Abstract:
A lead edge and/or trail edge sheet curl sensing and constraint method and system. First and second light emitters and detector pairs are aligned such that the light beams from the first light emitter and second light emitter cross at the transport media sheet substrate path, which constitutes the path of a media sheet substrate with zero curl. A media sheet substrate with either positive or negative curl on the lead edge of the sheet substrate interrupts light beams from the first and second light emitters, as detected by first and second light detectors. A similar approach can be used to detect the trail edge curl. The time delay between the light beam interruptions is proportional to the sheet substrate curl, and the order of interruptions indicates whether the sheet substrate curl is positive or negative. A first pair and a second pair of substrate constraint rollers can also be provided in the paper path upstream and downstream of the sensing system. The roller pairs closest to the sensor are made of relatively non-deformable materials or of materials of similar elasticity so that different media are constrained in the sensor zone with the same sheet trajectory relative to the nip.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 11/490,692, entitled “Lead Edge Sheet Curl Sensor”, which was filed with the U.S. Patent and Trademark Office on Jul. 20, 2006 the disclosure of which is incorporated herein by reference in its entirety. 
     
     TECHNICAL FIELD 
       [0002]    Embodiments are generally related to electrophotographic and inkjet printing machines. Embodiments are also related to the field of sheet substrate curl detection sensors utilized in rending devices such as printers. Embodiments are also related to sheet constraint systems and methods. 
       BACKGROUND 
       [0003]    The curling of print media sheets is a particular problem in the printing industry, and is exacerbated by high-density images and plural color printing. Sheet curling, however, can occur even in the context of unprinted sheets of paper due to changes in ambient humidity or moisture content of the paper. Sheet curling can interfere with proper sheet feeding, causing sheet feeding jams or delays. If sheet curl is present in the output, it can interfere with proper stacking or other finishing operations. For example, if printed sheets with curl do not lie flat when stacked together in sets, such as in the pages of booklets, an objectionable distortion of the resulting booklet may result. 
         [0004]    Furthermore, the amount of moisture in the sheet of paper can drastically change from the printing process itself, to cause or exacerbate curl. In particular, from water-based ink jet printing or the thermal fusing operation for toners in xerographic printing, and particular from high density image printing near the edges of the sheet. There is a further sheet curl problem in duplex printing, where the sheets are re-fed or recirculated for printing imaging material on their second sides, especially if that involves a second pass of the sheet through a thermal fuser and/or higher density images on one side than the other. 
         [0005]    In order to control or remove the amount of curl, the print media curl must be measured. Various paper curl sensors and control apparatus are known in the electrophotographic printing arts. One example is disclosed U.S. Pat. No. 6,668,155, entitled “Lead Edge Paper Curl Sensor,” which issued to Hubble, III, et al. on Dec. 23, 2003 and is assigned to the Xerox Corporation of Stamford, Conn. U.S. Pat. No. 6,668,155, which is incorporated herein by reference in its entirety, discloses a sheet curl sensor that remotely senses sheet curl without contacting or interfering with the motion of the sheets in their normal sheet path. This sensor operates on a portion of the moving sheet at an angle thereto and perpendicular thereto, with displacement insensitive optics, in both an angular direction substantially parallel to the sheet movement direction and an angular direction substantially transverse to the sheet movement direction, with rationing of the two input signals. In such a sheet curl sensor, the variable output control signals in response to the sensed illumination are a ratio of the output control signals from the photodetector system produced by the first and second illuminators. The ratio of the output signals from the photodetector system is then proportional to the amount of the paper curl sensed. 
