Abstract:
A system and method for correcting the timing and spacing between envelopes being serially processed in a high speed mail processing inserter system, whereby a pitch correcting module receives sensor input detecting unwanted pitch variation between envelopes and a transport mechanism in the pitch correcting module accelerates or decelerates an envelope according to a pitch correcting profile calculation performed by the pitch correcting module, the pitch correcting module being dimensioned to optimally perform pitch correction without interfering with high speed mail processing.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a module correcting pitch between documents traveling in a high speed mass mail processing and inserting system. The term “pitch” refers to the spacing between documents traveling in an inserter system. Properly controlled and predictable document pitch is necessary for reliable operation of such high speed inserter systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Inserter systems such as those applicable for use with the present invention, are typically used by organizations such as banks, insurance companies and utility companies for producing a large volume of specific mailings where the contents of each mail item are directed to a particular addressee. Additional, other organizations, such as direct mailers, use inserts for producing a large volume of generic mailings where the contents of each mail item are substantially identical for each addressee. Examples of such inserter systems are the 8 series and 9 series inserter systems available from Pitney Bowes Inc. of Stamford Conn.  
           [0003]    In many respects the typical inserter system resembles a manufacturing assembly line. Sheets and other raw materials (other sheets, enclosures, and envelopes) enter the inserter system as inputs. Then, a plurality of different modules or workstations in the inserter system work cooperatively to process the sheets until a finished mail piece is produced. The exact configuration of each inserter system depends upon the needs of each particular customer or installation.  
           [0004]    Typically, inserter systems prepare mail pieces by gathering collations of documents on a conveyor. The collations are then transported on the conveyor to an insertion station where they are automatically stuffed into envelopes. After being stuffed with the collations, the envelopes are removed from the insertion station for further processing. Such further processing may include automated closing and sealing the envelope flap, weighing the envelope, applying postage to the envelope, and finally sorting and stacking the envelopes.  
           [0005]    An inserter system may typically include a right angle transfer module to perform a 90-degree change of direction of documents flowing through the inserter system. The right angle transfer module allows for different configurations of modules in an inserter system and provides flexibility in designing a system footprint to fit a floor plan. Such a right angle transfer module is typically located after the envelope-stuffing module, and before the final output modules. Right angle transfer modules are well known in the art, and may take many different forms.  
           [0006]    During processing, envelopes will preferably remain a regulated distance (or “pitch”) from each other as they a transported through the system. Also, envelopes typically lie horizontally, with their edges perpendicular and parallel to the transport path, and have a uniform position relative to the sides of the transport path during processing. Predictable envelope positioning helps the processing modules perform their respective functions. For example, if an envelope enters a postage-printing module crooked, it is less likely that a proper postage mark will be printed. For these reasons it is important to ensure that envelopes do not lie askew on the transport path, or at varying distances from the sides of the transport path.  
           [0007]    For this purpose, envelopes, or other documents, are typically urged against an aligning wall along the transport path so that an edge of the envelope will register against the aligning wall thereby straightening the envelope and puffing it at a uniform position relative to the sides of the transport path. This aligning function may be incorporated into a right angle transfer module, whereby a document may impact against an aligning wall as part of performing a 90-degree change of direction.  
           [0008]    Typically the envelope edge that is urged against the aligning wall is the bottom edge, opposite from the top flapped edge of the envelope. Thus after coming into contact with the aligning wall and being “squared up,” the envelope travels along the transport path with the left or right edge of the envelope as the leading edge.  
           [0009]    The action of impacting the bottom edge of the envelope against the aligning wall may also serve the purpose of settling the stuffed collation of documents towards the bottom of the envelope. By settling the collation to the bottom of the envelope it is more likely that no documents will protrude above the top edge of the envelope, and that the envelope flap can be closed and sealed successfully.  
           [0010]    Current mail processing machines are often required to process up to 18,000 pieces of mail an hour. Such a high processing speed may require envelopes in an output subsystem to have a velocity as fast as 85 inches per second (ips) for processing. Envelopes will nominally be spaced 200 ms apart for proper processing while traveling through the inserter output subsystem. At such a high rate of speed, system modules, such as those for sealing envelopes and putting postage on envelopes, have very little time in which to perform their functions. If spacing is not maintained between envelopes, the modules may not have time to perform their functions, envelopes may overlap, and jams and other errors may occur.  
