Abstract:
An apparatus and method for advancing a receiver into registered relationship with a moving image-bearing member. A motor is provided that is responsive to motor drive pulses. A drive member engages the receiver, and a drive coupling connects the drive member and the motor. An encoder generates encoder pulses that correspond with movement of the image-bearing member. A pulse generator generates motor drive pulses in response to the encoder pulses to accelerate the receiver to a speed approximately equal to the speed of the image-bearing member speed. A timer determines an amount of delay time between detection of the receiver by an in-track sensor and the beginning of a subsequent movement of the motor. A delay mechanism delays the acceleration of the receiver to the image-bearing member speed by the amount of delay time.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electrophotographic reproduction apparatus and methods for registering sheets and more particularly to apparatus and methods for control of a stepper motor drive for controlling movement of a receiver sheet into transfer relationship with an image-bearing member that supports an image to be transferred to the receiver sheet. 
     2. Brief Description of Available Systems 
     In known electrophotographic copier, printers or duplicators the problem of accurate registration of a receiver sheet with a moving member supporting an image for transfer to the sheet is well known. In this regard, reference is made to U.S. Pat. No. 5,322,273, the contents of which are incorporated herein by reference. 
     Typically, an electrophotographic latent image is formed on the member and this image is toned and then transferred to a receiver sheet directly or transferred to an intermediate image-bearing member and then to the receiver sheet. In moving of the receiver sheet into transfer relationship with the image-bearing member, it is important to adjust the sheet for skew. Once the skew of the sheet is corrected, it is advanced by rollers driven by stepper motors towards the image-bearing member. During the skew control adjustment, the adjustment is implemented by selectively driving the stepper motor driven rollers, which are controlled independently of movement of the image-bearing member. Typically, movement of the receiver sheet and operations performed thereon by various stations are controlled using one or more encoders. Known registration control systems use a transfer roller with which an encoder wheel is associated. This encoder is used for controlling registration of the sheet. At some point in time after adjustment of the sheet for skew and prior to engagement of the sheet into transfer relationship with the image-bearing member, the control of the stepper motors that provide the drive to the rollers which advance the sheet, is transferred from simulated clock pulses of a microprocessor to the actual clocking pulses generated by the encoder wheel. 
     A problem with these systems is that in switching control of the stepper motors from synchronization with control signals in the skew correction device to that of the encoder wheel, a stepper motor driving pulse may be lost. This results in sufficient positional difference between receiver sheet and photoconductive belt that accurate registration is not accomplished. 
     An improved registration apparatus is disclosed in U.S. Pat. No. 5,731,680, the contents of which are incorporated herein by reference. However, even this improved apparatus relies upon a transfer of stepper motor control from simulated clock pulses to the clocking pulses generated by the encoder wheel. The relatively low resolution of the encoder wheels traditionally used in registration systems limits the precision that can be achieved during the transfer of stepper motor control. It is, therefore, an object of the invention to provide improved methods and apparatus for ensuring accurate registration of the receiver sheet and image-bearing member. 
     BRIEF SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with one aspect of the invention, there is provided an apparatus for advancing a receiver sheet into registered relationship with a moving image-bearing member. The apparatus includes a drive member that engages the receiver. A motor, which is responsive to motor drive pulses, is coupled to the drive member. The apparatus also includes an encoder that generates encoder pulses that correspond with movement of the image-bearing member. A pulse generator is provided to generate motor drive pulses. The pulse generator is connected to the motor for accelerating the receiver sheet to a speed approximately equal to the speed of the image-bearing member. 
     In accordance with another aspect of the invention, there is provided a method for advancing a sheet into registered relationship with a moving image-bearing member. An encoder is provided that tracks the movement of the image-bearing member. A motor is also provided. The motor is then driven in response to an output of the encoder to accelerate the receiver movement to a speed substantially equal to the speed of the image-bearing member. 
    