         [0006]    Another example of a sheet curl sensor is disclosed in U.S. Pat. No. 5,270,778, entitled “Sheet Curl Control Apparatus,” which issued to Andrew Wyer on Dec. 14, 1993 and is assigned to the Xerox Corporation of Stamford, Conn. U.S. Pat. No. 5,270,778, which is incorporated herein by reference in its entirety, discloses a sheet curl sensor comprising a radiation source, in the form of an infra-red emitter and two detectors in the form of infra-red sensors. The sensors are spaced apart adjacent a horizontal section of sheet path and are arranged whereby movement of the sheet material along the sheet path causes the infra-red light beams to be interrupted in succession by the lead edge of the sheet material. The time interval between interruptions of the infra-red light beams at the sensors is a function of the sheet curl. 
         [0007]    U.S. Pat. No. 5,751,443, entitled “Adaptive Sensor and Interface,” which issued to Borton et al on May 12, 1998 and assigned to the Xerox Corporation is an example of a precise lead edge sensing system. U.S. Pat. No. 5,751,443, which is incorporated herein by reference in its entirety, discloses a sensor which detects the presence of paper and transparencies in a sheet transporting path and includes a light source disposed near the transporting path for projecting light toward a reflector on the opposite side of the transporting path and a light detector located relative to the light source to receive light emitted by the light source and reflected b the reflector so that by such positioning the light path is interrupted by substrates passing through the transport path. The output signal of the light detector is proportional to the light received across the transport path. A control, electrically connected to the sensor, adjusts flux incident on the light detector to maintain the collector current in the linear portion of the light detector&#39;s operating range. The sensor is tilted at an angle with respect to the horizontal of a copy substrate to be able to detect transparencies. 
         [0008]    One problem encountered by prior art sheet curl sensors relates to the maximum resolution of the sensor. Expensive and complicated solutions have been used to measure to a resolution of less that 0.01 mm. The ability to properly measure and accurately control the lead edge sheet curl depends on the proper constraint of the leading edge of the sheet. A single elastomer roller is inadequate since the idler compresses the elastomer roller and the exit angle of the sheet therefore depends on the elastomer properties, the roller forces and the sheet media stiffness and weight. A constraint system is therefore required, which will not damage or mark the media sheets while ensuring that the sheets remain at a consistent height and angle. 
       BRIEF SUMMARY 
       [0009]    The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification claims, drawings, and abstract as a whole. 
         [0010]    It is, therefore, one aspect of the present invention to provide for a leading edge and trail edge sheet curl sensor with an improved constraint system. 
         [0011]    It is another aspect of the present invention to provide for an improved lead edge sheet curl sensor apparatus and method. 
         [0012]    It is another aspect of the present invention to provide for a lead edge sheet curl sensor that is robust to different paper weights and other properties. 
         [0013]    The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A lead edge sheet curl sensor and constraint method and system disclosed. In general, a first light emitter and a second light emitter can be aligned such that the fight beams from the first light emitter and second light emitter cross at the transport media sheet substrate path, which constitutes the path of a media sheet substrate with zero curl. First and second light detectors are also provided, which are aligned to receive the light beams from the first and second light emitters. A media sheet substrate with either positive or negative curl on the lead edge of the sheet substrate interrupts the light beams from first and second light emitters, as detected at first and second light detectors. Additionally, the time delay between the light beam interruptions is proportional to the sheet substrate curl and the order of interruptions indicates whether the sheet substrate curl is positive or negative. A first pair and a second pair of substrate constraint rollers are provided, such that each roller of the first and said second pair of substrate constraint rollers is disposed on opposite sides of the transport media substrate path. 
         [0014]    The method and system disclosed herein therefore can constrain the lead edge (and optionally also the trail edge) of each sheet so that it can be accurately detected by a curl height sensing system. When constraining the lead edge of the sheet, the disclosed embodiments ensure that the sheet enters the sensing zone at the same height and angle. In addition, such a method and apparatus ensures that the lead edge of the sheet does not have cross-process buckling or corrugation that could prevent the lead edge from curling up or downward as required to accurately sense the process direction curl. This can be accomplished using a dual nip system, where the nips are spaced closely together. The nip closest to the sensor can be implemented as a “hard” or relatively non-deformable nip and this hard nip should be designed to span the full width of the sheet. Alternatively, the nip closest to the sensor can be implemented as a deformable nip with both the upper and lower rollers having approximately the same hardness, thus yielding a consistent sheet ejection angle with a wide range of media. A second set of similar “constraint’ rolls can optionally be utilized to measure the tail edge curl with the same sensor. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein. 
