Patent Publication Number: US-7219888-B2

Title: Multi-pass deskew method and apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of application Ser. No. 10/298,500 filed on Nov. 18, 2002 now U.S. Pat. No. 6,834,853, which is hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     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 
     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. sheet-wise 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 aligning, trimming, scoring, folding, and stacking and stapling, as illustrated in  FIG. 1 . Each sheet is trimmed 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 total skew that is in the range of one sheet thickness (e.g., ˜100 microns) to two sheet thicknesses (e.g., ˜200 microns). For comparison, the typical acceptable skew for a printer is +/−1500 microns. 
     The skew of print media can be reduced by using buckle deskew methods or methods utilizing differentially driven nips. Some deskew mechanisms utilize multiple print media sensors in implementing print media deskew. 
     U.S. Pat. No. 6,374,075 to Benedict et al. teaches a method for correcting the skew of print media on a feedpath utilizing one or more pairs of differentially driven nips. The operating speeds of the individual nips are determined from data provided by print media sensors positioned along the edge of the feedpath. These sensors include point sensors and CCD arrays. The differentially driven nips re-orient the print media as it is fed along the feedpath. 
     U.S. Pat. No. 5,794,176 to Milillo also teaches a method for deskewing print media on a feedpath utilizing a pair of differentially driven nips. The operating speeds of the individual nips are determined from data provided by two print media sensors positioned immediately downstream on the feedpath from the nips and on an axis which is perpendicular to the feed direction of the feedpath. These sensors are positioned to detect the leading edge of the print media, with the time delay between detections of the edge by the two sensors being used to generate control signals for motors driving the individual nips. 
     U.S. Pat. No. 5,678,159 to Williams et al teaches a method for correcting the skew of print media on a feedpath which utilizes data from print media leading edge sensors positioned along the center of the feedpath and print media edge sensors positioned along the edge of the feedpath. This data is used to determine the operating speed of a pair of differentially driven nips which re-orient the print media as it is fed along the feedpath. 
     U.S. Pat. No. 5,466,079 to Quintana teaches a buckle deskew method which utilizes an optical interrupt sensor for print media leading edge detection. Print media is delivered from feed rollers and is passed through deskew rollers until the leading edge is detected. The print media is then reversed out of the deskew rollers, while being held by the feed rollers, until the leading edge is free to align in the nip of the deskew rollers. The alignment is assisted by a buckle which forms in the print media. Finally, the deskewed print media is again fed through the deskew rollers and along the feedpath. The sensor is mounted so that it can be shuttled across the feedpath to also detect a side edge of the print media. Detection of the leading and side edges allows the orientation of the print media to be determined. 
     Japanese Patent Abstract No. 57175643 teaches a buckle deskew method in which a buckle is formed in print media as it is fed into stalled deskew rollers, thus aligning the leading edge of the print media square to the nip of the deskew rollers. The deskew rollers are then activated, feeding the now deskewed print media along a feedpath. 
     These methods and apparatus are used in printing and copying applications in which the acceptable skew is much greater than for sheet-wise booklet making. What is needed is a deskew method and apparatus, that is suitable for use in applications in which precise alignment is a significant concern, such as sheet-wise booklet making. Furthermore, a deskew method and apparatus is needed which can be used with desktop printing and booklet making systems in which cost is a significant concern. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for aligning print media on a feedpath. A sheet of print media is processed through an alignment mechanism and the alignment is then measured. If the measured alignment is not satisfactory, the sheet is reversed through the alignment mechanism, allowing the process to be repeated. These steps are repeated until the measured alignment is satisfactory. More specifically, the method includes the steps of: (1) aligning a sheet of print media using a deskew mechanism; (2) measuring the skew of the sheet after it has been aligned; and (3) comparing the measured skew with a specified skew. If the measured value is greater than the specified value, the sheet is reversed through the deskew mechanism to allow a repetition of the aligning, measuring and comparing steps. 
     The invention provides an apparatus for implementing alignment of print media on a feedpath. The apparatus comprises a deskew mechanism and sensors positioned downstream on the feedpath from the deskew mechanism. The sensors are configured to detect a leading edge of a sheet of print media on the feedpath and are cooperative with a controller to return a sheet to the deskew mechanism upon determining that the sheet has not been adequately deskewed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates known process steps of sheet-wise booklet making. 
         FIG. 2  is a perspective view of a printer, which is one example of a device that may use the deskew apparatus in accordance with the invention. 