    
     The invention and its various advantages will become more apparent to those skilled in the art from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subsequent description of the preferred embodiments of the present invention refers to the attached drawings, wherein: 
     FIG. 1 is a side elevational view of a sheet registration mechanism, partly in cross-section, and with portions removed to facilitate viewing; 
     FIG. 2 is a view, in perspective, of the sheet registration mechanism of FIG. 1, with portions removed or broken away to facilitate viewing; 
     FIG. 3 is a top plan view of the sheet registration mechanism of FIG. 1, with portions removed or broken away to facilitate viewing; 
     FIG. 4 is a front elevational view, in cross-section of the third roller assembly of the sheet registration mechanism of FIG. 1; 
     FIG. 5 is top schematic illustration of the sheet transport path showing the actions of the sheet registration mechanism of FIG. 1 on an individual sheet as it is transported along a transport path; 
     FIG. 6 is a graphical representation of the peripheral velocity profile over time for the urging rollers of the sheet registration mechanism of FIG. 1; 
     FIGS. 7 a - 7   f  are respective side elevational views of the urging rollers of the sheet registration mechanism of FIG. 1 at various time intervals in the operation of the sheet registration mechanism; 
     FIG. 8 is a schematic of a circuit for controlling one or more stepper motors in accordance with one embodiment of the invention; 
     FIG. 9 is a schematic of a second circuit for controlling stepper motors in accordance with a second embodiment of the invention; 
     FIG. 10 is a flowchart describing operation of the circuit of FIG. 9; and 
     FIG. 11 is a flowchart further describing operation of the circuit of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Because electrophotographic reproduction apparatus are well known, the present description will be directed in particular to elements forming part of or cooperating more directly with the present invention. Apparatus not specifically shown or described herein are selectable from those known in the prior art. 
     Referring now to the accompanying drawings, FIGS. 1-3 best show the sheet registration mechanism, designated generally by the numeral  100 , according to this invention. The sheet registration mechanism  100  is located in association with a substantially planar sheet transport path P of any well known device where sheets are transported seriatim from a supply (not shown) to a station where an operation is performed on the respective sheets. For example, the device may be a reproduction apparatus, such as a copier or printer or the like, where marking particle developed images of original information, are placed on receiver sheets. As shown in FIG. 1, the marking particle developed images (e.g., image I) are transferred at a transfer station T from an image-bearing member such as a movable web or drum (e.g., web W) to a sheet of receiver material (e.g., a cut sheet S of plain paper or transparency material) moving along the path P. A transfer roller R guides the web W. 
     In reproduction apparatus of the above type, it is desired that the sheet S be properly registered with respect to a marking particle developed image in order for the image to be placed on the sheet in an orientation to form a suitable reproduction for user acceptability. Accordingly, the sheet registration mechanism  100  provides for alignment of the receiver sheet in a plurality of orthogonal directions. That is, the sheet is aligned, with the marking particle developed image, by the sheet registration mechanism by removing any skew in the sheet (angular deviation relative to the image), and moving the sheet in a cross-track direction so that the centerline of the sheet in the direction of sheet travel and the centerline of the marking particle image are coincident. Further, the sheet registration mechanism  100  times the advancement of the sheet along the path P such that the sheet and the marking particle image are aligned in the in-track direction as the sheet travels through the transfer station T. 
     In order to accomplish skew correction and cross-track and in-track alignment of the receiver with respect to the image-bearing member, one or more drive members are operable to engage the receiver. For example, to register the sheet S with respect to a marking particle developed image on the moving web W, the sheet registration apparatus  100  includes first and second independently driven roller assemblies  102 ,  104 , and a third roller assembly  106 . The first roller assembly  102  includes a first shaft  108  supported adjacent its ends in bearings  110   a ,  110   b  mounted on a frame  110 . Support for the first shaft  108  is selected such that the first shaft is located with its longitudinal axis lying in a plane parallel to the plane through the sheet transport path P and substantially perpendicular to the direction of a sheet traveling along the transport path in the direction of arrows V (FIG.  1 ). A first urging drive roller  112  is mounted on the first shaft  108  for rotation therewith. The urging roller  112  has an arcuate peripheral segment  112   a  extending about 180° around such roller. The peripheral segment  112   a  has a radius to its surface measured from the longitudinal axis of the first shaft  108  substantially equal to the minimum distance of such longitudinal axis from the plane of the transport path P. 
     One or more motors are operable to drive the drive members via a drive coupling. For example, a first stepper motor M 1 , mounted on the frame  110 , is operatively coupled to the first shaft  108  through a gear train  114  to rotate the first shaft when the motor is activated. The gear  114   a  of the gear train  114  incorporates an indicia  116  detectable by a suitable sensor mechanism  118 . The sensor mechanism  118  can be either optical or mechanical depending upon the selected indicia. Location of the sensor mechanism  118  is selected such that when the indicia  116  is detected, the first shaft  108  will be angularly oriented to position the first urging roller  112  in a home position. The home position of the first urging roller is that angular orientation where the surface of the arcuate peripheral segment  112   a  of the roller  112 , upon further rotation of the shaft  108 , will contact a sheet in the transport path P (see FIG. 7 a ). 