           [0011]    For example, if the space between contiguous envelopes has been shortened, a subsequent envelope may arrive at the postage metering device before the meter has had time to reset, or perhaps even before the previous envelope has left. As a result, the meter will not be able to perform its function on the subsequent envelope before a subsequent envelope arrives, and the whole system may be forced to a halt. At such high speeds there is very little tolerance for variation in the spacing between envelopes.  
           [0012]    Other potential problems resulting from excess variation in distance between envelopes include decreased reliability in diverting mechanisms used to divert misprocessed mail pieces, and decreased reliability in the output stacking device. Each of these devices have a minimum allowable distance between envelopes that may not be met when unwanted variation occurs while envelopes travel at 85 ips.  
           [0013]    Jam detection within the aligning module may become difficult to manage as a result of excess pitch variation. Jam detection is based on theoretical envelope arrival and departure times detected by tracking sensors along the envelope path. Variability in the aligner module will force the introduction of wide margins of error in the tracking algorithm, particularly for start and stop transport conditions, making jam detection less reliable for that module.  
           [0014]    Pitch variation occurs for a number of reasons. One source of variation can be an aligner module for a high-speed inserter system, as described above. As envelopes in a high speed mailing system impact the conventional aligner wall, the impact causes the envelopes to decelerate in a manner that may cause the gap between envelopes to vary as much as +/−30 ms. While such a variation might not be significant in slower machines, this variation can be too much for the close tolerances in current high speed inserter machines.  
           [0015]    In addition to variation resulting from impacts at the aligner module, variation may be the result of “dither” in the transport of stuffed envelopes. Different envelopes may be stuffed with different quantities of sheets that form the individual mail pieces. As a result, envelopes will vary in weight. Such variation in weight will cause envelopes to have different acceleration, momentum and frictional forces acting upon them as they are transported in the inserter output subsystem. For example, different envelopes will experience different slippage as transport mechanisms such as rollers and belts are used to transport them. Accordingly, such dither may result in an additional +/−30 ms variation in the spacing between envelopes.  
           [0016]    The problem of non-deterministic behavior at the aligning module is addressed in a co-pending patent application entitled DETERMINISTIC ALIGNER FOR AN OUTPUT INSERTER SYSTEM, by John Sussmeier, filed on Oct. 18. 2001, Serial No. ______, (Attorney Docket F-318) and commonly assigned to the assignee of the present application. That application is hereby incorporated by reference in its entirety. The aligner system described in that application may be used in conjunction with the system described in the present application in order to minimize variation in spacing between envelopes traveling in an inserter output subsystem.  
           [0017]    The present application describes a system and a method to reduce variation in envelope pitch to further meet the needs and shortcomings of the conventional art described above.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention addresses the problems of the conventional art by providing a pitch correcting module (“PCM”). The pitch correcting module is positioned upstream of modules that are sensitive to variation in pitch, in order that such variations may be corrected before the envelopes reach those modules. The pitch correction module includes a transport mechanism, such as hard nip rollers, or conveyor belts, to speed up or slow down the transport of envelopes in order to correct pitch variations. The relative spacing of envelopes is preferably detected by sensors which sense envelopes entering and leaving the pitch correcting module. Based on input from the sensors, a processing device controls the transport mechanism of the PCM to speed up or slow down the envelope in accordance with a predetermined algorithm.  
           [0019]    The pitch correcting module is dimensioned to accommodate the varying envelopes sizes that the inserter system is designed to process, while at the same time maintaining the capability of the inserter system to operate at its designed speed, and to correct the range of expected unwanted variation. The PCM is also designed to provide the necessary accelerations and decelerations to achieve corrections within a range of expected pitch variations. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a diagrammatic view of a pitch correcting module in relation to upstream and downstream modules.  
         [0021]    [0021]FIG. 2 is a graphical representation for velocity profiles for performing dynamic pitch correction on envelopes.  
         [0022]    [0022]FIG. 3 is a diagrammatic view of spacing of key input and output locations for the pitch correcting module. 
     
    
     DETAILED DESCRIPTION  
       [0023]    As seen in FIG. 1, the present invention includes a pitch correcting module (PCM)  1  positioned between an upstream module  2  and a downstream module  3 . An example of upstream module  2  could be a right angle transfer, or an aligner module such as that described in the co-pending U.S. patent application number ______ of Sussmeier (Attorney Docket, F-318), incorporated in its entirety. An exemplary downstream module  3  could be a diverting module, a metering module, or a stacking module, each of which includes a sensitivity to pitch variation. Besides these examples, upstream and downstream modules  2  and  3  can be any kinds of modules in an inserter output subsystem.  
