Abstract:
A sheet handling device including a sheet transport path, a transport mechanism adapted to advance a sheet along the transport path with a non-uniform speed, and a discharge mechanism arranged at the transport path for taking over the sheet from the transport mechanism and discharging it into a tray, wherein the discharge mechanism is adapted to convey the sheet with a momentary speed that is different from that of the transport mechanism; and wherein a portion of the transport path between the transport mechanism and the discharge mechanism is curved along a buffer space to allow the sheet to bend within the buffer space and thereby to absorb the speed difference between the transport mechanism and the discharge mechanism.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a sheet handling device including a sheet transport path, a transport mechanism adapted to advance a sheet along the transport path with a non-uniform speed, and a discharge mechanism arranged at the transport path for taking over the sheet from the transport mechanism and discharging it into a tray. 
     Ink jet printers, for example, often work on a scanning principle. That is, a carriage which includes a number of ink jet printheads reciprocates across a sheet. In each pass of the carriage, a number of pixel lines are printed on the sheet by means of the printheads which eject droplets of ink onto the sheet in accordance with image information supplied to the printheads. Between the printing passes, a transport mechanism advances the sheet by a suitable sheet advance step. In large format printers, the sheet advance movement is simultaneously performed in different parts of the printer. For example, while a leading part of a sheet has already been discharged onto the tray by the discharge mechanism, a rear part of the sheet may still be printed and advanced by the transport mechanism. 
     In order to achieve a high printing speed, the sheet advance movements have to be performed as quickly as possible. However, the printing quality depends on the accuracy of the sheet advance movements. When the sheet is simultaneously advanced by different transport means, one transport means may influence the accuracy of the movement of another transport means. For example, a discharge mechanism that discharges the printed sheet onto a tray may exert a force onto the sheet portion which is still being printed. This problem is even more pronounced in large format printers where the load that has to be exerted to move the sheet is higher. Moreover, the faster the sheet advance step is to be performed, the more power is required of the transport means. This leads to a higher load on the sheet and an increased noise generation. 
     From JP 62211263 A, a transport mechanism for discharging a sheet is known that transports the sheet from a pair of fixing rollers towards a discharge tray. A first driving roll is driven at a peripheral speed which is higher than that of the fixing rolls. Thereby, a waving of the sheet is reduced. However, the driving roll exerts a pulling force on the sheet during the fixing process. 
     JP 08268615 A also shows a transport mechanism for discharging a sheet that transports the sheet from a pair of fixing rollers towards a discharge tray. A sheet is held between the fixing rollers and first carrying rollers while a previous sheet is discharged by further carrying rollers at a speed which is higher than the paper carrying speed of the fixing rollers. The first carrying rollers are driven by means of a torque limiter, but nevertheless exert a pulling force on the sheet while it is held between the fixing rollers and the first carrying rollers. 
     From JP 2004196483 A, a transport mechanism is known which is adapted to appropriately maintain the amount of deflection of a sheet being conveyed along a curved path between two pairs of conveyance rollers. The amount of deflection of the sheet is measured by a pivotable contact arm, and the peripheral speed ratio of the two pairs of conveyance rollers is controlled to appropriately maintain the amount of deflection. However, a constant amount of deflection can only be maintained when the two pairs of conveyance rollers are continuously driven and their speed variations are small. 
     From WO 2004/041542 A1, a device for delivering a printed coupon is known wherein a web is advanced into a reserve chamber by a transport roller which participates in printing the web. The web is further advanced by a pair of discharge rollers at an discharge opening of the chamber. The transport rollers and the discharge rollers are operated to hold the web under tension against a resiliently pivotable arm during the printing process and to pull the web against a cutting edge after the printing is completed. Thus, a pulling force is exerted on the web during printing and also for cutting the web after printing. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a sheet handling device which allows a high printing speed and a high accuracy of sheet advance movements that are relevant to printing accuracy. 
     According to the present invention, this object is achieved by a sheet handling device of the type indicated above, wherein the transport mechanism is adapted to a stepwise advance of the sheet, wherein the discharge mechanism is adapted to convey the sheet with a momentary speed that is different from that of the transport mechanism; wherein a portion of the transport path between the transport mechanism and the discharge mechanism is curved along a buffer space to allow the sheet to bend within the buffer space and thereby to absorb the speed difference between the transport mechanism and the discharge mechanism, and wherein the transport mechanism and the discharge mechanism are adapted to be driven in such a way that a section of the sheet which is within the buffer space is mechanically stress-relieved, thus avoiding tension of the sheet between the transport mechanism and the discharge mechanism. 