     The second roller assembly  104  includes a second shaft  120  supported adjacent its ends in bearings  110   c ,  110   d  mounted on the frame  110 . Support of the second shaft  120  is selected such that the second shaft is located with its longitudinal axis lying in a plane parallel to the plane through the sheet transport path P and substantially perpendicular to the direction of a sheet traveling along the transport path. Further, the longitudinal axis of the second shaft  120  is substantially coaxial with the longitudinal axis of the first shaft  108 . 
     A second urging drive roller  122  is mounted on the second shaft  120  for rotation therewith. The urging roller  122  has an arcuate peripheral segment  122   a  extending about 180° around such roller. The peripheral segment  122   a  has a radius to its surface measured from the longitudinal axis of the first shaft  108  substantially equal to the minimum distance of such longitudinal axis from the plane of the transport path P. The arcuate peripheral segment  122   a  is angularly coincident with the arcuate peripheral segment  112   a  of the urging roller  112 . A second independent stepper motor M 2 , mounted on the frame  110 , is operatively coupled to the second shaft  120  through a gear train  124  to rotate the second shaft when the motor is activated. The gear  124   a  of the gear train  124  incorporates an indicia  126  detectable by a suitable sensor mechanism  128 . The sensor mechanism  128 , adjustably mounted on the frame  110 , can be either optical or mechanical depending upon the selected indicia. Location of the sensor mechanism  128  is selected such that when the indicia  126  is detected, the second shaft  120  will be angularly oriented to position the second urging roller  122  in a home position. The home position of the second urging roller is that angular orientation where the surface of the arcuate peripheral segment  122   a  of the roller  122 , upon further rotation of the shaft  120 , will contact a sheet in the transport path P (same as the angular orientation of the peripheral segment  112   a  as shown in FIG. 7 a ). 
     The third roller assembly  106  includes a tube  130  surrounding the first shaft  108  and capable of movement relative to the first shaft in the direction of the longitudinal axis thereof. A pair of third urging drive rollers  132  are mounted on the first shaft  108 , supporting the tube  130  for relative rotation with respect to the third urging rollers. The third urging rollers  132  respectively have an arcuate peripheral segment  132   a  extending about 180° around each roller. The peripheral segments  132   a  each have a radius to its respective surface measured from the longitudinal axis of the first shaft  108  substantially equal to the minimum distance of such longitudinal axis from the plane of the transport path P. The arcuate peripheral segments  132   a  are angularly offset with respect to the arcuate peripheral segments  112   a ,  122   a  of the first and second urging rollers. The pair of third urging rollers  132  are coupled to the first shaft  108  by a key or pin  134  engaging a slot  136  in the respective rollers (FIG.  4 ). Accordingly, the third urging rollers  132  will be rotatably driven with the first shaft  108  when the first shaft is rotated by the first stepper motor M 1 , and are movable in the direction along the longitudinal axis of the first shaft with the tube  130 . For the purpose to be more fully explained below, the angular orientation of the third urging rollers  132  is such that the arcuate peripheral segments  132   a  thereof are offset relative to the arcuate peripheral segments  112   a  and  122   a.    
     A third independent stepper motor M 3 , mounted on the frame  110 , is operatively coupled to the tube  130  of the third roller assembly  106  to selectively move the third roller assembly in either direction along the longitudinal axis of the first shaft  108  when the motor is activated. The operative coupling between the third stepper motor M 3  and the tube  130  is accomplished through a pulley and belt arrangement  138 . The pulley and belt arrangement  138  includes a pair of pulleys  138   a ,  138   b , rotatably mounted in fixed spatial relation, for example, to a portion of the frame  110 . A drive belt  138   c  entrained about the pulleys is connected to a bracket  140  which is in turn connected to the tube  130 . A drive shaft  142  of the third stepper motor M 3  is drivingly engaged with a gear  144  coaxially coupled to the pulley  138   a . When the stepper motor M 3  is activated, the gear  144  is rotated to rotate the pulley  138   a  to move the belt  138   c  about its closed loop path. Depending upon the direction of rotation of the drive shaft  142 , the bracket  140  (and thus the third roller assembly  106 ) is selectively moved in either direction along the longitudinal axis of the first shaft  108 . 
     A plate  146  connected to the frame  110  incorporates an indicia  148  detectable by a suitable sensor mechanism  150 . The sensor mechanism  150 , adjustably mounted on the bracket  140 , can be either optical or mechanical depending upon the selected indicia. Location of the sensor mechanism  150  is selected such that when the indicia  148  is detected, the third roller assembly  106  is located in a home position. The home position of the third roller assembly  106  is selected such that the third roller assembly is substantially centrally located relative to the cross-track direction of a sheet in the transport path P. 
     The frame  110  of the sheet registration mechanism  100  also supports a shaft  152  located generally below the plane of the sheet transport path P. Pairs of idler rollers  154  and  156  are mounted on the shaft  152  for free rotation. The rollers of the idler pair  154  are respectively aligned with the first urging roller  112  and the second urging roller  122 . The rollers of the idler roller pair  156  are aligned with the respective third urging rollers  132 , and extend in a longitudinal direction for a distance sufficient to accommodate for maintaining such alignment over the range of longitudinal movement of the third roller assembly  106 . The spacing of the shaft  152  from the plane of the sheet transport path P and the diameter of the respective rollers of the idler roller pairs  154  and  156  are selected such that the rollers will respectively form a nip relation with the arcuate peripheral segments  112   a ,  122   a , and  132   a  of the urging rollers. For example, the shaft  152  may be spring loaded in a direction urging such shaft toward the shafts  108 ,  120 , where the idler roller pair  154  will engage spacer roller bearings  112   b ,  122   b.    
