Abstract:
The skew of a sheet of print media is precisely controlled by intentionally inducing print media deformation, so that cooperative mechanical and gravitational forces may be used to align the leading edge of the sheet. The sheet is driven into a nip region of deskew rollers in a manner that causes the print media to deform, lifting a movable member in a direction substantially perpendicular to the feedpath into the nip region. The movable member applies a return force that is generally equal along a widthwise surface region of the print media. As a result, there is a reduced likelihood that the print media will be skewed relative to the nip region. In some applications, the return force is adjustable.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates generally to the field of media transport and more particularly to print media deskew methods and structural arrangements for use in print applications.  
         BACKGROUND ART  
         [0002]    In many print media handling applications, it is desirable to minimize skew, where “skew” is defined as the misalignment of print media as a leading edge approaches or reaches a position in which print media orientation affects operations. For applications in which the print media is a sheet of paper or a transparency, the skew will often vary from sheet to sheet. Booklet making is one example of an application in which minimizing skew is an important consideration. U.S. Pat. No. 6,099,225 to Allen et al., which is assigned to the assignee of the present invention, describes what is referred to as a sheet-wise method of booklet making, since the finishing operations are performed on a sheet-by-sheet basis. The finishing operations include cutting, scoring, folding, punching, and stapling. Each sheet is cut to a length that is determined by its sequence in the booklet and by the thickness of the sheets that form the booklet. A sheet that is folded to provide the outer pages of a booklet may not be trimmed at all, while the sheet that is folded to provide the center pages of the booklet will be trimmed by the greatest amount. Because sheets are individually trimmed prior to final assembly, random misalignment of sheets would result in a ragged, unfinished appearance to the booklet. The random skew that is considered to be allowable will vary with the expectations of the manufacturer, but is often a maximum skew that is in the range of one sheet thickness (e.g., ±100 microns) to two sheet thicknesses (e.g., 200 microns).  
           [0003]    Buckle deskew mechanisms are used in commercially available printers in which the acceptable skew is much greater. Such buckle deskew mechanisms are not capable of the accuracy required for booklet making. What is needed is a deskew mechanism that is suitable for use in applications in which cost is a significant concern, such as in desktop applications, and/or applications in which precise alignment is a significant concern, such as in sheet-wise booklet making.  
         SUMMARY OF THE INVENTION  
         [0004]    The degree of skew of print media progressing along a feedpath to a nip region between deskew rollers is precisely controlled by intentionally introducing print media deformation, with the major directional component of the deformation being perpendicular to the plane defined by the feedpath immediately upstream from the nip region. In one embodiment, mechanical and gravitational forces cooperate to provide the skew control. The print media is driven into the nip region of the deskew rollers in a manner that causes the print media to deform, lifting a movable upper member in a direction substantially perpendicular to the feedpath. However, the upper member applies a return force that is generally equal along a widthwise surface region of the print media, thereby reducing the likelihood that the print media is skewed relative to the nip region. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a perspective view of a printer, which is one example of a device that may use the deskew mechanism in accordance with the invention.  
         [0006]    [0006]FIG. 2 is a diagrammatic cross-sectional view in the vertical plane of one embodiment of a deskew mechanism in accordance with the invention.  
         [0007]    [0007]FIG. 3 is a side view of the deskew mechanism in a rest condition.  
         [0008]    [0008]FIG. 4 is a side view of the deskew mechanism shown in a condition in which print media is being fed along a feedpath.  
         [0009]    [0009]FIG. 5 is a side view of the coupling of a device which utilizes the deskew mechanism with other devices in which print media is manipulated.  
         [0010]    [0010]FIG. 6 is a block diagram of the integration of the deskew mechanism into a device in which print media is manipulated, such as in the printer of FIG. 1.  
         [0011]    [0011]FIGS. 7, 8 and  9  illustrate the use of the deskew mechanism in the process steps of sheet-wise booklet finishing.  
         [0012]    [0012]FIG. 10 shows a graph of the skew measured for paper sheets processed by the deskew mechanism of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    With reference to FIG. 1, a printer  10  is illustrated as merely one example of a device which may be adapted to include a deskew mechanism in accordance with the invention. Other devices may be similarly adapted. As used herein, “print media” refers to all types of paper, photographic paper, transparencies and other media used in devices such as printers and desktop publishing systems.  