         FIG. 3  is a top plan view of one embodiment of a deskew apparatus in accordance with the invention. 
         FIG. 4  is a cross-section through the deskew apparatus of  FIG. 3 , in the vertical plane containing  4 — 4 . 
         FIG. 5  is a flow chart for one embodiment of a sheet alignment method in accordance with the invention. 
         FIG. 6  is a side view of the deskew apparatus in a condition in which print media is being fed along a feedpath into a nip of stalled deskew rollers. 
         FIG. 7  is a side view of the deskew apparatus in a condition in which the skew of the print media is being measured. 
         FIG. 8  is a side view of the deskew apparatus in a condition in which the print media is being reversed through the deskew rollers. 
         FIG. 9  is a block diagram of the coupling of the deskew apparatus with other devices in which print media is manipulated. 
         FIG. 10  is a block diagram of the integration of the deskew apparatus into a device in which print media is manipulated, such as in the printer of  FIG. 2 . 
         FIG. 11  is a diagrammatic illustration of the integration of the deskew apparatus into a sheet-wise booklet finisher. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 2 , a printer  200  is illustrated as merely one example of a device which may be adapted to include a deskew apparatus 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. 
     The printer  200  includes a body  212  and a hinged cover  214 . Inkjet technology is employed, but other technologies may be used. An ink-jet printhead  216  is attached to a carriage  220  that moves back and forth along a carriage transport rail  222 . A flexible cable  224  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  218 . During each print operation, the print media is stepped in one direction, while the inkjet printhead  216  is moved along the transport rail  222  in the perpendicular direction. 
       FIGS. 3 and 4  show an embodiment of a deskew apparatus  300  located along a print media feedpath  370  for use in the printer  200 , as one example. The deskew apparatus includes feed roller axes  320 , feed rollers  322 , deskew roller axes  330 , deskew rollers  332  and  334 , a sensor alignment axis  340 , sensor components  342 ,  344 ,  346 ,  347  and  348 , a second sensor alignment axis  350 , second sensor components  352 ,  354 ,  356 ,  357  and  358  and a guide structure  360 . As will be described more fully below, sensor components  342 ,  346 ,  347  and  348  may be light emitters, while sensor component  344  may be a light detector in an array of detectors that corresponds with the array of light detectors. Thus, four sensors are available. Alternatively, the emitter/detector pairing may be reversed. This applies to the second sensor components as well. A sheet of print media  310  is shown being fed, by feed rollers  322 , in direction  315  through the deskew apparatus along feedpath  370 . A drive motor  378  is controlled to operate the feed rollers. While other embodiments are contemplated, the feedpath is substantially horizontal. The length of the feedpath between the feed rollers and the deskew rollers is less than the length of the sheet. While two feed rollers  322  are shown on each feed roller axis  320 , the number may be more or less. Similarly, there may be more or less than three deskew rollers  332  and  334  on each deskew roller axis  330 . Hereafter, a deskew apparatus is defined to comprise a deskew mechanism for aligning print media and sensors for measuring the alignment of the print media. 
     As shown in  FIG. 4 , the deskew rollers  332  and  334  are configured to form a nip in which print media are received from an upstream side of the feedpath. The deskew rollers are pinch rollers  334  and drive rollers  332  that are rotated by a reversible motor  380  of  FIG. 3 . In one embodiment, the drive rollers  332  are made of a hard material in order to reduce slipping of the print media while the print media are moving through, or being held by, the deskew rollers. Typically, drive rollers are formed of a compliant material. In another embodiment, the drive rollers are grit rollers in order to minimize slippage. Grit rollers are known in the art for their use in pen plotters. 
     The guide structure  360  guides print media into the nip of the deskew rollers. The guide structure is rigid, and in one embodiment is a wire frame. In certain embodiments, the guide structure is arcuate. This generally curved shape assists in the formation of a buckle in a sheet of print media, as discussed below. In other embodiments, the guide structure has upper and lower members (not shown), positioned above and below the feedpath. 
     In  FIG. 3 , the sensor components  342 ,  346 ,  347  and  348  are shown positioned along axis  340  as components of four sensors. Axis  340  is substantially perpendicular to direction  315  which is the direction of movement of print media along the feedpath and is also substantially parallel to the deskew roller axes  330 . A small misalignment of the axis  340  relative to the direction  315  and axes  330  can be compensated by an offset value for each sensor. The sensors provide data to a controller  382  to detect the leading edge of a sheet of print media as it emerges from the deskew rollers. The leading edge is detected by at least two sensors in order to provide data to calculate the skew of the sheet using the processing capability of the controller. Calculation of the skew is more accurate if data is available from two sensors which are spaced apart by approximately the width of the sheet. In some embodiments only two sensors are used. In order to accommodate