     With the above described construction for the sheet registration mechanism  100  according to this invention, sheets traveling seriatim along the sheet transport path P are alignable by removing any skew (angular deviation) in the sheet to square the sheet up with respect to the path, and moving the sheet in a cross-track direction so that the centerline of the sheet in the direction of sheet travel and the centerline C L  of the transport path P are coincident. Of course, the centerline C L  is arranged to be coincident with the centerline of the downstream operation station (in the illustrated embodiment, the centerline of a marking particle image on the web W). Further, the sheet registration mechanism  100  times the advancement of the sheet along the transport path P for alignment in the in-track direction (again referring to the illustrated embodiment, in register with the lead edge of a marking particle image on the web W). 
     In order to effect the desired skew removal, and cross-track and in-track sheet alignment, the mechanical elements of the sheet registration mechanism  100  according to this invention are operatively associated with a controller  220  (see FIG.  8 ). The controller  220  receives input signals from a plurality of sensors associated with the sheet registration mechanism  100  and a downstream operation station. Based on such signals and an operating program, the controller  220  produces appropriate signals to control the independent stepper motors M 1 , M 2 , and M 3  of the sheet registration mechanism. 
     For the operation of the sheet registration mechanism  100 , referring now particularly to FIGS. 5,  6  and  7   a - 7   f , a sheet S traveling along the transport path P is moved into the vicinity of the sheet registration mechanism by an upstream transport assembly including non-separable nip rollers (not shown). Such sheet may be oriented at an angle (e.g., angle a in FIG. 5) to the centerline C L  of the path P and may have its center A spaced a distance from the path centerline (e.g., distance d in FIG.  5 ). The angle α and distance d, which are undesirable, are of course generally induced by the nature of the upstream transport assembly and are variable sheet-to-sheet. 
     A pair of nip sensors  160   a ,  160   b  is located upstream of the plane X 1  (see FIG.  5 ). The plane X 1  is defined as including the longitudinal axes of the urging rollers ( 112 ,  122 ,  132 ) and the rollers of the idler roller pairs ( 154 ,  156 ). The nip sensors  160   a ,  160   b  may, for example, be of either the optical or mechanical type. Nip sensor  160   a  is located to one side (in the cross-track direction) of the centerline C L , while nip sensor  160   b  is located a substantially equal distance to the opposite side of the centerline C L . 
     When the sensor  160   a  detects the lead edge of a sheet transported along the path P, it produces a signal which is sent to the controller  220  for the purpose of activating the first stepper motor M 1 . In a like manner, when the sensor  160   b  detects the lead edge of a sheet transported along the path P, it produces a signal which is sent to the controller  220  for the purpose of activating the second stepper motor M 2 . If the sheet S is at all skewed relative to the path P, the lead edge to one side of the centerline C L  will be detected prior to detection of the lead edge at the opposite side of the centerline (of course, with no skew, the lead edge detection at opposite sides of the centerline will occur substantially simultaneously). 
     As shown in FIG. 6, when the first stepper motor M 1  is activated by the controller  220 , it will ramp up to a speed such that the first urging roller  112  will be rotated at an angular velocity to yield a predetermined peripheral speed for the arcuate peripheral segment  112   a  of such roller substantially equal to the entrance speed of a sheet transported along the path P. When the portion of the sheet S enters the nip between the arcuate peripheral segment  112   a  of the first urging roller  112  and the associated roller of the idler roller pair  154 , such sheet portion will continue to be transported along the path P in a substantially uninterrupted manner (see FIG. 7 b ). 
     Likewise, when the second stepper motor M 2  is activated by the controller  220 , it will ramp up to a speed such that the second urging roller  122  will be rotated at an angular velocity (substantially the same as the angular velocity of the first urging roller) to yield a predetermined peripheral speed for the arcuate peripheral segment  122   a  of such roller substantially equal to the speed of a sheet transported along the path P. When the portion of the sheet S enters the nip between the arcuate peripheral segment  122   a  of the second urging roller  122  and the associated roller of the idler roller pair  154 , such sheet portion will continue to be transported along the path P in a substantially uninterrupted manner. As seen in FIG. 5, due to the angle a of the sheet S, sensor  160   b  will detect the sheet lead edge prior to the detection of the lead edge by the sensor  160   a . Accordingly, the stepper motor M 2  will be activated prior to activation of the motor M 1 . 
     A pair of in-track sensors  162   a ,  162   b  is located downstream of the plane X 1 . As such, the in-track sensors  162   a ,  162   b  are located downstream of the nips formed respectively by the arcuate peripheral segments  112   a ,  122   a  and their associated rollers of the idler roller pairs  154 . Thus, the sheet S will be under the control of such nips. The in-track sensors  162   a ,  162   b  may, for example, be of either the optical or mechanical type. Sensor  162   a  is located to one side (in the cross-track direction) of the centerline C L , while sensor  162   b  is located a substantially equal distance to the opposite side of the centerline C L . 