         [0014]    The printer  10  includes a body  12  and a hinged cover  14 . Inkjet technology is employed, but this not critical to the invention. An inkjet printhead  16  is attached to a carriage  20  that moves back and forth along a carriage transport rail  22 . A flexible cable  24  connects the components of the print carriage to a print engine, not shown. The flexible cable includes electrical power lines, clocking lines, control lines, and data lines. Nozzles of the inkjet printhead are individually triggered to project droplets of ink onto print media delivered from a media supply  18 . During each print operation, the print media is stepped in one direction, while the inkjet printhead  16  is moved along the transport rail  22  in the perpendicular direction.  
         [0015]    [0015]FIG. 2 illustrates an embodiment of a deskew mechanism  100  for use in the printer  10 , as one example. The deskew mechanism includes a print media feedpath  110 , a deskew drive roller  122 , a deskew pinch roller  124 , a guide structure  130 , a fixed lower member  132 , a movable upper member  134 , a weight member  136 , a hinge  138 , a center of curvature  139 , aperture plates  140 , a fixed narrow aperture  141 , a light source  152 , a light detector  154 , and feed rollers  160 . In preparation for a print operation, print media is moved along the print media feedpath  110  through the deskew mechanism  100 . While other embodiments are contemplated, the feedpath is substantially horizontal.  
         [0016]    The print media is fed into the deskew mechanism  100  by the feed rollers  160 . The length of the feedpath between the feed rollers and the deskew rollers  122  and  124  should be less than the length of a sheet of the print media. As a result, the print media will deform while traveling along the feedpath, if the rotations of the rollers are properly coordinated. The leading edge of the sheet of print media enters the deskew rollers before the trailing edge of the sheet leaves the feed rollers. While not shown in FIG. 1, the feed rollers are typically multiple rollers that are spaced across the width of the feedpath, but are driven by a single drive shaft. As is well known in the art, the deskew rollers  122  and  124  are similarly configured.  
         [0017]    The deskew drive roller  122  (or the series of spaced apart deskew drive rollers on a single shaft) are made of a hard material in order to minimize slipping of the print media while the print media is moving through, or being held by, the deskew rollers  122  and  124 . Typically, deskew drive rollers are formed of a compliant material. Hard rollers and grit rollers potentially provide increased advantages when used as the deskew drive roller or series of drive rollers. Grit rollers significantly reduce slippage. Grit rollers are known in the art for their use in pen plotters.  
         [0018]    The guide structure  130  is located along the feedpath  110  between the feed rollers  160  and the deskew rollers  122  and  124 . The guide structure includes the fixed lower member  132 , the movable upper member  134 , the weight member  136  and the hinge  138 . The weight member  136  is positioned atop the movable upper member  134 . The weight member may be adjustable in order to accommodate different types of print media. For example, the optimal gravitational force that is exerted on a deformed transparency may be significantly greater than the optimal gravitational force on a deformed piece of paper. The weight member may be manually replaced depending upon the type of print media or the adjustment may be automatically triggered by an optical or user-input identification of the media type.  
         [0019]    As shown in FIG. 2, the axis of the hinge  138  is aligned perpendicular to the feed direction and is in a plane parallel to the feedpath. In the embodiment of FIG. 2, the fixed lower member  132  and the movable upper member  134  are curved and are conformal. The center of curvature  139  is represented below the fixed lower member  132 . The advantages of the conformality and curvature will be described below, while referring to FIGS. 3 and 4. The movable upper member  134  may be a rigid structure. In FIG. 2, the movable upper member is a wire frame.  