           [0016]      FIG. 1  illustrates a lead edge sheet curl sensor apparatus, which can be implemented in accordance with a preferred embodiment; 
           [0017]      FIG. 2  illustrates a lead edge sheet curl sensor apparatus, which can be implemented in accordance with an alternative embodiment; 
           [0018]      FIG. 3  illustrates a graph of the light beams received by the detectors of the sensor of  FIG. 1  verses time, in accordance with an embodiment; 
           [0019]      FIG. 4  illustrates a plot of time verses displacement of the lead edge and trail edge of paper in the sensor disclosed, in accordance with an embodiment; 
           [0020]      FIG. 5  illustrates a flow chart depicting a method of sensing the lead edge sheet curl, which can be implemented in accordance with a preferred embodiment; 
           [0021]      FIG. 6  illustrates a schematic diagram of a lead edge and trail edge sheet curl sensor system equipped with constraint rollers, in accordance with a preferred embodiment; 
           [0022]      FIG. 7  illustrates a schematic diagram of a lead edge sheet curl sensor with the lead edge constraint, drive and belt drive rollers illustrated; 
           [0023]      FIG. 8  illustrates a vertical perspective of the lead edge constraint and drive roller; and 
           [0024]      FIG. 9  illustrates a high level flow chart of operations depicting logical operational steps of an edge constraint method, which can be implemented in accordance with a preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. 
         [0026]      FIG. 1  illustrates a lead edge sheet curl sensor  100 , which can be adapted for use in accordance with a preferred embodiment. In general, the Lead edge sheet curl sensor  100  can be implemented in the context of measuring the leading edge position of a transport media substrate in a marking engine, such as paper or transparencies in a xerographic printer. As indicated in  FIG. 1 , two light emitters  101  and  102  can be disposed above a transport media path  103 . Located below the transport media path  103  are two light detectors  104  and  105 . Light emitter  101  can be aligned such that a light beam emitted from light emitter  101  is directed toward light detector  105 . Light emitter  102  can be aligned such that an emitted light beam is directed toward light detector  104 . 
         [0027]    Light emitters  101  and  102  can be mounted such that the light beams from light emitters  101 ,  102  cross the ideal transport media path  103  at approximately an angle of 45 degrees, although, other crossing angles are possible in accordance with other embodiments. Light emitters  101  and  102  can be mounted in the lead edge paper curl sensor  100  so that the light beams emitted from the light emitters  101  and  102  cross each other at close to the ideal transport media path  103  and at an angle of approximately 90 degrees to each other, although, other crossing angles are possible. Relative to the transport media path, light emitter  101  can be mounted before light emitter  102  such that the media substrate  108  transported on the transport media path  103  passes below light emitter  101  first and passes below light emitter  102  secondly. Both light emitters  101  and  102  are positioned such that the media substrate  108  can pass through both light emitter beams as it transverses the sensor  100 . 
         [0028]    A positive curl associated with the media substrate  108  is indicated in  FIG. 1  by arrow  106 . The “positive curl”, as defined by  FIG. 1 , can constitute any curl of the leading edge of the media substrate in the positive direction towards arrow  106 . “Negative curl” is defined similarly in the negative direction and shown as arrow  107  in  FIG. 1 . Media substrate  108  is shown in  FIG. 1  with a slight positive curl for illustrative purposes only. 
