Abstract:
Disclosed is a buffer disposed to receive cut sheets from a first workstation operating at a first slower speed, deskew the sheets and deliver them to a second workstation operating at a second faster speed. The buffer has a guide defining a path of travel extending from driven inlet rollers at a buffer inlet to driven exit rollers adjacent a buffer exit. The guide includes a trap that opens to accommodate a service loop formed of the cut sheet that is longer than the path of travel. The inlet rollers are driven at the first slower speed to move cut sheets into the buffer and up to the exit rollers and the exit rollers are selectively driven at the second faster speed to deliver cut sheets to the second workstation. The exit rollers are located at an elevation above the inlet rollers and this orientation together with the opening of the trap provides for a vertical separation between the trailing edge of a first cut sheet and the leading edge of a second cut sheet to insure there is non contact spacing between the sheets moving through the buffer.

Description:
CROSS REFERENCE TO RELATED CASES 
     This application is a Continuation of an application Ser. No. 10/061,142 filed Feb. 1, 2002 entitled “Buffer with Service Loop and Method” now U.S. Pat. No. 6,460,687. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to paper handling apparatus and more particularly to a buffer mechanism used in a photofinishing system for conveying a cut sheet from one work station of the system operating at a first speed to a second workstation of the system operating at a faster speed. 
     BACKGROUND OF THE INVENTION 
     In a photo finishing system of the type for which this invention relates, prints created by an ink jet printer are dried, cut into sheets that may include one or a number of prints, and then the sheets are laminated to protect the images. The lamination process is continuous in that the laminating material is applied to the cut sheets from a continuous roll of donor material. In order to minimize waste of the laminating material, it is desirable that the space between cut sheets be as small as possible. The desire for as small a space as possible between adjacent sheets is balanced against the need to prevent the leading edge of a following sheet from contacting the trailing edge of a leading sheet during the processing of the sheets. Accordingly, while it is necessary to maintain some gap or spacing between the cut sheets, this gap preferably is as small as possible. 
     The lamination process requires the application of heat and pressure to the sheets to effectively carry out the lamination process. Typically, this is done with lamination rollers that provide both the heat and pressure and this dual function makes the rollers somewhat bulky. The lamination process further requires that these rollers operate at an essentially constant speed and that the rollers operate continuously. It is important that the rollers not stop during the lamination process as this could destroy a sheet of prints. 
     From the laminator, the sheets may be fed to an embosser to provide the photographs with a desired matte finish. Like the laminator, the embosser uses relatively heavy rollers that carry out the embossing process by producing a textured surface on the laminate. The embosser rollers may be heated and need to operate continuously to produce a satisfactory surface. Also, the embossing process tends to proceed faster than the laminating process so there is a difference in the speed at which the laminating and embossing rollers operate. Thus, the problem exists of delivering the cut sheets from the laminator to an embosser operating at a faster speed. 
     The cut sheets emerging from the laminator may not be perfectly aligned when they are delivered to the embosser. Passing a skewed laminated sheet through the embosser will adversely effect the quality of the finished print. Accordingly, if the sheets are skewed leaving the laminator, it is necessary to realign or “deskew” the sheets before delivering them to the embosser. 
     One method for deskewing is to provide a pair of transport rollers positioned to form a nip. If no corrective action is taken, a sheet of paper entering the nip at an angle will engage the rollers first at one point on the leading edge. The paper then will progressively enter the nip along the leading edge until the other side is reached so the sheet will pass through the nip in a skewed orientation. However, if the transport rollers are stopped the leading edge will butt against the nip and will align itself with the nip as the portion of the sheet immediately behind the leading edge forms a buckle or curled portion that takes up the skew. The transport rollers can then be energized to advance the now aligned sheet through the nip. 
     Stopping the transport rollers for a time sufficient to remove the skew at the leading edge of a first sheet causes the following sheet to close the gap between it and the trailing edge of the first sheet. Accordingly, the transport rollers must operate at a speed higher than a speed at which the cut sheets are delivered from the laminator or the buffer will fall behind, which is not acceptable. 