         [0020]    Aperture plates  140  are situated on the feedpath  110  between the guide structure  130  and the deskew rollers  122  and  124 . The aperture plates are parallel to the feedpath, defining a fixed narrow aperture  141  for guiding the sheets of print media into the deskew rollers. The narrow aperture  141  is intended to restrict the print media deformation to an area spaced apart from the nip region  164  between the deskew rollers. In embodiments in which it is desired to measure the skew of print media, if any, or to detect the position of the leading and trailing edges of the sheets of print media, optical sensors are incorporated into the deskew mechanism  100 . The light source  152  and the light detector  154  are positioned downstream of the feedpath  110  from the deskew rollers  122  and  124  for this purpose. As the print media passes between the light source and the detector, a light beam from the light source is interrupted, so that the edge of the sheet is detected. In some applications, two or more sensors may be required should it be desired to measure the skew of print media. The light source  152  may be a light emitting diode (LED) and the light detector  154  may be a photodiode. The components of the deskew mechanism  100  are attached to a rigid frame, such as the body  12  of the printer  10  of FIG. 1.  
         [0021]    In FIG. 3, the guide structure  130  is shown in its rest condition in which there is no print media traveling along the feedpath  110 . As a result, the movable upper member  134  rests on the fixed lower member  132  at the downstream end of the guide structure. On the other hand, FIG. 4 shows the guide structure in a media-feed condition. In this condition, the relative operations of the feed rollers  160  and the deskew rollers  122  and  124  induce a deformation of a sheet  210  of print media. For example, the deskew rollers may be stalled temporarily while the feed rollers progress the sheet  210  through the feedpath to the nip region of the deskew rollers. It should be noted that the feeding of the sheet results in a single buckle  215 , which lifts the movable upper member  134 . The movement of the upper member is about the axis of the hinge  138 . Therefore, with the spacing between the lower and upper members  132  and  134  remaining generally constant and with the spacing between the aperture plates  140  remaining constant, the buckle  215  of the sheet  210  has a controlled position that is near the nip region without adversely affecting the entrance of the sheet into the nip region.  
         [0022]    Arrow  230  in FIG. 4 represents the force that is applied to the sheet  210  at the buckle region  215 . The gravitational force applied by the movable upper member  134  will result in a larger force driving the sheet into the nip of the deskew rollers  122  and  124 , as compared to the force of the feed rollers  160  alone. Moreover, the force will be generally equal across the width of the sheet. In one embodiment, the sheets are driven into the nip region of the deskew rollers such that only a single buckle is formed in the sheet. In accordance with this embodiment, the guide aperture  220 , which is formed by lifting the movable upper member  134 , is kept relatively narrow. Moreover, the lower and upper members  132  and  134  can be substantially conformal, allowing for a narrow and uniform guide aperture. As can be seen in FIGS. 3 and 4, the upper and lower members are arcuate, so as to encourage the formation of a single buckle  215  in the sheet  210  of print media. In one embodiment, the arcuate members have relatively gentle curvature. The aperture plates  140  guide the sheet into the nip of the deskew rollers  122  and  124  without any further buckling.  
         [0023]    Referring now to FIGS. 2, 3 and  4 , the force applied by the movable upper member  134  on the buckle  215  in the sheet  210  of print media is determined by the mass of the movable upper member, the mass of the weight member  136 , if used, and the spring constant if the hinge  138  is spring biased to the rest condition shown in FIG. 3. The deskew mechanism  100  can be optimized for the alignment of a particular type of print media by the choice of the mass of the weight member or the choice of the spring constant of the spring at the hinge. Thus, the mass of the weight member may be changed and the spring constant of the spring may be adjusted in order to accommodate different types of print media. The adjustments may be manual, but may be automatically imposed upon the selection of a print media during the selection of preferences for a particular print operation.  
         [0024]    In the testing of a deskew mechanism  100  in accordance with the invention, the input of the mechanism was connected to a paper tray. Sheets of paper were aligned in the deskew mechanism and the skew was measured using optical sensors. The results for thirty sheets of paper are shown in FIG. 10. The mean skew was-0.8 microns (-0.03 mils) and the standard deviation was 70 microns (2.75 mils). This data shows that the deskew mechanism of the invention is capable of aligning a succession of sheets of print media to within a tolerance substantially less than ±100 microns (±4 mils), which is often used as the standard in booklet making.  