         [0029]    The lead edge sheet curl sensor  100  operates by measuring any time difference between an interruption of the light beams from light emitters  101  and  102  as detected at the detectors  104  and  105 . Media substrate  108  traveling on the ideal transport path  103  with zero curl of the leading edge will pass through the intersection of the beams from light emitters  101  and  102 , interrupting the light beams sensed at the detectors  104  and  105  simultaneously. Media substrate  108  with a positive curl of the leading edge towards light emitters  101  and  102  will interrupt the light beam from light emitter  101  as sensed at detector  105  before the light beam from light emitter  102  is sensed at detector  104 . Similarly, media substrate  108  with a negative leading edge curl away from the emitters  101  and  102  will interrupt the light beam from light emitter  102  as sensed at detector  104  before the light beam from light emitter  101  is sensed at detector  105 . The amount of time elapsed between the two interruptions of the light beams as sensed at detectors  104  and  105  is generally the measure of media substrate  108  leading edge curl. The direction of the media substrate curl, either positive or negative, is indicated by order of the interruption of the light beams as detected at light detectors  104  and  105 . 
         [0030]    The output signals from the detectors  104  and  105  can be processed utilizing a microprocessor such as that disclosed in U.S. Pat. No. 5,751,443 to Borton et al. The lead edge paper curl sensor  100  can utilize the known self calibration techniques of U.S. Pat. No. 5,751,443. The curl measurement resolution is a function of timer clock speed. Increased timer clock speeds will result in a higher curl measurement resolution and increased sensor sensitivity and precision. A preferred embodiment may possess a maximum sensing resolution of less than 0.01 mm. Of course, other resolution values are also possible. One parameter that must be known and controlled is the media transport velocity. The timing of the interruption of the light beams at detectors  104  and  105  is directly proportional to the transport media velocity as the media substrate  108  transverses the lead edge paper curl sensor. 
         [0031]    Alternatively, if the length of the media substrate  108  is known, the media substrate velocity may be calculated utilizing the timing of the interruption of the light beam caused by the leading edge of the media substrate  108  and the resumption of light detection at either detector  104  or  105  after the trailing edge of the media substrate passes. It can be appreciated that the techniques and devices discussed in U.S. Pat. No. 5,751,443 are referenced herein for illustrative and edification purposes only and do not constitute limiting features of the disclosed embodiments. It can be appreciated that other types of calibration techniques can be adapted for use with alternative embodiments. 
         [0032]      FIG. 2  illustrates an additional embodiment of a lead edge sheet curl sensor  200 . As indicated in  FIG. 2 , lead edge sheet curl sensor  200  comprises two light emitters  201  and  202  disposed on opposite sides of the transport media path  203 . Also disposed on opposite sides of the transport media path are two light detectors,  204  and  205 . Light emitter  201  can be aligned such that a light beam emitted from light emitter  201  is directed toward light detector  205 . Light emitter  202  can be aligned such that an emitted light beam is directed toward light detector  204 . 
         [0033]    Lead edge curl sensor  200  operates similarly to lead edge curl sensor  100 . Light emitters  201  and  202  can be mounted such that the light beams from light emitters  201 ,  202  cross the ideal transport media path  203 . As shown in  FIG. 2 , light emitters  201  and  202  are mounted in lead edge paper curl sensor  200  so that the light beams cross each other at close to the ideal transport media path  203  and at an angle of approximately 90 degrees to each other, although, other crossing angles are possible in accordance with other embodiments. Relative to the transport media path, light emitter  201  can be mounted above light emitter  202  such that the media substrate  208  transported on the transport media path  203  passes between light emitters  201  and  202 . Both light emitters  201  and  202  are positioned such that the media substrate  208  can pass through both light emitter beams as it transverses the sensor  200 . 