     When the sensor  162   a  detects the lead edge of a sheet transported along the path P by the urging roller  112 , it produces a signal which is sent to the controller  220  for the purpose of deactivating the first stepper motor M 1 . In a like manner, when the sensor  162   b  detects the lead edge of a sheet transported along the path P by the urging roller  122 , it produces a signal which is sent to the controller  220  for the purpose of deactivating the second stepper motor M 2 . Again, if the sheet S is at all skewed relative to the path P, the lead edge at one side of the centerline C L  will be detected prior to detection of the lead edge at the opposite side of the centerline. 
     When the first stepper motor M 1  is deactivated by the controller  220 , its speed will ramp down to a stop such that the first urging roller  112  will have zero angular velocity to stop the engaged portion of the sheet in the nip between the arcuate peripheral segment  112   a  of the first urging roller  112  and the associated roller of the idler roller pair  154  (see FIG. 7 c ). Likewise, when the second stepper motor M 2  is deactivated by the controller  220 , its speed will ramp down to a stop such that the first urging roller  112  will have zero angular velocity to stop the engaged portion of the sheet in the nip between the arcuate peripheral segment  122   a  of the second urging roller  122  and the associated roller of the idler roller pair  154 . Again referring to FIG. 5, due to the angle α of the sheet S, sensor  162   b  will detect the sheet lead edge prior to the detection of the lead edge by the sensor  162   a . Accordingly, the stepper motor M 2  will be deactivated prior to deactivation of the motor M 1 . Therefore, the portion of the sheet in the nip between the arcuate peripheral segment  122   a  of the second urging roller  122  and the associated roller of the idler roller pair  154  will be held substantially fast (i.e., will not be moved in the direction along the transport path P) while the portion of the sheet in the nip between the arcuate peripheral segment  112   a  of the first urging roller  112  and the associated roller of the idler roller pair  154  continues to be driven in the forward direction. As a result, the sheet S will rotate substantially about its center A until the motor M 1  is deactivated. Such rotation, through an angle β (substantially complementary to the angle α) will square up the sheet and remove the skew in the sheet relative to the transport path P to properly align the lead edge thereof. 
     The in-track sensor  162   a  and/or  162   b  establishes a known position of the receiver by sensing the receiver, for example a leading edge. A set of stepper motor pulses may be sent to the stepper motors to establish a known position downstream from the in-track sensor  162   a  and/or  162   b  since a set of stepper motor pulses sent to the stepper motors moves the receiver a fixed distance, an inherent property of stepper motors and the geometry of the urging rollers. 
     Once the skew has been removed from the sheet, as set forth in the above description of the first portion of the operative cycle of the sheet registration mechanism  100 , the sheet is ready for subsequent cross-track alignment and registered transport to a downstream location. A sensor  164 , such as a set of sensors (either optical or mechanical as noted above with reference to other sensors of the registration mechanism  100 ) aligned in the cross-track direction (see FIG.  5 ), detects a lateral marginal edge of the sheet S and produces a signal indicative of the location thereof. 
     The signal from the sensor  164  is sent to the controller  220  where the operating program will determine the distance (e.g., distance d shown in FIG. 5) of the center A of the sheet from the centerline C L  of the transport path P. At an appropriate time determined by the operating program, the first stepper motor M 1  and the second stepper motor M 2  will be activated. The first urging roller  112  and the second urging roller  122  will then begin rotation to start the transport of the sheet toward the downstream direction (see FIG. 7 d ). The stepper motors will ramp up to a speed such that the urging rollers of the roller assemblies  102 ,  104 , and  106  will be rotated at an angular velocity to yield a predetermined peripheral speed for the respective portions of the arcuate peripheral segments thereof Such predetermined peripheral speed is, for example, substantially equal to the speed of the web W. While other predetermined peripheral speeds are suitable, it is important that such speed be substantially equal to the speed of the web W when the sheet S touches down at the web. 
     Of course, in view of the above coupling arrangement for the third roller assembly  106 , rotation of the third urging rollers  132  will also begin when the first stepper motor Ml is activated. As will be appreciated from FIGS. 7 a - 7   d , up to this point in the operative cycle of the sheet registration mechanism  100 , the arcuate peripheral segments  132   a  of the third urging rollers  132  are out of contact with the sheet S and have no effect thereon. Now the arcuate peripheral segments  132   a  engage the sheet (in the nip between the arcuate peripheral segments  132   a  and the associated rollers of the idler roller pair  156 ) and, after a degree of angular rotation, the arcuate peripheral segments  112   a  and  122   a  of the respective first and second urging rollers leave contact with the sheet (see FIG. 7 e ). The control over the sheet is thus handed off from the nips established by the arcuate peripheral segments of the first and second urging rollers and the idler roller pair  154  to the arcuate peripheral segments of the third urging rollers and the idler roller pair  156  such that the sheet is under control of only the third urging rollers  132  for transport of the sheet along the path P. 