         [0025]    [0025]FIG. 5 illustrates the integration of the deskew mechanism  100  with other stand-alone devices which utilize sheet feedpaths  110 . The print media feedpath is shown as starting in a first device  310 , passing through the deskew mechanism  100 , where the sheets of print media are aligned, and finishing in a second device  320 . Examples of the first and second print media devices include (1) a printer and a sheet-wise booklet finisher that individually trims sheets depending upon their position within a booklet, (2) a paper tray loaded with pre-printed print media and a sheet-wise booklet finisher, and (3) a paper tray and a full bleed printer. Examples (1) and (2) are different embodiments of sheet-wise booklet makers. In FIG. 4, the deskew mechanism is shown as including the feed rollers  160 . However, in some applications it may be preferred to instead use the mechanism that is the sheet exit mechanism for the first print media device  310 . In such applications, the first device  310  supplies the sheets to the guide structure of the deskew mechanism  100 , so that the feed rollers  160  are no longer necessary within the deskew mechanism. Feed rollers, sheet exit mechanisms and any other mechanism that supplies the sheets to the guide structure of the deskew mechanism  100  are referred to as feed mechanisms.  
         [0026]    In FIG. 6, the deskew mechanism is shown as being incorporated into a print media device  330  having a feedpath  110 . By incorporating the deskew mechanism into the print media device, the sheets of print media may be accurately aligned. Possible examples of the third media device include the printer  10  of FIG. 1, sheet-wise booklet finishers, and full bleed printers.  
         [0027]    Perhaps the application with the lowest skew tolerance is sheet-wise booklet making, since finishing operations are often performed on a sheet-by-sheet basis. For booklets that are formed by folding each sheet at its center and stapling the folded sheets together, sheets at the center of the booklet should be shorter than those that are away from the center. Thus, the sheet trimming is carried out as a function of the size of the booklet, the thickness of the individual sheets, and the positions of the individual sheets within the booklet. The deskew mechanism of the present invention can be integrated with a sheet-wise booklet finishing apparatus to enable the aligning, trimming to length, scoring, and folding steps of the process, which are carried out on sheets of print media individually; the final step of stacking and stapling the sheets in a booklet. Shown in FIGS. 7, 8 and  9  are the deskew rollers  122  and  124 , the guide structure  130 , the fixed lower member  132 , the movable upper member  134 , the weight member  136 , the hinge  138 , the feed rollers  160 , the print media sheet  210 , a sheet trimming  211 , a trimmed sheet  212 , a folded sheet  214 , the buckle  215 , a trimming station  410 , a scoring and folding station  420 , and a stacking and stapling station  430 . In FIG. 7, the deskew mechanism is used to align the sheet  210 . The sheet is fed part way through the deskew rollers and then held in position while the trimming station  410  trims the trailing edge of the sheet, as represented in FIG. 8; the sheet trimming  211  is discarded. In FIG. 9, the trimmed sheet  212  of FIG. 8 is again held in position by the deskew rollers while the scoring and folding station  420  scores and folds the sheet. Finally, the folded sheet  214  is fed through the deskew rollers and progressed to the stacking and stapling station  430 . In one embodiment, aperture plates for guiding the sheet of print media into the nip of the deskew rollers are integrated with the trimming station, which is not represented in the figures.  
         [0028]    While the deskew mechanism in accordance with the invention has been described and illustrated as having upper and lower members, so that gravitational and mechanical forces cooperate to provide the deskew capability, there may be applications in which the invention uses spring force or other biases to localize the print media deformation directly adjacent to the entrance of the print media into the nip region. On the basis of the described deskew mechanism, the buckle is both proximate to and perpendicular to the nip region. However, aperture plates or similar structures may be used to ensure that the neighboring buckle is isolated from the ability of the deskew rollers to properly channel the leading edge of the print media.  
         [0029]    Another possible modification to the deskew approach described with reference to FIGS. 3 and 4 relates to the technique for inducing the buckle  215  in the sheet  210 . Rather than stalling the paper travel at the deskew rollers  122  and  124 , the buckle may be induced by reverse driving. For example, the sheet of print media may be fed partially through the deskew rollers, whereafter stalling of the feed rollers  160  and reversal of the deskew rollers will cause the buckling to occur. The reversal of the deskew rollers should continue until the leading edge of the sheet resides within the nip of the deskew rollers. Subsequently, all operations will be identical to those that were previously described.