         [0034]    A positive curl associated with the media substrate  208  is indicated in  FIG. 2  by arrow  206 . The “positive curl”, as defined by  FIG. 2 , can constitute any curl of the leading edge of the media substrate in the positive direction towards arrow  206 . “Negative curl” is defined similarly in the negative direction and shown as arrow  207  in  FIG. 2 . Media substrate  208  is shown in  FIG. 2  with a slight positive curl for illustrative purposes only. The lead edge sheet curl sensor  200  operates by measuring any time difference between an interruption of the light beams from light emitters  201  and  202  as detected at the detectors  204  and  205 , just as in lead edge curl sensor  100 . The output signals from the detectors  204  and  205  can be processed in a microprocessor such as that disclosed in U.S. Pat. No. 5,751,443 to Borton et al. The lead edge paper curl sensor  200  can utilize the known self calibration techniques of U.S. Pat. No. 5,751,443. It can be appreciated that the techniques and devices discussed in U.S. Pat. No. 5,751,443 are referenced herein for illustrative and edification purposes only and do not constitute limiting features of the disclosed embodiments. It can be appreciated that other types of calibration techniques can be adapted for use with alternative embodiments. 
         [0035]      FIG. 3  illustrates a schematic diagram of representative output signals emitted from light detectors  104  and  105  over a particular period of time as the media substrate  108  travels at a velocity of 1 meter per second through the lead edge curl sensor  100 , in accordance with a preferred embodiment. Plot  301  of  FIG. 3  shows an example of the output signals from detectors  104  and  105  in a condition of zero curl of the media substrate  108 . The zero curl condition causes the media substrate  108  to interrupt the light beams from light emitters  101  and  102  simultaneously, resulting in a detector timer difference of zero time. Plot  302  of  FIG. 3  illustrates an example of the output signal from detectors  104  and  105  in a condition of positive curl of the media substrate  108 . The positive curl condition causes the media substrate  108  to interrupt the light beams from light emitter  101  first and light emitter  102  second, resulting in a measurable time difference in the light detector output signals. Similarly, plot  303  of  FIG. 3  depicts the negative curl condition, wherein the downward leading edge curl of the media substrate  108  causes the light beam of emitter  102  to be interrupted first followed by interruption of the light beam of light emitter  101 . A schematic diagram of representative output signals from light detectors  204  and  205  in lead edge curl sensor  200  would be similar to those in  FIG. 3 . 
         [0036]      FIG. 4  illustrates an example of test data provided by a lead edge curl sensor  100  wherein the light emitter beams cross the media transport media path  103  at approximately an angle of 45 degrees, in accordance with a preferred embodiment. In  FIG. 4 , plot  400  indicates the linear function of the measurement of the time differences between the light beams interrupted by the media substrate  108  and the linear displacement of the leading edge of the substrate. Plot  401  indicates the measurement of time differences between the light beams interrupted by the media substrate  108  and the displacement of the media substrate trailing edge. 
         [0037]    The alignment of the light beams from light emitters  101  and  102  wherein the beams cross exactly at the ideal media transport path  103  would be the condition requiring no further calibration. However, the slight misalignment of the light emitters may be calibrated out by using a reference delay time between the signals, achieving the maximum media substrate curl resolution even with slightly misaligned light emitter beams. Additionally, reduction of stray light and shaping of the beams can improve signal to noise ratio at the light detectors  104  and  105  by increasing the “on” to “off” detector contrast. 
         [0038]      FIG. 5  illustrates a flow chart of operations depicting logical operational steps of lead edge sheet curl sensing method  500 , which can be implemented in accordance with a preferred embodiment. First, a transport media sheet substrate path can be provided, as depicted at block  501 . Next, first and second light emitters are provided, as illustrated at block  502 . Block  503  depicts an operation for of aligning the light beams of the first and second light emitters so that the light beams cross at the transport media sheet path. Thereafter as described at block  504 , first and second light detectors can be aligned with the first and second light emitters. The first and second light emitters are generally disposed on opposite respective sides of the transport media sheet substrate path from the first and second light detectors, as depicted next at block  505 . The final step includes timing the output signals from the first and second light detectors in a timing device (e.g., a microcontroller, microprocessor or other timing device) such that the time differential of the output signals is proportional to the lead edge sheet positive or negative curl, as depicted at block  506 . 
