ( 50 ), a sprocket ( 12 ) for delivering a sheet from a sheet roll ( 16 ), a platen roller ( 13 ) used in printing on the sheet, a thermal head ( 51 ) for printing on the sheet, a drive mechanism for rotating the sprocket ( 12 ) and platen roller ( 13 ), and a gear mechanism for rotating, by the drive mechanism, the ribbon core of an ink ribbon cassette ( 60 ) mounted on the cover ( 50 ), the printer main body ( 11 ) being provided with the sprocket ( 12 ) and platen roller ( 13 ), the cover ( 50 ) being provided with the thermal head ( 51 ), wherein the platen roller ( 13 ) is disposed downstream of the sprocket ( 12 ) and it is arranged that when the cover ( 50 ) is dosed, the core gear ( 65 ) of the ink ribbon cassette ( 60 ) meshes with a gear ( 34 ) to elliminate the backlash of the platen gear ( 15 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a printer for printing characters and the like indicating a route or location.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventionally, for example, a printer shown in FIG. 16 is known. The printer shown in FIG. 16 includes a sprocket  1 , a platen roller  2 , a thermal head  3 , a sheet roll  4  on which a sheet is wound. The thermal head  3  is arranged on a cover  5 . A ribbon cassette (not depicted) is mounted on this cover  5 . Moreover, the cover  5  is mounted on a printer main body  7  in such a manner that the cover  5  can swing around a shaft  6 .  
           [0003]    The sprocket  1  and the platen roller  2  are arranged in the printer main body  7 . A reference symbol  8  denotes a cutter for cutting the sheet. The cutter  8  is arranged in the printer main body  7 . A drive gear mechanism for rotating the sprocket  1  and the platen roller  2  is also arranged in the printer main body  7 .  
           [0004]    In this printer, since the sprocket  1  is arranged downstream of the platen roller  2 , a distance W between the thermal head  3  and the cutter  8  is significantly large. This results in a long blank space between the leading edge of the sheet to the position where a character is printed.  
           [0005]    This blank space can be reduced by arranging the platen roller  2  downstream of the sprocket  1 . However, in this case, to eliminate loosening of the sheet between the platen roller  2  and the sprocket  1 , the platen roller  2  should have a circumferential speed slightly larger than the circumferential speed of the sprocket  1  when feeding the sheet.  
           [0006]    However, upon printing start, the sprocket  1  starts rotation earlier than the platen roller  2  because of backlash and the sheet is loosened between the platen roller  2  and the sprocket  1 . Thus, when the platen roller  2  has a fast circumferential speed, the feed-direction length is undesirably increased. That is, the printing distance accuracy becomes unstable.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the present invention to provide a printer comprising a printer main body, a cover arranged on the printer main body in such a manner that the cover can be opened, a sprocket for feeding a sheet from a sheet roll arranged in the printer main body, a platen roller for printing on the sheet, a printing head for printing on the sheet, a drive mechanism for rotating the sprocket and the platen roller, and a gear mechanism for rotating a ribbon core of an ink ribbon cassette mounted on the cover using the drive mechanism, wherein the sprocket and the platen roller are arranged in the printer main body, the printing head is arranged on the cover, and the sprocket is arranged downstream of the sprocket, and wherein backlash eliminating means is further provided for eliminating backlash of the platen gear when the cover is dosed is.  
           [0008]    According to the present invention, the platen roller is arranged downstream of the sprocket. This enables to eliminate backlash of the platen gear when the cover is closed. and to reduce the blank space. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a perspective external view of a printer according to the present invention.  
         [0010]    [0010]FIG. 2 shows arrangement of main component of the printer.  
         [0011]    [0011]FIG. 3 shows arrangement of gears used in the printer.  
         [0012]    [0012]FIG. 4 is a front view of a sprocket used in the printer.  
         [0013]    [0013]FIG. 5 is a front view of a platen roller used in the printer.  
         [0014]    [0014]FIG. 6 shows configuration of the printer when a cover of the printer is opened.  