         [0024]    PCM  1 , upstream module  2 , and downstream module  3 , all include transport mechanisms for moving envelopes along the processing flow path. In the depicted embodiment, the modules use sets of upper and lower rollers  10 , called nips, between which envelopes are driven in the flow direction. In the preferred embodiment rollers  10  are hard-nip rollers to minimize dither. As an alternative to rollers  10 , the transport mechanism may comprise overlapping sets of conveyor belts between which envelopes are transported.  
         [0025]    The rollers  10  for PCM  1 , and modules  2  and  3  are driven by electric motors  11 ,  12 , and  13  respectively. Motors  11 ,  12 , and  13  are preferably independently controllable servo motors. Motors  12  and  13  for upstream and downstream modules  2  and  3  drive their respective rollers  10  at a constant velocity, preferably at the desired nominal velocity for envelopes traveling in the system. Accordingly, upstream and downstream modules  2  and  3  will transport envelopes at 85 ips in the flow direction.  
         [0026]    Motor  11  drives rollers  10  in the PCM  1  at varying speeds in order to provide pitch correction capabilities. When no pitch correction is required PCM  1  will transport envelopes at the same velocity as the upstream and downstream modules  2  and  3 . PCM motor  11  is controlled by controller  14  which in turn receives sensor signals including signals from upstream sensor  15  and downstream sensor  16 . Sensors  15  and  16  are preferably used to detect the trailing edges of consecutive envelopes passing through the PCM  1 . By receiving sensor signals indicating the trailing edges of envelopes, controller  14  can calculate the pitch between consecutive envelopes and adjust the speed of PCM motor  11  to correct variance from a nominal desired pitch.  
         [0027]    While a single sensor could be used to detect the pitch between consecutive envelopes, the preferred embodiment of the present invention utilizes at least two sensors  15  and  16 , one positioned near each of the boundaries between PCM  1  and the upstream and downstream modules  2  and  3 . Such sensors are preferably photo sensors that detect the trail edge of envelopes. By comparing sensor signals corresponding to consecutive envelopes, actual pitch between envelopes is calculated in terms of time and/or displacement. The preferred positioning of the sensors, and the utilization of signals received from the sensors is discussed in more detail below.  
         [0028]    One aspect of the present invention relates to the relative positioning of the transport mechanisms between PCM  1  and the other modules. Referring to FIG. 1, the location of the output of the transport for upstream module  2  is location A. The location for the input to the transport of PCM  1  is location B, and the output of the transport mechanism for PCM  1  is location C. The input for the transport of downstream module  3 .  
         [0029]    In the exemplary embodiment shown in FIG. 1, the transport mechanisms are nip rollers  10  for each of the modules. Accordingly locations A, B, C, and D correspond to the respective locations of input and output nip rollers  10  in that embodiment. The modules may also include other rollers  10  at other locations, such as the set depicted in FIG. 1 between locations B and C, also driven by motors  11 ,  12 , and  13  for the respective modules. In the example depicted in FIG. 1, the three nip rollers sets  10  in PCM  1  will be driven by motor  11 . To maintain control over envelopes traveling through the system, consecutive distances between rollers  10  must be less than the shortest length envelope expected to be conveyed. In the preferred embodiment, it is expected that envelopes with a minimum length of 6.5″ will be conveyed. Accordingly and the rollers  10  will preferably be spaced 6.25″ apart, so that an envelope can be handed off between sets of rollers  10  without giving up control transporting the envelope at any time.  
         [0030]    Upstream sensor  15  is preferably located at or near location A, while downstream sensor  16  is preferably located at or near location C. As mentioned above, pitch computation could be accomplished using one sensor, however in the preferred embodiment pitch correction is calculated after a downstream envelope has received its pitch correction via PCM  1 , and has exited PCM  1  from the nip rollers  10  at location C. In that way, PCM can perform corrections on envelopes one-at-a-time and perform pitch correction operations separately for consecutive envelopes. This arrangement simplifies the calculations to be done by controller  14  in adjusting the speed of PCM  1  to make the appropriate corrections between consecutive envelopes.  