     As noted above, the operating speed of the embosser preferably is faster than the operating speed of the laminator. This arrangement allows the embosser to accept sheets from the transport roller without creating a backup. However, with the embossing process and laminating process operating at different speeds, the buffer must be long enough to accommodate the longest sheet being processed so that a sheet does not simultaneously engage the embosser rollers and the lamination rollers. 
     Accordingly, it is an object of this invention to provide a buffer for receiving cut sheets from a laminator in a photo finishing machine, exit the sheets, and delivering the sheets to an embosser. 
     Another object of this invention is to provide an apparatus and method for buffering sheets moving from one work station operating at a first speed to a second work station operating at a second, faster speed. 
     A further object of this invention to provide a buffer that accommodates the different operating speeds of the laminator and the embosser and provide an efficient transfer from one to the other while minimizing the gap between sheets. 
     While this invention is particularly well suited for the purpose just described, it will be understood that the nature of the processes that immediately precede and follow the buffer is not an element of the invention. The buffer can be used between any two processes where the requirements for receiving sheets for one process and delivering them to a second process are similar to those presented by the laminator and embosser described herein. 
     SUMMARY OF THE INVENTION 
     Briefly stated and in accordance with the presently preferred embodiment of the invention, a buffer of the present invention is disposed between a laminator and an embosser in a photo finishing machine. The buffer includes exit rollers forming a nip for receiving a leading edge of a cut sheet in a skewed orientation and registering the edge parallel to the rollers. Driven inlet rollers deliver the cut sheets to the exit rollers along a guide track at a speed equal to the output speed of the laminator. 
     A controller, operatively connected to the exit rollers and the inlet rollers, stops the exit rollers to allow the inlet rollers to move the leading edge of the cut sheet against the nip and form a slight buckle in the cut sheet. The controller then starts the exit rollers to advance the cut sheet partly through the exit roller and again stops the exit rollers. During all this time the inlet rollers continue to feed the cut sheet to the buffer from the laminator. Stopping the exit rollers prevents the possibility that the leading edge of the cut sheet will enter the embosser while the trailing edge of the sheet is still in the grip of the laminator or of the inlet rollers. 
     The guide track has a trap that opens on command from the controller so continued operation of the inlet rollers (while the exit rollers are stopped) forms a service loop of the cut sheet that extends through the open trap. In this fashion the buffer can accommodate a length of cut sheet that is longer than the guide track. A sensor signals the controller upon the passage of the trailing edge of the cut sheet past a fixed point. This indicates that the trailing edge of the cut sheet is free of the laminator. When this happens, the exit rollers are activated to turn at a faster speed that matches the operating speed of the embosser. The timing is such that the trailing edge of the cut sheet will clear the inlet rollers before the service loop is depleted so the cut sheet is not put into tension by rollers operating at different speeds. 
     As the trailing edge of the first sheet clears the inlet rollers, it drops away from the inlet rollers and through the open trap to create vertical clearance between the inlet rollers and the trailing edge. Due to the length of the service loop and the speed at which it is drawn by the exit rollers, it is possible that the leading edge of the following sheet entering between the inlet rollers will overtake the trailing edge of the first sheet. Keeping the trap open and creating the vertical clearance as noted above prevents the leading edge of the following sheet from contacting the trailing edge of the first sheet. The controller keeps the trap open until a gap between the first sheet and the following sheet is reestablished and then the trap is closed so the guide track can direct the leading edge of the following sheet to the exit rollers. 