print media of different widths, while having sensors spaced by a distance approaching that of the different widths of sheets, a plurality of sensors are used in spaced relationship along the axis  340 . It is well known in the printer art for print media to be aligned to one side of the feedpath, irrespective of the print media size. The embodiment shown in  FIG. 3  has sensors configured for print media aligned to the left hand side of the feedpath, such that sensor component  342  is positioned near the left hand edge for all different print media. Sensor components  346 ,  347  and  348  are spaced apart such that one of them will be close to the right hand edge for a selection of widths of print media. For example, the embodiment of  FIG. 3  could have the sensor components  346 ,  347  and  348  spaced so as to accommodate US letter (8.5 inch×11 inch), A4 (210 mm×297 mm), and US executive (7.25 inch×10.5 inch) size print media. Other embodiments may utilize a larger number of sensors or a different configuration. For example, an alternative embodiment in which print media is aligned to the left hand side of the feedpath has one fixed sensor, on the left hand side, and one movable sensor that is moved to be close to the right hand side edge of the print media, thus accommodating different print media widths. The movable sensor can be a sensor mounted on a carriage which moves along a carriage transport rail aligned to the sensor axis. Such carriages are well known in the art and are used in printers, such as the printer of  FIG. 2 . In  FIG. 2 , a sensor  226  is shown as being mounted for movement as the carriage  220  moves along the carriage transport rail  222 . 
     In  FIG. 3 , the second sensor components  352 ,  356 ,  357  and  358  are shown positioned along axis  350  to define positions of second sensors. Axis  350  is substantially perpendicular to direction  315  and substantially parallel to the deskew roller axes  330 . The second sensors provide data to the controller  382  to detect the trailing edge of a sheet of print media before it enters the deskew rollers. The same considerations apply to the trailing edge sensors as to the leading edge sensors regarding measurement of skew and accommodation of print media of different widths. Measurement of sheet alignment by both leading edge and trailing edge sensors provides data to the controller in order to determine the parallelism of these two edges. One or more of the second sensors can be used to detect a trailing edge of a sheet of print media. When combined with leading edge detection by the leading edge sensors, this provides the data to calculate the length of the sheet. Both the length of the sheet and the parallelism of the leading and trailing edges are useful measurements for applications such as sheet-wise booklet making. 
     The sensors and second sensors are typically optical sensors which are configured to detect edges of print media. In some embodiments the sensors are optical interrupt sensors, having a light emitting member and a light detecting member positioned facing each other on opposite sides of the feedpath. For example, referring to  FIG. 4 , a light emitting member  346 , a beam of light  345 , and a light detecting member  344  are shown. When a sheet of print media passes between the emitter and detector, the sheet interrupts the light beam and hence its edge is detected. In other embodiments, not shown, the emitter and detector may both be positioned above the feedpath, such that a sheet of print media is detected when light from the emitter is either reflected or scattered back to the detector. Examples of light emitting members are a light emitting diode (LED) and a laser. An example of a detecting member is a photodiode. 
       FIG. 5  is a flow chart for a general embodiment of a method for aligning print media along a feedpath in accordance with the invention. The first step, aligning step  410 , uses a deskew mechanism to align a sheet of print media to the feedpath. In  FIG. 3 , the drive motor  378  and the reversible motor  380  rotate rollers  322  and  332 , respectively, to progress the sheet  310  along the feedpath. In the second step, measuring step  420 , the skew of the sheet is measured at the output of the deskew mechanism. Skew may be measured using the sensors and the controller  382 . In the third step, represented by  430  and  440 , the measured skew value calculated by the controller for the sheet is compared with a specified skew, and if the measured skew is greater than the specified skew, then the sheet is reversed (by operation of the motor  380 ) through the deskew mechanism, and steps  410 ,  420  and  430  are repeated. When the measured skew is found to be less than or equal to the specified skew, the sheet continues along the feedpath, as represented by  450 . 
     Some embodiments of the method limit the number of times that the sheet is reversed through the deskew mechanism in order to attempt to attain a desired alignment of the sheet. For example, after ten passes through the deskew mechanism the sheet is allowed to continue along the feedpath even though a desired alignment has not been attained. Alternatively, the sheet is rejected on failing to attain the desired alignment after ten passes. The maximum number of passes can be set depending on throughput requirements. The controller  382  of  FIG. 3  can be programmed to incorporate this limit in the method. 