         [0031]    Downstream sensor  16  detects the departure of an envelope from PCM  1  as it exits the rollers  10  at location C. Subsequently, upstream sensor  15  detects the arrival of a new envelope for which control is being transferred from the upstream module  2  to PCM  1 . Controller  14  receives the sensor information and, based on the desired nominal speed and spacing of the envelopes, determines a variation in the measured pitch from the nominal expected pitch.  
         [0032]    Envelopes that arrive later than the desired pitch are accelerated by PCM  1  and then decelerated back to the constant velocity of the downstream module  3  before the lead edge of the envelope reaches location D. This motion has the effect of advancing the envelope closer to the previous downstream envelope.  
         [0033]    Conversely, envelopes that arrive earlier than the desired pitch are decelerated by PCM  1  and then accelerated back to the constant velocity of the downstream module  3  before the lead edge of the envelope reaches location D. This motion has the effect of retarding the envelope relative to the previous downstream envelope.  
         [0034]    The necessary advancing and retarding action of PCM  1  is controlled according to a motion profile calculated by controller  14 . Motion profiles are individually calculated for each envelope as a function of the pitch information collected by sensors  15  and  16 .  
         [0035]    Referring to FIG. 2, exemplary motion profiles are illustrated for both an envelope advance profile and an envelope retard profile. This figure depicts graphs showing the velocity of the envelope as a function of time while passing through PCM  1 . Acceleration of the envelope is represented by the slope of the lines. V transport  represents the nominal velocity of the transports in the system, preferably 85 ips. T correction  represents the time during which pitch correction is executed by PCM  1 . The area under the velocity curve during T correction  represents the displacement of the envelope during pitch correction.  
         [0036]    In FIG. 2, the area represented by the rectangle below V transport  represents the displacement of the envelope (X nominal ) as if it were traveling at nominal speed. However, this displacement must be increased or decreased in order to perform pitch correction. Accordingly, in FIG. 2, X correction  represents the area of the increased or decreased displacement above or below the X nominal  value resulting from the corresponding acceleration and deceleration.  
         [0037]    The retard profile is illustrated in FIG. 2 using accelerations that are less than that of the advance profile to illustrate a correction that is allowed to occur over a longer pitch correction time, T correction .  
         [0038]    It should be noted that although FIG. 2 depicts pitch correction motion profiles having constant acceleration and deceleration values of equal magnitudes, it is not necessary that a motion profile have those characteristics. Rather, the motion profile may take any form, so long as it achieves the required displacement correction within the limited time and space available.  
         [0039]    The preferred embodiment of the present invention, however, does use constant acceleration and deceleration in the manner depicted in FIG. 2. Accordingly, in the preferred embodiment an envelope undergoing pitch correction will undergo acceleration and deceleration of equal magnitudes for half of the envelope travel distance within PCM  1 . Using the motion profile with linear segments, the calculation for determining accelerations for achieving displacements can be calculated easily by calculating the slope of the lines representing velocity necessary to achieve the desired displacement. If non-linear acceleration is used, the appropriate calculations can be more complicated, but may be achieved using known integration algorithms.  
         [0040]    The pitch correcting profiles as depicted in FIG. 2 are designed to begin when the tail end of the envelope to be pitch corrected exits the upstream module  2  at location A and to end when the lead edge of the envelope reaches the downstream modules  3  at location D. This methodology minimizes the accelerations and deceleration required during the pitch correction profile, thereby minimizing the heating of PCM motor  11 .  
         [0041]    When performing pitch correction on an envelope, PCM  1  must have total control of the envelope. For example, the envelope cannot reside between nip rollers  10  at location A or D during execution of the pitch correcting profile. Additionally, in the preferred embodiment, envelopes upstream and downstream of the envelope being pitch corrected must be completely out of PCM  1 , i.e., they cannot reside anywhere between nip rollers  10  between locations B and C during the execution of the pitch correcting profile. Accordingly, in the preferred embodiment, PCM  1  will only perform the pitch correcting profile (1) after the trail edge of the envelope to be pitch corrected has exited upstream module  2  at location A; and (2) after the trail edge of the downstream envelope has exited PCM  1 . Similarly, in the preferred embodiment, PCM  1  must complete the pitch correcting profile (1) before the lead edge of the upstream envelope has reached PCM at location B; and (2) before the lead edge of the envelope to be pitch corrected has reached the downstream module  3  at location D.  