     Accordingly, the present invention may be characterized in one aspect thereof by a buffer disposed between two workstations. The buffer receives cut sheets from a first work station operating at a first speed and delivers the cut sheets to a second work station operating at a second speed faster than the first speed. The buffer acts to maintain a separation or gap between the trailing edge of a first sheet and a leading edge of a following sheet and comprises: 
     a) inlet drive rollers for moving consecutive first and second spaced apart sheets into the buffer and in a forward direction along a path of travel to a buffer exit at a first sped; 
     b) exit inlet rollers for moving the sheets from the buffer at a second faster speed; 
     c) a controller operable to selectively stop the exit inlet rollers thereby stopping the forward progress of the first sheet while continuing to operate the inlet rollers such that a leading end of the second sheet overtakes a trailing end of the first sheet thereby eliminating the space between the sheets and creating an overlap of the sheets at some point in the buffer; and 
     d) the controller operable to drive the exit inlet rollers at the second faster speed to restore a spacing between the sheets and resolve the overlap prior to the leading end of the second sheet reaching the buffer exit. 
     In another aspect, the invention may be characterized by a method for buffering cut sheets moving from a first workstation operating at a first speed to a second workstation operation at a faster speed, the method comprising: 
     a) moving consecutive first and second spaced apart sheets from a first station and into a buffer inlet at a first speed, the sheets moving in a forward direction along a path of travel through the buffer to a buffer exit; 
     b) stopping the forward progress of the first sheet along the path of travel while continuing to move the second sheet at the first speed resulting in a leading end of the second sheet overtaking a trailing end of the first sheet thereby eliminating the space between the sheets and creating an overlap of the sheets at some point in the buffer; and 
     c) moving the first sheet through the buffer exit to a second station at a second faster speed and restoring a spacing between the sheets to resolve the overlap prior to the leading end of the second sheet reaching the buffer exit. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the position of the buffer of the present invention; 
     FIG. 2 is a view in cross section showing laminated sheets prior to separation; and 
     FIGS. 3-11 are views showing a portion of the buffer at successive operational steps. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, FIG. 1 shows a buffer according to the present invention generally indicated at  10  disposed between a first upstream workstation  12  and a second downstream workstation  14 . The operation of the buffer is under control of a controller  16  as further described hereinbelow. The buffer as described herein is for use in a photofinishing operation wherein the first workstation is a laminator and the second workstation is an embosser, all components being part of a photofinishing system. Neither the laminator nor the embosser is part of the present invention so they will not be described in detail. It is sufficient to say that when an inkjet printer is used in a photofinishing operation, it is preferred that individual printed sheets be laminated with a clear protective material, usually a plastic sheet. 
     For efficient operation, the lamination material is drawn from a roll and laid onto one or both surfaces of printed sheets individually fed to the laminator. To minimize waste of the lamination material, the gap or spacing between the individual printed sheets fed to the laminator is kept as small as possible. The result, as shown in FIG. 2 is a continuous piece comprising two adjacent printed sheets  4  and  6  connected by a layer of the laminate material  8 . On leaving the laminator the continuous piece is cut between the adjacent printed sheets  4 ,  6  to again separate the individual printed sheets. 
     After the lamination is applied and the sheets are cut apart, the cut sheets are delivered to an embosser that puts a matte finish to the cut sheet. Generally the embosser operates at a faster speed than the laminator so one function of the buffer is to receive cut sheets from the laminator (first workstation) operating at a first speed and deliver the cut sheets to the embosser (second workstation) operating at a faster speed. Another function of the buffer is to insure that a cut sheet is completely free of the laminator before being delivered to the embosser. This is because damage can result to a cut sheet having one end in the grip of the laminator operating at one speed and another end in the grip of the embosser operating at a faster speed. 
     As shown in FIG. 3, the buffer has a set of driven inlet rollers  18 , a set of driven exit rollers  20  and a set of driven outlet rollers  22 . The inlet rollers  18  are driven at the same operational speed as the laminator. The outlet rollers  22  are driven at the same operational speed as the embosser and the exit rollers are driven at a selected speed. Preferably, inlet rollers  18  are disposed at a lower elevation in the buffer than the exit and outlet rollers. 