         [0015]    [0015]FIG. 7 is a side view of an ink ribbon a cassette used in the printer.  
         [0016]    [0016]FIG. 8 is a plan view of the ink ribbon cassette when its cover is removed.  
         [0017]    [0017]FIG. 9 shows engagement relationships between respective gears.  
         [0018]    [0018]FIG. 10 shows engagement relationships between gears.  
         [0019]    [0019]FIG. 11A shows a state of backlash; and FIG. 11B shows a state where backlash is eliminated.  
         [0020]    [0020]FIG. 12 is a block diagram showing configuration of a control system of the printer.  
         [0021]    [0021]FIG. 13A shows a positional relationship between a sheet, a cutter, and a thermal head;  
         [0022]    [0022]FIG. 13B shows a state where the sheet has been fed by preparatory feed; and  
         [0023]    [0023]FIG. 13C shows a state when the sheet has been fed by empty feed.  
         [0024]    [0024]FIG. 14A shows a state when a character printing of a first color has been completed,  
         [0025]    [0025]FIG. 14B shows a state when the sheet has been fed backward by a predetermined amount; and  
         [0026]    [0026]FIG. 14C shows a state when the sheet has been fed by preparatory feed.  
         [0027]    [0027]FIG. 15 shows a state when the sheet has been fed by empty feed from the state of FIG. 14C;  
         [0028]    [0028]FIG. 15B shows a state when a character printing of a second color has been completed, and  
         [0029]    [0029]FIG. 15C shows a state when the sheet has been fed backward by a predetermined amount.  
         [0030]    [0030]FIG. 16 shows configuration of a conventional printer 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]    Description will now be directed to a printer according to an embodiment of the present invention with reference to the attached drawings.  
         [0032]    A printer  10  shown in FIG. 1 to FIG. 3 includes a printer main body  11  and a cover  50  mounted on the printer main body  11  in such a manner that the cover  50  can be opened and dosed.  
         [0033]    In the printer main body  11 , there are arranged a sprocket  12 , a platen roller  13 , a drive mechanism  20  for rotating the sprocket  12  and the platen roller  13 , a gear mechanism  30  for rotating a ribbon core (See FIG. 8) of an ink ribbon cassette  60  which will be detailed later, and a cutter  18  for cutting a sheet. The platen roller  13  has a circumferential speed set slightly faster than a circumferential speed of the sprocket  12 .  
         [0034]    As shown in FIG. 4, a sprocket gear  14  is ranged on the sprocket  12 . As shown in FIG. 5, a platen gear  15  and a torque limiter  19  are ranged at one end of the platen roller  13 . When a load exceeding a predetermined value is applied tot he platen roller  13 , the torque limiter  19  makes the platen gear  15  idle against the platen roller  13 . Moreover, a sheet roll  16  can be arranged at a rear portion of the printer main body  11 . A sheet  16 S which will be detailed later is wound on the sheet roll  16 . The sheet  16 S has feed holes (not depicted) at both sides thereof and the sprocket  12  has protrusions  12   a  to be engaged with the feed holes. A reference symbol  17  is a guide member for guiding a printed sheet portion forward.  
         [0035]    The drive mechanism  20  includes a drive gear  21  arranged on a drive shaft  53   a  of a stepping motor  53  which will be detailed later, a gear  22  engaged with the drive gear  21 , a gear  23  arranged on this gear  22 , a gear  24  engaged with the gear  23 , and a gear  24 A arranged on the gear  24 . The gear  24 A is engaged with a sprocket gear  14  while the gear  24  is engaged with a gear  15 .  
         [0036]    The gear mechanism  30  includes a gear  31  engaged with the platen gear  15 , a gear  32  engaged with the gear  31 , a gear  33  engaged with the gear  32 , and a gear  34  arranged on the gear  33 . The gear  34  is engaged with a core gear  65  which will be detailed later.  