         [0042]    In practice, these requirements will limit the range of lengths for PCM  1  in order that it can process envelopes of the desired sizes at the desired speed. The pitch correcting system must be able to process minimum and maximum specified envelope lengths and correct the pitch in the anticipated worst case error condition.  
         [0043]    [0043]FIG. 3 depicts relative locations of elements in the pitch correcting system for determining an appropriate size for PCM  1  to achieve the desired functionality. As discussed previously, the nip rollers  10  at locations B and C are the input and output to the transport mechanism for PCM  1 . The nip rollers  10  at locations A and D are the output from the upstream module  2  and the input to the downstream module  3 , respectively. FIG. 3 further depicts a maximum size envelope  20  as it comes under full control of PCM  1 .  
         [0044]    In the preferred embodiment, the minimum and maximum expected envelope lengths are 6.5 and 10.375 inches respectively. As discussed above, in order to always maintain control of the smallest envelope, the distance between location A and B (L up ) and the distance between location C and location D (L down ) will be 6.25″ in the preferred embodiment of the present invention. Additionally the analysis for determining the length of PCM  1  in the preferred embodiment assumes that the maximum anticipated correction is 30 ms, that the minimum desired period between envelopes is 200 ms, and that the velocity of the transports in upstream and downstream modules  2  and  3  is 85 ips.  
         [0045]    To determine the minimum length of PCM  1  (L pcmmin  in FIG. 3), PCM  1  must be able to complete the longest pitch correction profile to advance the envelope if it requires the larges anticipated correction. This calculation takes into account the longest envelope, because the longer the envelope, the shorter the available space within the PCM to perform the correction. The determination of L pcmmin  also depends on the maximum allowable acceleration based on the maximum torque characteristics of PCM motor  11  and the frictional characteristics of rollers  10  in PCM  1 .  
         [0046]    Based on the arrangement depicted in FIG. 3, the equation for determining minimum length for PCM  1  is:  
         
       L 
       pcmmin 
       =L 
       envmax 
       +X 
       travelreq 
       −L 
       up 
       −L 
       down  
     
         [0047]    X travelreq  is the total required distance traveled during the longest pitch correction profile as a function of the maximum allowable acceleration. Since the maximum expected correction is 30 ms at 85 ips, the necessary correction will require the envelope to be advanced an additional 2.55 inches over the nominal displacement while traveling in PCM  1 . Assuming a maximum acceleration of 8 G&#39;s, based on typical conservative limits for DC brushless motor systems, X travelreq  can be calculated by referring to the motion profile as shown in FIG. 2, and calculating the total distance to be traveled within PCM  1 . This calculation results in X travelreq  being 7.433 inches. Inserting the other values given above into the above equation for L pcmmin , the minimum length for PCM  1  is calculated to be 5.308 inches under the preferred conditions described herein.  
         [0048]    Although a maximum acceleration of 8G&#39;s has been selected for the preferred embodiment, this maximum may be increased or decreased based on the needs of the system. For example, if it is required that PCM  1  be capable of correcting variations greater than +/−30 ms, then a more robust, and more costly, electric motor may be used to achieve that greater acceleration. Conversely, if PCM  1  is to be used in a system that is intended to only correct lesser variations, a less robust, and potentially less expensive, electric motor may be used. It should be noted, however, that the acceleration characteristics of PCM motor  11  impact the minimum size of PCM  1 .  
         [0049]    Again referring to FIG. 3, the maximum length of PCM  1 , (L pcmmax  on FIG. 3), is determined by calculating the maximum length of PCM  1  before the tail end of an upstream envelope will exit the upstream module  2  at location A before the tail end of the downstream envelope exits PCM  1  at location C. Expressed as an equation:  
           L   pcmmax   =X   pitchmin −L up ,  
         [0050]    where X pitchmin  is the minimum expected distance between envelopes resulting from unwanted variation.  
         [0051]    Substituting in the quantities for the preferred embodiments given above, the value of L pcmmax  is 8.200 inches. It should be noted that this calculation does not depend on the size of the envelope, but rather the expected minimum pitch between consecutive envelopes.  
         [0052]    Controller  14  of PCM  1  is programmed to determine an appropriate pitch correcting profile, as shown, for example, in FIG. 2, for pitch variations detected by sensors  15  and  16 . Based on the calculated pitch correcting profile rollers  10  of PCM  1  are controlled to accelerate and decelerate accordingly in order to achieve the desired displacement correction.  