     At a predetermined time, once the sheet is solely under the control of the third urging rollers  132 , the controller  220  activates the third stepper motor M 3 . Based on the signal received from sensor  164  and the operating program of the controller  220 , the stepper motor M 3  will drive the third roller assembly  106 , through the above-described belt and pulley arrangement  138 , in an appropriate direction and for an appropriate distance in the cross-track direction. Accordingly, the sheet in the nips between the arcuate peripheral segments of the third urging rollers  132  and the associated rollers of the idler roller pair  156  is urged in a cross-track direction to a location where the center A of the sheet coincides with the centerline C L  of the transport path P to provide for the desired cross-track alignment of the sheet. 
     The third urging rollers  132  continue to transport the sheet along the transport path P at a speed substantially equal to the speed of the web W until the lead edge touches down on the web, in register with the image I carried by the web. At this point in time, the angular rotation of the third urging rollers  132  brings the arcuate peripheral segments  132   a  of such rollers out of contact with the sheet S (see FIG. 7 f ). Since the arcuate peripheral segments  112   a  and  122   a  of the respective first and second urging rollers  112  and  122  are also out of contact with the sheet, such sheet is free to track with the web W undisturbed by any forces which might otherwise have been imparted to the sheet by any of the urging rollers. 
     At the time the first, second and third urging rollers are all out of contact with the sheet, the stepper motors M 1 , M 2 , and M 3  are activated for a time, dependent upon signals to the controller  220  from the respective sensors  118 ,  128 , and  150 , and then deactivated. As described above, such sensors are home position sensors. Accordingly, when the stepper motors are deactivated, the first, second, and third urging rollers are respectively located in their home positions. Therefore, the roller assemblies  102 ,  104 ,  106  of the sheet registration mechanism  100  according to this invention are located as shown in FIG. 7 a , and the sheet registration mechanism is ready to provide skew correction and cross-track and in-track alignment for the next sheet transported along the path P. 
     As noted above, a problem with the registration control mechanism of known systems is that control of the stepper motor drives during ramp-up of the sheet speed is not synchronized with exact movement of the web. Because the web speed changes, improved registration requires that control of the drive to the sheet be synchronized with the movement of the web. The synchronization method of U.S. Pat. No. 5,731,680 achieves synchronization through use of an encoder associated with the transfer roller R. The encoder produces an output of electrical pulses that are synchronized with the movement of the transfer roller R. The encoder pulses are used to drive the urging rollers  112 ,  122  once the sheet S has been ramped up to a speed approximately equal to that of the moving web W. However, due to the limited precision of the encoder output, a separate high-frequency timer must be used to drive the urging rollers  112 ,  122  during ramp-up and synchronization with the encoder output. Moreover, the limited precision of the encoder output results in a margin of error of up to one step of the stepper motor during the skew correction and in-track alignment process. The improved registration method of the current invention reduces the margin of error by driving all stages of the registration process with an encoder having a higher resolution. 
     With reference to FIG. 8, a schematic of one form of a stepper motor controller for use in the apparatus and method of the invention is illustrated. An encoder wheel  200  is provided that is associated with the transfer roller R (FIG. 1) and as the roller rotates, the indicia on the encoder wheel move and interrupt light from a light source  202 , which light or absence of same is sensed by a phototransducer  204 . Other forms of encoders that use magnetic indicia or are linear rather than rotating may be used since the encoder details are not critical to the invention. Electrical pulses  206  are generated by the phototransducer on line  208  and these pulses are synchronized with movement of the transfer roller  9  and the moving web W. The logic and control unit LCU  210 , which may be a microprocessor functioning in accordance with an operating program, commences a programmed control over line  212  of a programmable pulse generator  214  that generates a series of stepper motor pulses  216  over a line  218 . Collectively, the LCU  210  and the pulse generator  214  may constitute a registration system controller  220 . 
     As described above, the stepper motor Ml is mechanically coupled by a drive coupling to a drive member such as the first drive roller  112  that is in engagement with the receiver sheet S. The second stepper motor is similarly connected to the second drive roller for providing similar drive to the sheet S. The programmed drive of the stepper motors, as will be more fully described below, is provided to correct any skew in the sheet, to drive the sheet to a speed approximate to that of the image-bearing member, and to deliver the sheet to the image-bearing member at the proper time to ensure accurate in-track registration. A third stepper motor is provided for driving the third roller assembly for obtaining cross-track registration as noted above. 
     In one presently preferred embodiment of the invention, a programmable timer may serve as the pulse generator. This embodiment will now be discussed with reference to the schematic of FIG.  9  and the flowchart of FIG.  10 . 
     With reference to FIG. 9, there is shown a schematic of one presently preferred embodiment of the invention wherein a registration system controller  220  includes a programmable timer  302 , such as a  9513  System Timing Controller manufactured by Advanced Micro Devices, or the equivalent. Attached as an Appendix A is an ASIC Specification for a system timing controller suitable for use with the present invention. Two output lines, Out  1 , Out  2  are associated with the timer. Line Out  1  is connected to a drive input of a first stepper motor M 1  via line  218   a . Similarly, line Out  2  is connected to a drive input of a second stepper motor M 2  via line  218   b . The timer includes at its input a line  208  which carries encoder pulses  206  that are generated in synchronism with rotation of the transfer roller R as described above. 