         [0037]    An arc-shaped prolonged hole  11 H is provided on two side plates  11 A of the printer main body  11 , i.e., at a position above a shaft  11 J. A pin  54  provided on the cover  50  is inserted into the prolonged holes  11 H in such a manner that the pin  54  can move along the prolonged holes  11 H. The spring  55  has one end attached to the pin  54  and the other end attached to an engagement portion  11 K provided on the printer main body  11 . The cover  50  is always urged by the spring  55  to an open-state direction. Because of this urging force, the cover  50  will not be dosed with bump by its weight when dosing the cover  50 .  
         [0038]    As shown in FIG. 7 and FIG. 8, the ink ribbon cassette  60  has ribbon cores  62  and  63  arranged in a cassette  61  and core gears  64  and  65  mounted on the ribbon cores  62  and  63 . An ink ribbon  66  is wound on the ribbon core  62  and this ink ribbon  66  is wound up onto the ribbon core  63 .  
         [0039]    When the cover  50  is opened, the core gear  65  of the ink ribbon cassette  60  is disengaged from the gear  34  and when the cover  50  is dosed, the core gear  65  of the ink ribbon cassette  60  is engaged with the gear  34 . As shown in FIG. 9, the core gear  65  has its center located at a position slightly lower than the center position of the gear  34 . Here, as shown in FIG. 10, the gear  34  is slightly rotated counterclockwise by the core gear  65 . This rotation rotates the gears  32 ,  31 , and  15  in directions indicated by arrow in FIG. 9. By this, backlash BR present between the platen gear  15  and the gear  24  shown in FIG. 11A is eliminated as shown in FIG. 11B.  
         [0040]    [0040]FIG. 12 is a block diagram showing configuration of a control system of the printer  10 . In FIG. 12, the control system includes a control unit for controlling the thermal head  51 , a display device  52 , the stepping motor  53 , and the cutter  18  according to operation of keys K. This control unit  71  is composed of a CPU and the like and has a function of preparatory feed means for preparatory feed of the sheet by L 1  (first predetermined amount), a function of empty feed means for empty feed of the sheet L 2  which has been fed in advance, by L 2  (second predetermined amount), and a function of backward feed means for feeding back the sheet by the total length of the preparatory feed amount, the empty feed amount, and the actual printing range.  
         [0041]    Next, explanation will be given on operation of the printer according to the aforementioned embodiment.  
         [0042]    Firstly, as shown in FIG. 6, the cover  50  is opened, the ribbon cassette  60  is mounted on the cover  50 , and the cover  50  is dosed. When the cover  50  is dosed, the core gear  65  slightly rotates the gear  34  counterclockwise, thereby eliminating the backlash B of the platen gear  15 . Here, the sheet roll  16  is already set on the printer main body  11  and the previous printing has been completed and the sheet  16 S has been cut by the cutter  18 . Accordingly, as shown in FIG. 13A, it is assumed that the leading edge  16   a  of the sheet  16 S is located at the position of the cutter  18 .  
         [0043]    Next, operation key K are operated to enter a desired character and the like. The characters and the like entered are displayed on the display device  52 . When printing is started by operating the operation keys K, the control unit  71  rotates the stepping motor  53 . Rotation of the stepping motor  53  rotates via the drive mechanism  20  the platen roller  13  and the sprocket  12 , thereby feeding the sheet  16 S from the sheet roll  16 .  
         [0044]    Here, since the backlash is eliminated, there is no danger of rotation of the sprocket  12  prior to the platen roller  13 . Thus, it is possible to prevent generation of loosening of the sheet  16 S by the backlash BR between the platen roller  13  and the sprocket  12 .  
         [0045]    As shown in FIG. 13B, feeding out of the sheet  16 S is performed firstly as a preparatory feed of length L 1 . This preparatory feed can eliminates the possible loosening of the sheet  16 S caused by a factor other than the backlash BR between the platen roller  13  and the sprocket  12 . This is because the circumferential speed of the platen roller  13  is set faster than the circumferential speed of the sprocket  12 .  