     A specific embodiment of the alignment method will be described with reference to  FIGS. 6 ,  7  and  8 .  FIG. 6  shows a sheet of print media  310  being driven by feed rollers  322  into the nip formed by deskew rollers  332  and  334 . The deskew rollers are stalled. Consequently, a buckle  312  forms as the sheet continues to be driven into the nip. The buckle assists in driving the sheet into the nip, encouraging the leading edge of the sheet to align squarely in the nip. Sensor components  344  and  346  form an optical interrupt sensor, where sensor component  346  emits a beam of light  345  which is detected by sensor component  344 . Next, the deskew rollers are activated in the forward direction, feeding the leading edge of the sheet through the deskew rollers. Activation of the deskew rollers can be based on detection of the buckle  312  (sensors not shown), or the leading edge of the sheet can be detected by the second sensors (see  FIG. 4 ) as the sheet approaches the deskew rollers, triggering activation of the deskew rollers after a suitable time delay. 
       FIG. 7  shows the sheet  310  being fed by both the feed rollers  322  and the deskew rollers  332  and  334 . The leading edge of the sheet is shown as having just interrupted the light beam  345 , and consequently will have been detected. Detection of the sheet at several points (at least two) along its leading edge allows the skew of the sheet to be calculated. Should the skew of the sheet be unsatisfactory, as determined by comparing the measured skew with a specified skew, then the sheet is reversed through the deskew rollers, as shown in  FIG. 8 . The sheet is shown as being fed in the forward direction by the feed rollers  322 , even when the leading section of the sheet is being reversed. The combination of the reversing of the sheet through the deskew rollers and the feeding forward through the feed rollers gives rise to a buckle  312  in the sheet. The sheet is reversed until the leading edge is free of the rollers and is sitting in the nip, which is the approximate configuration shown in  FIG. 6  (the only difference being that more of the sheet is now situated between the feed rollers and the deskew rollers, giving rise to a larger buckle). The alignment method is then repeated. 
     In  FIGS. 6 ,  7  and  8 , the feed rollers are shown to be feeding the sheet forward at all steps. In some embodiments, the feed rollers are operated continuously at the same rate. This mode of operation will limit the number of times the alignment steps can be repeated, since once the sheet is completely fed through the feed rollers, it can no longer be aligned using the method of the invention. Clearly, the faster the operation of the deskew rollers and the sensors, the larger the number of repeats of the alignment steps that can be accommodated. In other embodiments, the feed rollers are stalled after the first alignment step, and are only reactivated in the forward direction when an acceptable skew is measured. This allows for as many repeats of the alignment steps as an application can tolerate. 
     As discussed previously, the sheet shown in  FIG. 8  is reversed through the deskew rollers  332  and  334  until the leading edge is free of the rollers and is sitting in the nip. In embodiments in which the drive roller  332  is a grit roller, there is the possibility that the leading edge of the sheet may be caught on a grit particle and therefore not be free to align in the nip. To overcome this problem, the deskew rollers can be vibrated or “buzzed” after the sheet has been reversed through the deskew rollers. Another solution to this problem is to continue to operate the deskew rollers in reverse for some time beyond the time at which the leading edge of the sheet reaches the nip of the rollers. Furthermore, the rollers can be rotated in reverse until the leading edge of the sheet, when sitting in the nip, is in contact with a different part of the rollers for each time the alignment is repeated. This procedure is effective in mitigating the effect a large grit particle can have on the alignment process (an undesirable skew can result from a large grit particle being positioned in the nip of the drive roller during the alignment of a sheet). 
     Using either an encoder in conjunction with the reversible motor  380  or a reversible motor which is a stepper motor will facilitate the methods described above. These configurations of the reversible motor allow the length of sheet that has been fed through the deskew rollers, in either direction, to be monitored. 
     In the testing of an alignment method in accordance with the invention, an alignment apparatus as in  FIG. 3  was used. The apparatus was connected to a paper tray at its input. Two leading edge optical interrupt sensors were used, positioned downstream from the deskew rollers and approximately 1 cm from each side edge of the sheet. The deskew drive roller axis was driven by a servo motor with an encoder, calibrated to give the length of the sheet fed through the deskew rollers. When the leading edge sensors detected the sheet, the encoder reading was captured. Subtraction of the encoder readings from the two sensors (including an offset due to a small misalignment of the axis of the sensors relative to the axes of the deskew rollers) gave a length measurement that was representative of the skew of the sheet. Table 1 has the skew data collected for ten sheets. The specified acceptable skew value was +/−50 microns and the number of times the alignment steps had to be repeated is given by the “Try Number.” This data shows that the alignment method of the invention is capable of aligning a succession of sheets of print media to within a tolerance of less than +/−100 microns, which is often used as the standard in sheet-wise booklet making. For comparison, the typical acceptable skew for a printer is +/−1500 microns. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Performance Data for An Embodiment of 
               