     A guide  24  composed of spaced upper and lower members  26 ,  28  respectively defines a path of travel between the inlet rollers and the nip  21  formed by the exit rollers. Since the exit rollers are disposed above the inlet rollers  18 , the guide  24  defines a path of travel that curves upward to exit rollers  20  from the inlet rollers  18 . Thus a cut sheet  4  passing from the laminator  12  and entering the buffer through inlet rollers  18 , has its leading edge  30  directed to the exit rollers  20 . 
     The lower member  28  of the guide has an end  32  adjacent the exit rollers  20  fixed for rotation about an axis  34 . Adjacent its end  32 , the guide member  28  is formed with a dogleg  36  for purposes set out hereinbelow. Completing the structure is a sensor  33  that issues a signal to controller  16  upon the passage of the trailing edge of a cut sheet. The location of the sensor is such that passage of the trailing edge of a cut sheet signifies that the cut sheet is free of the grip of the laminator. 
     The operation of the buffer will be described as beginning with FIG. 3, which shows the leading edge  30  of a laminated cut sheet  4  as passing between the driven inlet rollers  18  and entering the buffer. As noted hereinabove, inlet rollers  18  operate at the same speed as the laminator. This insures that the portion of the cut sheet  4  in the grip of inlet rollers  18  moves at the same speed as a trailing edge  38  of the cut sheet that may still be in the grip of the laminator. Also shown in FIG. 3 for purposes of illustration is a second cut sheet  6  that follows the first sheet and has its leading edge  40  spaced from the trailing edge  38  of the first sheet by a distance “x”. 
     The guide  24  leads the sheet  4  to the exit rollers  20 . At this time the exit rollers are stopped. Eventually the leading edge  30  of the sheet contacts the nip  21  formed by the exit rollers while the inlet rollers  18  continue to operate. This causes a portion  42  of the sheet adjacent the leading edge to buckle as shown in FIG.  4 . The buckle is accommodated by the dogleg  36  in the lower portion of the guide. As the sheet buckles, the buckle takes up any skew of its leading edge relative to the nip  21  so that the leading edge becomes aligned with the nip. In this fashion a sheet that may be skewed upon entering the buffer is deskewed by the buffer exit rollers  20  and the leading edge is realigned parallel to the nip so that a properly orientated sheet is delivered to the outlet rollers  22  and to the embosser or downstream workstation. 
     After the deskewing operation is complete, the controller  16  momentarily activates the exit rollers  20  to allow the leading edge  30  and a small portion of the sheet to pass through nip  21  (FIG.  5 ). For this operation the exit rollers are driven at the same speed as the inlet rollers  18 . After the momentary activation, the exit rollers are stopped. In this fashion the nip of the exit rollers holds the sheet while the inlet rollers continue to operate to move the cut sheet  4  into the buffer at the same speed as the operational speed of the laminator. The exit rollers  20  are stopped while the inlet rollers continue to operate to avoid a situation where a long cut sheet might extend into the outlet rollers  22  (that move at a faster speed) while part of the cut sheet is still in the grip of the inlet rollers  18 . 
     Since a length of a cut sheet may be longer than the length of the path of travel defined by guide  24 , some room must be made for the length of sheet being moved into the buffer while the exit rollers are stopped. Accordingly, the controller acts to rotate the lower portion  28  of the guide about the axis  34  so a trap in the guide is opened (FIG.  6 ). With the trap open, a length of the sheet driven into the buffer can bow out into the space created by the opening. In this way the buffer can accommodate a length of the sheet by causing a service loop  5  to form that is much longer than the length of the path of travel defined by the guide  28 . The length of the service loop (L SL ) between the inlet rollers  18  and the nip  21  equals the length of the path of travel with the trap closed (L C ) plus the speed (V L ) of the inlet rollers  18  multiplied by the time that the exit rollers are stopped (T 1 ) or L SL =L C +(V L ×T 1 ). 
