Patent Publication Number: US-8109508-B2

Title: Method and system for determining improved correction profiles for sheet registration

Description:
BACKGROUND 
     In printing environments, the transport of paper, or other sheets upon which text and images are rendered, is one of many important components in the overall quality of the printed sheet. In this regard, accuracy and precision of registration of the sheets to the text and images printed thereon contribute to the print quality. If the sheets are not transported in an acceptable manner, then the registration process could be adversely impacted. 
     In high speed, high end printing environments, a technique of agile registration has developed. Agile registration relates to registration techniques that involve high speed, adaptive, closed loop processes. 
     More particularly, with reference to  FIG. 1 , a document processing device  10  is illustrated. The device  10  includes a controller  12  that controls a variety of functions of the device including the paper path. In this regard, the paper path includes stationary nips A and B which impart x-direction velocity vectors V A  and V B  on a sheet  14 . The average (V A +V B )/2 provides an x-direction (process direction) motion to the sheet  14 . The difference (V A −V B ) provides a rotation of the sheet  14 . The sheet  14  is to be delivered to a device downstream. This device can be a photoreceptor or a drum (where it can receive an image) or any other appropriate device, inclusive of another set of nips. 
     In known processes, before the sheet  14  enters the nips A and B, the velocities V A  and V B  are typically set equal to the paper velocity of the upstream paper path V 0 . This should assure correct hand-off of the sheet from the upstream path to the paper registration device. 
     In this regard, agile registration commences shortly after the paper arrival as detected by sensors LEA and LEB. The sensors report the time-of-arrival t 0  and the process position x 0  and angle β 0  of the sheet. The side edge, or lateral, sensor reports the lateral position y 0 . In many cases, the lead-edge-center or lead-edge-side is considered the point that is being registered. Simple geometric calculation will yield values for the initial conditions of the registration point from sensor measurements. 
     Typically, delivery strategies calculate velocity profiles V A (t) and V B (t) to deliver the sheet  14  from these initial conditions to an end condition. The velocity profiles V A (t) and V B (t) must be calculated to deliver the sheet to position xf, yf, βf at a time tf with a velocity vf. As noted above, the velocity vf usually matches the velocity of the downstream device. However, in actual implementation, there are many factors that detract from these expectations. 
     In this regard, agile registration processes using polynomial profiles have been used. However, while polynomial agile registration typically exhibits accurate registration results, a large tail wag is generated. Triangular profiles have also been used. These profiles typically result in a small tail wag, but have less accurate results. Use of trapezoidal profiles has advantages, but typically leads to unpredictable nip forces. 
     It is desired that velocity profiles be calculated more accurately than is presently known to obtain precise delivery of sheets at various points in the paper path to achieve desired paper registration. 
     INCORPORATION BY REFERENCE 
     U.S. Pat. No. 5,678,159 is hereby incorporated by reference. 
     BRIEF DESCRIPTION 
     In one aspect of the presently described embodiments, the method comprises determining a lateral position of a sheet entering the nips of a paper path, determining a skew of the sheet as it enters the nips of the paper path, establishing a registration time, establishing a nominal velocity of the sheet on the paper path, determining an amplitude of a process direction correction velocity, computing a first value based on the lateral position, the skew, the registration time, the average velocity and the amplitude of the process direction correction velocity, determining a second value based on the first value, determining a peak of the lateral correction profile based on the second value, determining a velocity profile based on the peak, and, controlling the document processing device based on the profile. 
     In another aspect of the presently described embodiments, determining the lateral position of the sheet is based on detecting by a lateral sensor. 
     In another aspect of the presently described embodiments, determining the skew is based on detecting of the sheet by leading edge sensors. 
     In another aspect of the presently described embodiments, establishing the registration time is based on a target delivery time. 
     In another aspect of the presently described embodiments, establishing the registration time is based on a difference between a first time when the sheet engages leading edge sensors and a second time when the sheet should reach a target. 
     In another aspect of the presently described embodiments, determining the nominal velocity of the sheet comprises calculating an average velocity of the sheet. 
     In another aspect of the presently described embodiments, determining the amplitude of a process direction correction velocity is accomplished in closed form. 
     In another aspect of the presently described embodiments, the first value is computed using y=y+C*skw*Tee*(Vel Nom+VPro/2)/2. 
     In another aspect of the presently described embodiments, the second value is computed by dividing the first value by Tee 1.5 . 
     In another aspect of the presently described embodiments, the controlling comprises applying the velocity profile to the drive wheels of the document processing device. 
     In another aspect of the presently described embodiments, suitable means are provided to implement the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphic representation of an image rendering device into which the presently described embodiments may be incorporated; 
         FIG. 2  is another graphic illustration of an image rendering device into which the presently described embodiments may be incorporated; 
         FIG. 3  illustrates velocity profiles utilized in connection with the presently described embodiments; 
         FIG. 4  is a graph illustrating peak velocities; 
         FIG. 5  is a graph illustrating normalized peak velocities; 
         FIG. 6  is a graph illustrating peak velocities normalized for registration time; 
         FIG. 7  is a graph showing approximation errors; and, 
         FIG. 8  is a flow chart illustrating a method according to the presently described embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The velocity registration problem can be transposed. That is, rather than prescribing the motion of the sheet, one can prescribe the motion of the center of the wheels on the sheet. 
     This approach is illustrated in the  FIG. 2 . It should be understood that the system  10  of  FIG. 2  is substantially the same as that of  FIG. 1 . For ease of viewing, the sensors are not shown in  FIG. 2 , but are understood to be incorporated in the system. 
     The equations that describe the path are as follows: 
     