     The timer  302  is controlled via line  212  by the LCU  210 . The LCU  210  includes a central processing unit, memory and various attendant input/output devices for communicating control data to the timer  302 . The LCU receives input data from nip sensors  160   a ,  160   b  and in-track sensors  162   a ,  162   b . The timer includes a first register (REG 1 ) and a first counter (CTR 1 ) that is associated with the register. In order to generate stepper motor pulses that are spaced at programmed intervals, it is known to provide a programmed count value that is stored in a counter. The counter then counts high speed clock pulses and when it matches the count, a single stepper motor drive pulse is generated. Typically, the counts may work by downcounting the number of clock pulses starting with the count value until zero is reached before emitting the stepper motor drive pulse. A new count value is then loaded into the counter from the associated register which in turn receives the count from the LCU. The counting process repeats for generating the next stepper motor drive pulse. By changing the count values a programmed series of stepper motor drive pulses may be generated at non-uniform intervals. Uniform intervals of stepper motor drive pulses may be provided by either retaining the same count value in the counter or the register or continually reloading the same count value from the LCU to the associated register which stores the count value and is used to load or preset the counter. The programmable counter (CTR 1 ) is responsive to encoder pulses  206  from the transducer  204  on line  208 . The series of stepper motor drive pulses generated by the counter (CTR 1 ) are output on line Out  1 . A second register (REG 2 ) and second programmable counter (CTR 2 ) are also provided for counting encoder pulses on line  208 . Because register (REG 2 ) can be loaded with different count values by the LCU, the stepper motor pulses generated by the second counter (CTR 2 ) may be of different spacing when output on line Out  2  from those output on line Out  1 . The LCU controls the timer  302  by providing appropriate count values for controlling the stepper motors M 1 , M 2 . The timer  302  counts down from each count value provided by the LCU  210 , then emits a stepper motor drive pulse on the appropriate output line. In generating stepper motor drive pulses responsive to encoder pulse the timer  302  is set in a mode wherein the rising edge of the appropriate encoder pulse on line  208  generates a stepper motor pulse on an output line such as Out  1 . 
     The operation of this presently preferred embodiment of the invention will now be discussed with reference to FIG.  10 . Initially, an encoder index pulse signal (F-PERF) is detected (step S 102 ) and a count is commenced (S 104 ) of encoder pulses in a counter associated with the LCU. In step S 106 , the receiver sheet has been transported or fed into the skew registration device  10  and a determination is made in response to the nip sensors  160   a ,  160   b  as to whether or not the sheet is detected. Upon detection of a sheet, the two stepper motors M 1 , M 2  are activated to run in accordance with programmed profiles (step S 108 ). As described above, the stepper motors may be run with a controlled profile by having the LCU input different count values into registers provided in the programmable timer  302 . When a count value is loaded into one of the timer&#39;s counter registers, a counter in the timer counts the encoder pulses and decrements the count in the register. Upon the count in the register reaching zero, an output pulse is provided on the appropriate output line which serves as a pulse to drive the corresponding stepper motor. At this time, a new count may be then loaded into the register. As this is repeated, a controlled series of stepper motor drive pulses  216   a ,  216   b  at predetermined time spacings may be generated by selecting the individual count values that are placed in the register through signals from the LCU. Other means for generating-non-uniformly spaced pulses are known. For example, a shift register may be provided with a programmed series of digital ones and zeros as data. In this example, the LCU may generate clock pulses that are used to shift the data from the register onto the shift register&#39;s output line that is connected to the stepper motor. The digital one values, for example, may serves as stepper motor drive pulses. 
     The LCU is programmed to load serially into each of the registers a predetermined set of digital numbers representing count values. These numbers may be serially loaded into each register which is known to activate each stepper motor to provide a drive profile that will cause a receiver sheet to be advanced within the registration device. Each stepper motor M 1 , M 2  is driven independently of the other, with stepper motor M 1  being driven by pulses on the timer&#39;s output line Out  1  to which stepper motor M 1  is connected. The output on line Out  1  is generated by pulses produced by the counter (CTR 1 ) that is programmed with count values stored in the register (REG 1 ). Similarly, stepper motor M 2  is driven by step pulses on the timer&#39;s output line Out  2  to which stepper motor M 2  is connected. The output on line Out  2  is generated by pulses produced by the counter (CTR 2 ) that is programmed with count values stored in the register (REG 2 ). 