               
                 the Alignment Method of the Invention 
               
            
           
           
               
               
               
            
               
                 Sheet Number 
                 Try Number 
                 Skew Value/microns 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 1 
                 −100 
               
               
                   
                 2 
                 −430 
               
               
                   
                 3 
                 −1015 
               
               
                   
                 4 
                 25 
               
               
                 2 
                 1 
                 −25 
               
               
                 3 
                 1 
                 −125 
               
               
                   
                 2 
                 −355 
               
               
                   
                 3 
                 −230 
               
               
                   
                 4 
                 −75 
               
               
                   
                 5 
                 −150 
               
               
                   
                 6 
                 −25 
               
               
                 4 
                 1 
                 −50 
               
               
                 5 
                 1 
                 0 
               
               
                 6 
                 1 
                 0 
               
               
                 7 
                 1 
                 −100 
               
               
                   
                 2 
                 −405 
               
               
                   
                 3 
                 −785 
               
               
                   
                 4 
                 965 
               
               
                   
                 5 
                 815 
               
               
                   
                 6 
                 50 
               
               
                 8 
                 1 
                 −50 
               
               
                 9 
                 1 
                 −150 
               
               
                   
                 2 
                 −405 
               
               
                   
                 3 
                 −125 
               
               
                   
                 4 
                 −125 
               
               
                   
                 5 
                 0 
               
               
                 10 
                 1 
                 180 
               
               
                   
                 2 
                 75 
               
               
                   
                 3 
                 −455 
               
               
                   
                 4 
                 −50 
               
               
                   
               
            
           
         
       
     
       FIG. 9  illustrates the integration of the deskew mechanism  520  and plurality of sensors  530  with other stand-alone print media devices which utilize a print media feedpath  370 . The print media feedpath is shown as starting in a first print media device  510 , passing through the deskew mechanism and plurality of sensors, where sheets of the print media are aligned, and finishing in a second print media device  540 . 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  FIGS. 3 and 4 , the deskew mechanism is shown as including the feed rollers  322 . 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  510 . In such applications, the first device  510  supplies the sheets to the deskew mechanism  520  in such a way that the feed rollers  322  are no longer necessary within the deskew mechanism; in which case the first device  510  is comprised of a feed mechanism coupled to a supply of sheets of print media. 
     When an alignment apparatus is integrated with a printer and a sheet-wise booklet finisher, forming a sheet-wise booklet maker, communication between the component devices may be desirable. For example, if numerous attempts are required to successfully align a particular sheet, then a signal can be sent from the alignment apparatus to the printer to delay the printing of the next sheet. Furthermore, if it is determined that a particular sheet cannot be aligned to specification, then this sheet can be rejected and a signal can be sent to the printer to produce a replacement sheet. 
     In  FIG. 10 , the deskew mechanism  520  and plurality of sensors  530  are shown as being incorporated into a print media device  550  having a feedpath  370 . 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  200  of  FIG. 2 , sheet-wise booklet finishers, and full bleed printers. 
     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 apparatus of the present invention can be integrated with a booklet making apparatus to enable the aligning, trimming to length, scoring, and folding steps of the process. The process flow  100  in  FIG. 1  and the apparatus of  FIG. 11  are both referred to in the description that follows. Shown in  FIG. 11  are the feed rollers  322 , deskew rollers  332  and  334 , the print media feedpath  370 , a trimming station  610 , a plurality of sensors  640 , a scoring and folding station  620  and a stacking and stapling station  630 . The feed rollers, the deskew rollers and the plurality of sensors are used to feed and align a sheet moving along the feedpath, step  110 . Next, the sheet is fed part way through the deskew rollers and held in position while the trimming station trims the sheet to length, step  120 . The trimming is discarded of at step  150 . The trimmed sheet is fed further through the deskew rollers to the scoring and folding station, where it is scored and folded, steps  130  and  140 , again while being held within the deskew rollers. Finally, the folded sheet is fed completely through the deskew rollers and progressed to the stacking and stapling station, where it is stacked with other finished sheets and then stapled to make a booklet, step  160 . 
     A possible modification to the alignment method described with reference to  FIG. 6  relates to the technique for inducing the buckle  312  in the sheet  310 . Rather than stalling the deskew rollers  332  and  334 , 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  322  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. 
     Other possible modifications of the invention relate to the deskew rollers and the sensors. Non-optical sensing members may be substituted. Similarly, deskew members other than rollers may be used without diverging from the invention.