         [0046]    Moreover, the preparatory feed can make the engagement between the gears  22 ,  23 ,  24 , and  14  constant. That is, it is possible to eliminate backlash of these gears  22 ,  23 ,  24 , and  14 .  
         [0047]    Next, as shown in FIG. 13C, empty feed of length L 2  is performed. The preparatory feed and the empty feed are performed by counting pulses to the stepping motor  53 . After this empty feed, printing is performed by the thermal head  52  on the sheet  16 S.  
         [0048]    Thus, since loosening of the sheet  16 S due to backlash BR is prevented and the preparatory feed is performed prior to the empty feed, it is possible to surely eliminate loosening of the sheet  16 S and to obtain constant engagement between the respective gears. Accordingly, it is possible to perform printing accurately at a predetermined position.  
         [0049]    As shown in FIG. 14A, upon completion of printing, drive of the stepping motor is stopped. After this, the stepping motor  53  is rotated in a backward direction, so that the sheet  16 S is fed backward as shown in FIG. 14B. The backward feed length LR is defined by the total of the preparatory feed length L 4  performed for the second color and after, the empty feed length L 2 , and the sheet feed length L 3  upon printing. The backward feed length is determined by counting pulses to the stepping motor  53 .  
         [0050]    When the sheet  16 S is fed backward by the length LR, the backward rotation of the stepping motor is stopped. After this, the stepping motor  53  is drive to rotate in forward direction and as shown in FIG. 14C, the sheet  16 S is fed by the length LA as the preparatory feed. Then, as shown in FIG. 15A, empty feed is performed by the length L 2 . After this, as shown in FIG. 15B, printing of the second color is performed.  
         [0051]    Upon this printing of the second color as well the preparatory feed is performed to correct the loosening of the sheet  16 S and engagement between the respective bears to be constant. Accordingly, it is possible to feed the sheet  16 S in such a manner that a character printed by the first color comes to the position of the thermal head  51 . Thus, it is possible to perform printing of the second color accurately on the character printed by the first color. Thus, it is possible to prevent printing by different colors at different positions.  
         [0052]    As shown in FIG. 14B, upon completion of printing of the fist color, the sheet  16 S is to be fed backward by the length LR′ (L 1 +L 2 +L 3 ). However, the actual backward feed length is the length deleted by the backlash amounts (maximum total backlash Ba) of the respective gears  22  to  24 A, i.e., LR=LR′−Ba. Upon printing of the second color, the length LR′−Ba, ie., a value deleted by the total backlash Ba of the respective gears  22  to  24 A is actually fed out. Accordingly, there is no affect of backlash.  
         [0053]    As shown in FIG. 15B, upon completion of the second color, the sheet  16 S is fed backward by the length LR as shown in FIG. 15C. In the same way as the aforementioned, the preparatory feed and the empty feed are performed before performing printing of the third color. This printing of the third color is also performed accurately at the position of the second color. Thus, there is no danger that different colors are printed at different positions.  
         [0054]    According to this embodiment, the platen roller  13  is arranged downstream of the sprocket  12  and accordingly, it is possible to reduce the distance between the platen roller  13  and the cutter  18 . Therefore, it is possible to reduce the blank space between the leading edge of the sheet  16 S and the position where a character is printed.  
         [0055]    Moreover, according to this embodiment, the torque limiter  19  is provided between the platen roller  13  and the platen gear  15 . Accordingly when a load exceeding a predetermined value is applied to the platen roller  13 , the platen gear  15  per runs idle against the platen roller  13 . Thus, it is possible to feed the sheet  16 A while applying a a predetermined tension between the platen roller  13  and the sprocket  12 .  
         [0056]    In the aforementioned embodiment, when the cover  50  is closed, the core  65  of the ink ribbon cassette  60  is engaged with the gear  34  so as to prevent generation of backlash BR. It is also possible to correct loosening of the sheet  16 S by the preparatory feed alone prior to the empty feed without preventing generation of the backlash BR.  
         [0057]    As has been described above, according to the present invention, it is possible to reduce the blank space and eliminate the backlash.