       
         
           
             
               
                 ⅆ 
                 s 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 
                   
                     V 
                     0 
                   
                   + 
                   
                     V 
                     1 
                   
                 
                 2 
               
               = 
               
                 V 
                 AVG 
               
             
           
         
       
       
         
           
             
               
                 ⅆ 
                 β 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 
                   V 
                   0 
                 
                 - 
                 
                   V 
                   1 
                 
               
               D 
             
           
         
       
       
         
           
             
               
                 ⅆ 
                 x 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 V 
                 AVG 
               
               ⁢ 
               cos 
               ⁢ 
               
                   
               
               ⁢ 
               β 
             
           
         
       
       
         
           
             
               
                 ⅆ 
                 y 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 V 
                 AVG 
               
               ⁢ 
               sin 
               ⁢ 
               
                   
               
               ⁢ 
               β 
             
           
         
       
         
         
           
             s=progress along the path of wheel center 
             β=angle of the path of wheel center 
             D=distance between the wheels 
             x=coordinate of the path of wheel center 
             y=coordinate of the path of wheel center 
           
         
       
    
     These equations can be integrated in closed form only for small values of the angle β. 
     The presently described embodiments are directed to a method and system for improving sheet registration in a document processing device. The presently described embodiments implement a technique the produces accurate results with merely a small tail wag. To do so, the method ultimately establishes or determines a variety of parameters (e.g. lateral position of a sheet, skew, registration time, nominal sheet velocity, and correction velocity). These parameters are then used by the system to calculate a lateral velocity profile. In this regard, the calculated velocity profiles (such as that determined using the method of  FIG. 8 ) are applied to the wheels or nips, in the paper path. Thus, the wheels can be controlled and will allow for improved sheet registration in the document processing device. 
     With reference to  FIG. 3 , examples of profiles used in a method of the presently described embodiments are shown. From these profiles, and the processes described in connection with  FIGS. 4-6 , sufficient information regarding the velocity profile of the paper path can be determined and used in connection with a method, such as that described in  FIG. 8 . It should be understood that the graph generated as a result of this analysis in  FIG. 6 , is used in the method of  FIG. 8  to determine a corrected velocity profile for the system. This, of course, improves the sheet registration process, as noted above. 
     The profile of  FIG. 3  comprises the following elements: 
     1. A nominal velocity Vel Nom (line A) 
     2. A process direction correction velocity (line B) with amplitude, VPro 
     3. A skew correction velocity (lines C and D) for inboard and outboard wheels, Skew Vel 0  and Skew Vel 1 ; 
     4. A lateral correction velocity (lines E and F) for inboard and outboard wheels, Lat Vel 0  and Lat Vel 1 . 
     The sum of these elements result in the velocities of the inboard and outboard wheels (lines G and H), Total Vel 0  and Total Vel 1 . Note that the amplitude of the skew correction velocity (lines C and D) and process correction velocity (line B) can be calculated in closed form. This is not true for the lateral profile. Also note that the acceleration of the lateral profile is by far the largest. It is the dominant contributor to the inertial forces that the sheet exhibit onto the wheels. Hence, this lateral acceleration is selected to be a constant and its value is set by maximum sheet force requirements. 
     A solution method for agile profile generation is presented below. 
     1. The lateral acceleration ‘acc’ is assumed constant. 
     2. Vary the process direction registration time ‘Tee’ from the nominal (160 ms in this example) by [−20, 0, +20] ms. 
     3. Calculate amplitude of process direction correction (line B), VPro. 
     4. Calculate skew profile amplitude (lines C and/or D) for a range of input skew ‘skw’=[−25, 0, +25] mrad. 
     5. Impose a range of lateral correction amplitudes (lines E and/or F) from −acc*Tee/4 to +accTee/4 in increments of accTee/128. Note that accTee/4 is the maximum amplitude that can be obtained. 
     The lateral corrections are then calculated for the above variation from the equations noted above. The results are shown in  FIG. 4 . 
     The different shapes of the data points correspond to different registration times (diamonds=140 ms, squares=160 ms and circles=180 ms) in  FIGS. 4-7 . The clusters of data points in three apparent lines for each color correspond to different skew (skw) values (−25, 0, +25 mrad).  FIG. 4  shows how the amount of lateral correction varies with the peak of the lateral correction triangular profile. 
     Next, with reference to  FIG. 5 , the following operation is performed on all the y-values (Vel Nom is the nominal velocity, C is a factor found by trial and error to give the best fit, y new  is a new lateral position, y old  is an old or current lateral position).
 