         [0039]      FIG. 6  illustrates a high-level schematic diagram depicting an edge restraint system  600 , which can be implemented in accordance with a preferred embodiment. System  600  can include the use of the lead edge curl sensor  100  depicted in  FIG. 1 . Note that in  FIGS. 1-9 , identical or similar parts are generally indicated by identical reference numerals. Although the lead edge curl sensor  100  of  FIG. 1  is also depicted in  FIG. 6 , for purposes of illustration, it can be appreciated that an alternative embodiment can be implemented, which utilizes the lead edge curl sensor  200  of  FIG. 2 . The edge constraint system  600  comprises both a lead edge constraint system  601  and a trailing edge constraint system  602 . The lead edge constraint system  601  constrains the edge of the substrate media  608  such that the curt sensor  100  is able to accurately measure the sheet media curl. The process direction of the substrate media is indicated by arrow  603  in  FIG. 6 . 
         [0040]    The lead edge constraint system  601  maintains the media substrate sheet  608  as it proceeds along the substrate sheet media path  603  to the lead edge curl sensor  100 . The constraint of the substrate media  608  can be achieved through the functionality of drive rollers  609  and  610 , which can be positioned on opposite sides of the substrate media path  603  and drive the substrate  608  towards the lead edge curl sensor  100 . In addition, there are two constraint rollers  611  and  612  further positioned between the drive rollers and the lead edge curl sensor  100 . The substrate  608  travels between the constraint rollers  611  and  612 . In one embodiment of lead edge constraint system, the drive rollers  609  and  610  can be positioned close to the constraint rollers  611  and  612  at an exemplary distance of less than 100 mm. It can be appreciated, of course, that the parameter of 100 mm is merely a suggested value and is not considered a limiting feature of the disclosed embodiments. 
         [0041]    The constraint rollers  611  and  612  are composed of a non-deformable material which provides a light clamping force to the substrate  608  while not damaging the substrate  608 . One example of a non-deformable material for the composition of the constraint rollers  611  and  612  would be a hard non-deformable plastic, although the composition of the constraint rollers  611  and  612  could be composed of any non-deformable material capable of providing a light clamping force to the substrate media  608 . The non-deformable constraint rollers  611  and  612  ensure that the substrate media  608  is held tangent to the constraint rollers  611  and  612  on the substrate media path  603 . The drive speed of the constraint rollers  611  and  612  is controlled such that the lead edge constraint rollers  611  and  612  are driven at a slightly higher speed than that of the drive rollers  609  and  610  to keep the substrate media  608  taunt and under tension as it enters the curl sensor  100 . 
         [0042]    Alternatively, in an additional embodiment, drive rollers  611  and  612  can be composed of a deformable material such that each roller  611  and  612  has similar elastic modulus or stiffness. The nip thus formed by rollers  611  and  612  provides a consistent sheet ejection angle over a wide range of substrates and nip normal forces. 
         [0043]    The trailing edge constraint system  602  functions in a manner that is similar to the lead edge constraint system  601 . The trailing edge constraint system  602  holds the substrate media  608  as it exits the curl sensor  100  such that the trailing edge curl of the substrate media  608  can be accurately measured. The trailing edge constraint system includes the drive rollers  615  and  616  positioned on opposite sides of the substrate media path  603  wherein the substrate media  603  is pulled forward after exiting the curl sensor  100 . Between the curl sensor  100  and the drive rollers  615  and  616  are positioned the trailing edge constraint rollers  613  and  614 . The trailing edge constraint rollers  613  and  614  could be composed of any non-deformable material capable of providing a light clamping force to the substrate media  608 , or of a deformable material such that each roller  613  and  614  has similar elastic modulus or stiffness, as in the lead edge constraint rollers  611  and  612 . 