     When the lead edge of the receiver sheet is detected by the in-track sensors  162   a ,  162   b , a signal is generated to the LCU (step S 110   a , S 110   b ). In response to this signal, a set of programmed count values is then serially placed in the appropriate timing register to cause a series of pulses on the corresponding stepper motor drive line, i.e., either  118   a  or  118   b , thereby causing a ramp down speed profile effect to be generated to stop the respective stepper motor (step S 112   a , S 112   b ). When both stepper motors are stopped, the sheet has been corrected for skew to within one stepper motor drive step (step S 114 ). The system is then prepared to ramp the sheet up to the approximate speed of the moving web W. Ramping to web speed begins a predetermined number of encoder pulses after the initial detection of F-PERF. By way of example, this predetermined number may be  2000  encoder pulses. The predetermined value is stored in non-volatile memory within the LCU  210 . When the LCU has detected (steps S 116   a , S 116   b ) the predetermined number of pulses after F-PERF, a set of programmed count values is serially placed in the appropriate timing registers to cause a series of pulses on the corresponding stepper motor drive lines  118   a ,  118   b , thereby causing the stepper motors M 1 , M 2  to ramp up movement (steps S 118   a , S 118   b ) of the receiver sheet S to web speed. For example, a series of four count values may be used to ramp the sheet S to film speed. The fourth and final value that is loaded into each of the counter registers is five, which will cause a stepper motor pulse to be generated after five encoder pulses. At this rate, the sheet S advances at approximately the speed of the moving web W. The count value of five is then retained, causing the timer to generate a series of uniformly spaced stepper motor drive pulses because the counter is continually downcounting the count of encoder pulses starting at the same count value and emitting a stepper motor drive pulse when reaching zero. Thus, the stepper motors M 1 , M 2  are driven to maintain a speed of the sheet S that approximates that of movement of the image I on the photoconductive web. The registration assembly maintains this drive speed until the sheet S is delivered to the image-bearing member. 
     Cross-track registration is provided along an independent logic flow path. As may be seen in step S 120 , a count is commenced of step pulses to stepper motor M 1 . When  280  step pulses are counted (step S 122 ) drive by a third stepper motor to the third drive roller assembly is provided to begin cross-track registration (step S 124 ). This typically would be expected to occur after steps S 118   a , S 118   b . Correction of cross-track registration (steps S 126 ) would be completed prior to the sheet engaging the moving web W. 
     Yet another presently preferred embodiment of the present invention reduces the margin of error in the registration process by accounting for potential over-correction in the de-skewing stage. As described above, skew correction is accomplished by ramping down the stepper motors M 1 , M 2  after detection of the lead edge of the sheet by the in-track sensors  162   a ,  162   b . The ramp-down is accomplished in an integral number of steps of each stepper motor, each step occurring during a programmed number of encoder pulses. Because each step of a stepper motor requires a finite amount of time (approximately equal to the duration of five encoder pulses), it is possible for in-track detection to occur during a step. However, the ramp-down program will not initiate until the beginning of the next step. In such a case, the sheet S travels a fraction of a step past the optimal stopping point. This may result in residual skew and positional or timing errors that remain uncorrected. This problem is addressed by determining the difference in time between in-track detection and the actual initiation of the ramp-down program. The ramp-up program is then delayed by an appropriate amount of time to account for the error. This process is discussed in further detail with reference to the flowchart of FIG.  11 . 
     When the in-track sensors  162   a ,  162   b  detect (steps S 210   a , S 210   b ) the lead edge of the receiver sheet S, the LCU  210  starts a high-frequency timer to determine the amount of time between in-track detection and the beginning of the next stepper motor drive step, which is coincident with initiation of the ramp-down program (steps S 212   a , S 212   b ). The delay-timing step (S 211   a , S 211   b ) is performed independently for each of the stepper motors M 1 , M 2 . The amount of delay time is then converted (steps S 215   a , S 215   b ) to an integral number of encoder pulses. The number Y 1 , Y 2  of encoder pulses is determined independently for each of the stepper motors M 1 , M 2  respectively. The appropriate number Y 1 , Y 2  of corrective encoder pulses is then added to the delay counter for each stepper motor in steps S 216   a , S 216   b , so as to further delay initiation of the ramp-up program (steps S 218   a , S 218   b ) by an additional Y 1  or Y 2 encoder pulses. For example, the period of time between successive stepper motor drive pulses  216  may be 253 microseconds. This corresponds to five consecutive encoder pulses. Conversely, each encoder pulse corresponds to one quintile of a stepper motor drive pulse period, or approximately 50 microseconds. Accordingly, the following associations between delay times and corresponding number Y 1 , Y 2  of corrective encoder pulses may be established: 
     
       
         
               
               
             
           
               
                   
               
               
                                          Delay Time 
                 Y Value 
               
               
                   
               
             
             
               
                             0-50 microseconds  
                 1 encoder pulse 
               
               
                   51-100 microseconds 
                 2 encoder pulses 
               
               
                 101-150 microseconds 
                 3 encoder pulses 
               
               
                 151-200 microseconds 
                 4 encoder pulses 
               
               
                 201-253 microseconds 
                 5 encoder pulses 
               
               
                   
               
             
          
         
       
     
     By delaying the ramp-up program in this way, the registration mechanism compensates for variation between in-track detection and initiation of the ramp-down program, thereby further increasing the precision of both skew correction and in-track alignment. 
     Although the invention is described with specific reference to electrophotographic apparatus and methods, the invention has broader applicability to other fields wherein registration of a moving sheet is to be made with an image-bearing member. 
     The invention has been described in detail with particular reference to preferred embodiments thereof and illustrative examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.