     At some point, the sensor  33  will identify the passing of the trailing edge  38  of the cut sheet signifying that the cut sheet is out of the laminator. When this occurs, controller  16  activates the exit rollers  20  for operation at a speed equal to the processing speed of the embosser or downstream workstation. This action takes up the slack provided by the service loop  5  and begins to move the cut sheet  4  through the driven outlet rollers  22  and into the embosser (FIG.  7 ). It is possible that the exit rollers can be activated to turn at the faster operating speed of the embosser before the trailing edge of the cut sheet is clear of the slower moving inlet rollers. However, the controller insures that the slack provided by the service loop, that is the length of the service loop, contains a length sufficient to prevent the slack from being depleted prior to the time the trailing edge  38  of the first cut sheet clears the inlet rollers. This avoids a situation where the cut sheet is put into tension between the inlet rollers  18  operating at one speed and the exit rollers  20  operating at a faster speed. 
     As described above, the exit rollers draw the cut sheet from the service loop at a speed faster than the speed at which the following sheet  6  is delivered to the buffer. However, the length of the first sheet may be such that time does not permit the removal of a sufficient length of the first sheet  4  to prevent the following sheet  6  from catching up to the first sheet at some point along the path of travel. In other words the length of the gap ‘x” between the sheets could be reduced to a negative number before the first sheet is out of the buffer. This means that the leading edge  40  of the second or following sheet  6  will run into the trailing edge  38  of the first or leading sheet  4 . Keeping the trap open avoids this situation. 
     As shown in FIG. 8, the trailing edge  38  of the first sheet  4  as it clears the inlet rollers  18  will drop from the inlet rollers and leave the defined path of travel. This is because the location of the exit rollers  20  at a higher elevation than the inlet rollers  18  and the curvature of the path of travel cause the trailing edge  38  of the cut sheet to spring downwards and away from the inlet rollers. Now when the leading edge  40  of the following sheet passes through the nip at the inlet rollers  18 , it will be vertically displaced from the trailing edge of the first sheet as shown in FIG.  9 . This displacement avoids an overlap that could cause the two sheets to contact. 
     If the length of the leading sheet  4  is such that no over lap is created with the leading edge of the following sheet, the trap can be closed. Conversely, if the length of the first sheet is such that there is an overlap with the following sheet, the trap will remain open to allow time for the first sheet to “run away” from the following sheet before the trap is closed. The over lap can be calculated using the formula 
     
       
         Overlap=( L   R   −L   C )−( x )( V   E )/ V   L   
       
     
     Where 
     L R =length of service loop when the trailing edge of the first sheet leaves the laminator 
     L C =the length of the path of travel with the trap closed 
     “x”=length of the initial gap between the sheets 
     V E =the speed of the embosser and 
     V L =the speed of the laminator. 
     If the overlap is calculated to be a negative number, there is no over lap and the trap can close as soon as the length of the service loop is less than the length of the path of travel with the trap closed. If the calculation yields a positive number, the sheets would overlap so the trap must remain open and allow the first sheet to move away from the following sheet. 
     After a time, there is a removal of the first sheet by the exit rollers  20  sufficient to reestablish a gap between the two sheets. When this happens the controller causes the lower guide portion  28  to close as shown in FIG.  10 . This reestablishes the path of travel for guiding the leading edge  40  of the second sheet  6  to the exit rollers  20 . After passage of the first sheet from the buffer, these rollers are stopped and the action repeated to deskew the following sheet  6  as shown in FIG.  11 . 
     Thus it should be appreciated that the present invention provides a buffer disposed between workstations that have different operational speeds that can accommodate a cut sheet entering at one operational speed and then pass it out of the buffer at a second operational speed. The buffer further is able to stop and deskew a sheet while preventing a trailing sheet from running into a leading sheet. The buffer is able to accommodate sheets of various lengths including sheets longer than a path of travel through the buffer.