     Accordingly, the sheet is buffered between the transport mechanism and the discharge mechanism. Thus, the sheet does not couple the transport mechanism to the discharge mechanism. In particular, the discharge mechanism does not exert a force on the transport mechanism through the sheet, because tension of the sheet between the transport mechanism and the discharge mechanism is avoided. For example, the buffer space is situated in the sheet transport path between a sheet support element and the discharge mechanism. The sheet support element supports the sheet during the printing process. The transport mechanism and the discharge mechanism are adapted to be driven in such a way that the sheet section in the buffer space is mechanically stress-relieved. For that purpose, the transport mechanism and the discharge mechanism may be driven in such a way that the length of the sheet section in the buffer space is always larger than a minimal length, unless the leading or the trailing edge of the sheet is yet within the buffer space. 
     In a preferred embodiment, the portion of the transport path comprises a curved guide plate, said guide plate passing in approximately a one-half turn around the buffer space. When the sheet is present in the buffer space and is conveyed by the discharge mechanism, the length of the sheet section that is accommodated in the buffer space will be reduced, and the sheet will be lifted from the curved guide plate. Then, the buffer space offers room for a further advance of the sheet by the transport mechanism. 
     Preferably, a path sensor is arranged at the buffer space. The path sensor is adapted to detect the distance between the portion of the transport path and the sheet when it is bent into the buffer space and a signal of the path sensor is input to a drive controller of the discharge mechanism. When the sheet traverses the buffer space, the path sensor is sensitive to the length of the sheet section that is within the buffer space. For example, the path sensor may detect a situation where the path of the sheet section within the buffer space has reached a certain minimum length, or the path sensor may detect a situation where the sheet section within the buffer space has reached a certain tension. In a straightforward example, the discharge mechanism is operated as long as a further reduction of the length of the sheet section between the sheet support element and the discharge mechanism is possible without inducing an unallowable tension in the sheet. 
     The transport mechanism is adapted to a stepwise advancement of the sheet. This applies to printers or copiers that work on a scanning principle as has been described above. For example, the discharge mechanism may also utilize a stepwise conveyance of the sheet. However, due to the buffering of the sheet, the discharge mechanism does not influence the sheet advance accuracy of the transport mechanism. 
     Preferably, the discharge mechanism is adapted to continuously convey the sheet. For example, the discharge mechanism may continuously advance the sheet while the transport mechanism stepwise advances the sheet over the sheet support element during the printing process. When the discharge mechanism continuously advances the sheet, the power of the discharge mechanism can be reduced as compared to the requirements for stepwise advancing the sheet. Moreover, a continuous discharging of the sheet into the tray is more convenient and gives an impression of a higher printing speed. Preferably, the speed of the discharge mechanism is adapted or adaptable to different printing modes and average sheet advance speeds of the transport mechanism for each printing mode. By continuously conveying the sheet, the acceleration forces that are applied to the discharge mechanism and to the sheet are significantly reduced. This has also the additional advantage of reducing the noise generation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present invention will now be described in conjunction with the drawings, in which: 
         FIG. 1  is a schematic partial cross-sectional view of a printer; and 
         FIG. 2  shows a detail of a sheet handling device of the printer shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in  FIG. 1 , an ink jet printer comprises a platen  10  which is intermittently driven to rotate in order to advance a sheet  12 , e.g. a sheet of paper, in a direction indicated by an arrow A over the top surface of a sheet support plate  14 . A number of transport rollers  16  are rotatably supported in a cover plate  18  and form a transport nip with the platen  10 . The transport rollers  16  and the platen  10  form a transport mechanism for stepwise advancing the sheet  12 . Thereby, the sheet  12 , which is supplied from a reel  19  via a guide plate  20 , is paid out through a gap formed between an edge of the cover plate  18  and the surface of the sheet support plate  14 . 
     A carriage  22  which includes a number of ink jet print heads (not shown) is mounted above the sheet support plate  14  so as to reciprocate in a direction that is perpendicular to the plane of the drawing across the sheet  12 . In each pass of the carriage  22 , a number of pixel lines are printed on the sheet  12  by means of the print heads which eject droplets of ink onto the sheet in accordance with image information supplied to the print heads. For the sake of simplicity, guide and drive means for the carriage  22 , ink supply lines and data supply lines for the print heads, and the like, have not been shown in the drawing. 
     The top surface of the sheet support plate  14  has a regular pattern of suction holes (not shown) through which the sheet  12  is drawn against the flat surface of the support plate  14  and is thereby held in a flat condition, especially in the area which is scanned by the carriage  22 , so that a uniform distance between the nozzles of the printheads and the surface of the sheet  12  is established over the whole width of the sheet, and a high print quality can be achieved. 