         [0044]    The drive speed of the trailing edge constraint rollers  613  and  614  is controlled such that the constraint rollers  613  and  614  are driven at a slightly slower speed than that of the drive rollers  615  and  616  to keep the substrate media  608  taunt and under tension as it exits the curl sensor  100 . This allows the curl sensor  100  to make an accurate measurement of the amount of curl of the trailing edge of the substrate media  608 . As in the lead edge constraint rollers, the trailing edge constraint rollers  613  and  614  are positioned closely to the trailing edge drive rollers  615  and  616 . One embodiment has the distance between the trailing edge rollers at a distance of less than 110 mm in order to keep the substrate media tangent to the constraint rollers  613  and  614 . 
         [0045]    All four constraint rollers  611 ,  612 ,  613 ,  614  for both the leading and trailing edges can be formed such that the rollers  611 ,  612 ,  613 ,  614  span the full width of the substrate media (i.e., in the cross process direction). This full width span of the constraint rollers  611 ,  612 ,  613 ,  614  prevents the substrate media from buckling or corrugating in the cross-process direction, which can affect the accuracy of the curl sensor measurement. Also illustrated in  FIG. 6  are lead-in baffles  618 ,  619 ,  620 ,  621 ,  622 ,  623 ,  624 , and  625 . The lead-in baffles minimize the possibility of the substrate media  608  stubbing along the substrate sheet media path  603 . 
         [0046]      FIG. 7  illustrates a high-level schematic diagram depicting only the lead edge portion  601  of the edge curl constraint system  600  of  FIG. 6 , which can be implemented in accordance with a preferred embodiment. The configuration depicted in  FIG. 7  is similar to that illustrated in  FIG. 6  with the addition of drive belt  701  and drive motor  702 . The configuration of the trailing edge constraint rollers is not depicted, but is similar to the lead edge drive system. Motor  702  drives the drive belt  701  which in turn drives the rollers  610  and  612 . As noted above, the lead edge constraint roller  612  is driven by the drive belt  701  at a rotational speed slightly higher than that of the drive roller  610 . It should be further noted that  FIG. 7  illustrates only the drive system for the lower set of rollers  610  and  612 . A similar drive system would be utilized for the rollers  609 ,  611 ,  613 ,  614 ,  615  and  616 . 
         [0047]      FIG. 8  illustrates a vertical perspective of drive roller  609  and constraint roller  611 . The full width span of the constraint roller  611  is shown as a plurality of constraint nips  801  on the constraint roller  611 . The important point of the full width span of the constraint rollers  611  is that the media substrate sheet is prevented from buckling or corrugating in the cross-process direction by the full span width of the constraint nips  801 . The drive roller  609  comprises two drive nips  802 , as depicted in this example. The drive nips  802  are not required to span the full width, only remain in contact with the media sheet substrate  608  in order to drive the media sheet substrate forward towards the lead edge curl sensor system  100 . 
         [0048]      FIG. 9  illustrates a flow chart of operations depicting logical operational steps of an edge sheet substrate constraint method  900  that can be implemented in accordance with an alternative embodiment. The process beings as indicated at block  901 . First, a media sheet substrate path can be provided, as illustrated at block  902 . Next, as described at block  903 , a first pair of drive rollers can be provided to transport the media substrate. Next, as depicted at block  904 , a first pair of constraint rollers formed of a non-deformable material, can be provided to constrain the media substrate. Thereafter as illustrated at block  905 , the constraint rollers can be driven at a higher rotational speed than the drive rollers. Next, as described at block  906 , the media substrate then enters a substrate curl detector. Upon exiting the substrate curl detector, the media substrate can be further constrained as indicated at block  907  by providing a second pair of constraint rollers formed of a non-deformable material. The media substrate is then driven forward by providing a second pair of drive rollers, as illustrated at block  908 . The second pair of drive rollers can then be driven at a high rotational speed than the second pair of constraint rollers, as described at block  909 . The method  900  can then terminate, as depicted at block  910 . 
         [0049]    It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.