 y   new   =y   old   −C *skw*Tee*(Vel Nom+ V Pro/2)/2
 
     As shown in  FIG. 5 , this makes all the plots for the different skews overlap. The dashed line  51  corresponds to a 180 ms registration time. The dash-double dot line  53  corresponds to 160 ms registration time, And, the dash-dot line  55  corresponds to a 140 ms. 
     Next, with reference to  FIG. 6 , the peak of the lateral correction profile is normalized by (x-axis in the figure above) diving it by the registration time ‘Tee’. Also, the y-values are divided by Tee 1.5 . The result is shown in  FIG. 6 . 
     Note that the normalization process makes data points almost coincide. For convenience, this is illustrated as large dots that coincide with no apparent distinction. If the y-values are averaged at different x-locations, then there is a single curve that is useful. 
     It should be understood that the methods and techniques of the presently described embodiments may be implemented using a variety of software routines and/or hardware configurations. For example, software routines reflecting, for example, the method set forth in  FIG. 8  or others according to the presently described embodiments (such as methods described in connection with  FIGS. 2-6 ), may reside in and be implemented by a controller, such as controller  12  of  FIGS. 1 and 2 . Of course, the software may also reside on other elements in a printing environment or be distributed among suitable elements in such a printing environment. 
     With reference now to  FIG. 8 , a procedure or method  800  to obtain the amplitude of the lateral correction profile for a given set of input conditions is as follows: 
     Initially, a variety of input parameters are provided to the controller. For example, input conditions of lateral position of the sheet y (lat) (at  804 ), a skew of the sheet (skw) (at  806 ), a desired registration time, Tee (at  802 ), a nominal velocity, Vel Nom (at  808 ) and a process correction velocity (at  810 ). 
     It should be understood that the lateral position y is determined through implementation of the side edge or lateral sensor illustrated in  FIG. 1 . The output of this sensor provides a lateral position of the sheet  14  as it progresses down the paper path. In one form, the lateral position is measured at a point in time when the wheels or nips of the paper path obtain control of the sheet. 
     The measured skew (skw) is computed by determining the difference in times that the leading edge sensors detect the sheet  14 . So, sensors LEA and LEB provide the time at which the sheet  14  is detected by the sensors. The difference in time detected by these sensors is then multiplied by the sum (V A +V B )/2, and then divided by the spacing between the sensors LEA and LEB. This provides a measure that is in radians, or an angle of the skew. 
     The registration time, Tee, is established to be the target delivery time from the point at which the leading edge sensors detect the sheet  14  to the arrival time (i.e., delivery time) to the appropriate downstream device in the paper path. The nominal velocity, Vel Nom, is an average of the speed of travel of the sheet on the paper path. The process correction velocity, VPro, is an amplitude of process correction velocity. 
     Also, a constant, C, is used in the equation above and below. This constant is determined through experimentation and varies by families of machines. The constant is dependent upon the geometry of the system, wheel spacing, . . . etc. 
     Referring back to  FIG. 8 , these input parameters are then used to compute a first value: y new =y old +C*skw*Tee*(Vel Nom+VPro/2)/2 (at  812 ) where skw is measured. y new  is a new lateral position and y old  is an old or current lateral position. Next, a second value is computed: y/(Tee 1.5 ) (at  814 ). Using  FIG. 6 , a corresponding value Vstar on the x-axis is determined by linear interpolation. A peak of the lateral correction profile is then calculated by multiplying by Tee (at  816 ). 
     Thus, the lateral profile can then be determined (at  818 ). Note that skew and process correction velocities were calculated in closed form. Since the acceleration is held constant, this profile can be constructed. 
     With reference now to  FIG. 7 , error analysis was performed. The results are shown in the curve. The dots represent different values of skew and registration time. The diamonds represent a registration time of 140 ms. The squares represent a registration time of 160 ms. And, the circles represent a registration time of 180 ms. It should be appreciated that the error is less than 350 um for lateral moves up to 10 mm. 
     It will be appreciated that various 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.