     The sheet  12  is further advanced along a curved guide plate  112  that turns the sheet upside down and reverses the transport direction of the sheet  12 . As is shown in  FIG. 1  and, in more detailed view, in  FIG. 2 , the sheet  12  is guided to a discharge nip  114  formed between a plurality of upper discharge rollers  116  and lower discharge rollers  117 , that are mounted on common axles  116   a  and  117   a , respectively. The discharge rollers  116 ,  117  form a discharge mechanism for continuously conveying the sheet, as will be described below. 
     From the discharge nip  114 , the sheet  12  is discharged onto a tray  124 . The tray  124  has a top surface  126  for supporting the sheets and has stops  132  at which the trailing edges of the sheets  12  will be aligned. 
     A discharge sensor  136  is arranged near the discharge nip  114  to indicate when the trailing edge of the sheet  12  has been discharged from the discharge nip  114 . The discharge sensor  136  is of conventional design and comprises an arm that is pivotable about an axis. 
     A top frame member  138  of the tray  124  carries a tray-full sensor  140  which is also of conventional design containing an arm that is pivotably mounted on the frame member  138 . 
     The curved guide plate  112  surrounds a buffer space  160  for the sheet  12  and passes in approximately a one-half turn around the buffer space  160 . When a leading edge of the sheet  12  has reached the discharge rollers  116 ,  117 , the discharge rollers  116 ,  117  engage the sheet  12  and advance the sheet  12  towards the tray  124 . Thereby, sheet  12  may be lifted from the guide plate  112  when its trailing portion is still held on the sheet support plate  14 . Thereby, a section  12   p  of the sheet  12  traverses the buffer space  160  on a bent path that has a shorter length than the path along the guide plate  112 . The buffer space  160  is adapted to accommodate varying lengths of the sheet section  12   p  between the sheet support plate  14  and the discharge rollers  116 ,  117 . Thereby, the sheet  12  is buffered so that speed differences between the transport rollers  16  and the platen  10  on the one side and the discharge rollers  116 ,  117  on the other side are absorbed, so that the sheet section  12   p  is mechanically stress-relieved. Thus, the discharge rollers  116 ,  117  will not exert a force on that part of the sheet that is held on the sheet support plate  14  nor on the platen  10  and the transport rollers  16 . 
     At the guide plate  112 , a path sensor  162  is mounted having a pivotable arm  162   a  that extends into the buffer space  160 . At the end of the arm  162   a , a rod  162   b  extends transverse to the plane of the drawing of  FIG. 2 . When the sheet  12  is lifted from the guide plate  112 , the sheet  12  engages the rod  162   b  and pivots the arm  162   a . Thereby, the path sensor  162  detects how far the sheet  12  is lifted from the guide plate  112 , indicating the length of the path of the sheet section  12   p  that traverses the buffer space  160 . 
     A signal of the path sensor is input to a drive controller  164  for the discharge rollers  116 ,  117 . When the sheet is discharged by the discharge rollers  116 ,  117 , the discharge rollers  116 ,  117  are continuously driven to convey the sheet  12  with a speed corresponding to an average advance speed of the sheet along the sheet support plate  14 . Thereby, a continuous discharge movement of the sheet  12  is combined with the stepwise advancing of the sheet over the sheet support plate  14  during the printing process. 
     Due to the input of the path sensor  162  to the drive controller  164 , the speed of the discharge rollers  116 ,  117  can be controlled to assure that the sheet section  12   p  is always lifted from the guide plate  112  in a degree so as to be mechanically stress-relieved. When the length of the path of the sheet section  12   p  becomes to short, the speed of the discharge rollers  116 ,  117  is reduced, and vice versa. Thereby, the sheet  12  is discharged onto the tray  124  with an almost constant speed. 
     The discharge rollers  116 ,  117  may be driven via free wheel clutches, so that they may temporarily rotate at a higher speed when the leading edge of a new sheet is pushed into the discharge nip. 
     Although an example has been described, where the sheet is stepwise advanced during the printing process and is continuously advanced through the discharge nip, the invention may be applied to any combination of continuous or stepwise discharge movements with continuous or intermittent movements during the printing process. In any case, the buffering of the sheet assures that the discharge rollers  116 ,  117  do not exert a force via the sheet section  12   p  onto that part of the sheet that is being printed at the sheet support plate nor on the transport rollers  16  nor the platen  10 . Thereby, a high printing accuracy is achieved. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.