         [0053]    In the preferred embodiment controller  14  calculates the pitch correcting profile based on the physical constants of PCM  1  and the detected pitch variation. The algorithm for the preferred embodiment assumes that upstream and downstream sensors  15  and  16  are located at or near locations A and C respectively. If the upstream sensor is located upstream of location A, the pitch correcting profile begins when the tail end of the envelope reaches location A. If the upstream sensor  15  is located downstream of location A, then the pitch correcting profile begins when the tail end of the envelope reaches upstream sensor  15 .  
         [0054]    The following are fixed physical variables for all pitch correcting profile calculations:  
         [0055]    L pcm =distance from the transport mechanism input to the transport mechanism output in PCM  1 ;  
         [0056]    L up =separation distance between the output of the upstream module  2  transport to the input of PCM  1 ; preferred value=6.25″;  
         [0057]    L1=distance upstream sensor  15  is located downstream of location A (negative value if located upstream of A);  
         [0058]    L2=distance downstream sensor  16  is located of location C (negative value if located upstream of C);  
         [0059]    For L1&gt;0; L upmod =L up −L1 (and pitch correcting profile begins when the tail end of the envelope reaches the upstream sensor  15 ; otherwise L upmod =L up  (and pitch correcting profile begins when the tail end of the envelope reaches location A).  
         [0060]    The following are fixed physical variables and calculations for a job run, and their preferred values, are:  
         [0061]    T desiredperiod =desired period between envelope leading edges; preferred value=200 ms;  
         [0062]    T dithermax =maximum anticipated time between envelopes under normal conditions expected at PCM  1 ; preferred value=230 ms;  
         [0063]    T dithermin =minimum anticipated envelope between envelopes under normal conditions expected at PCM  1 ; preferred value=170 ms;  
         [0064]    V transport =nominally constant velocity of upstream and downstream modules  2  and  3 ; preferred value=85 ips;  
         [0065]    L sensors =L up +L pcm +L2−L1;  
         [0066]    X pitchnom =V transport *T desiredperiod    
         [0067]    X pitchmax =V transport *(T desiredperiod −T dithermax )  
         [0068]    X pitchmin =V transport *(T desiredperiod −T dithermin )  
         [0069]    X travel =L upmod +L pcm +L down −L env    
         [0070]    Input variable that changes for every envelope processed:  
         [0071]    X=distance the upstream module motor  12  translated from the instant the tail end of downstream envelope reached the downstream sensor  16  to the instant the upstream envelope tail end reached upstream detector  15 .  
         [0072]    Calculation for determining the actual pitch between envelopes:  
         [0073]    X pitchactual =L sensors +X  
         [0074]    Finally, the following calculations provide the preferred embodiment for determining the accelerations to perform a pitch correcting motion profile of the type as shown in FIG. 2.  
         [0075]    If X pitchactual ≧X pitchmax , then Accel1=maximum acceleration, and Accel2=−Accel1; or  
         [0076]    If X pitchactual ≦X pitchmin , then Accel1=maximum deceleration, and Accel2=−Accel1; otherwise  
         Accel1   =       (       X   pitchactual     -     X   pitchnom       )         (       (       X   travel     -     X   pitchactual     +     X   pitchnom       )     /     (     2   *     V   transport       )       )     ^   2         ;     and                           
 
         [0077]    Accel2=−Accel1; and  
         [0078]    X1=X2=X travel /2  
         [0079]    As shown in FIG. 2, Accel1 and Accel2 are the accelerations used for each of the two segments of the pitch correcting profile and X1 and X2 are the corresponding total distances traveled during each acceleration segment.  
         [0080]    It should be noted that although the above described embodiment preferably calculates displacement, a time based methodology can be substituted. A displacement based methodology is preferred because distance relationships between envelopes and modules can be preserved, even during start-up and stopping conditions.  
         [0081]    The above algorithm for correcting pitch assumes that distances between consecutive envelopes is being measured. However, during a start up of a new series of envelopes, there will be no prior envelope. Under those circumstances, the controller  14  is programmed to recognize the first envelope of a series of envelopes in a job run. Similarly, if an envelope is diverted upstream of PCM  1 , a larger than expected gap may be encountered before a subsequent envelope arrives. Accordingly, in the preferred embodiment, any envelope that arrives at PCM  1  one or more cycles late will be defined as a first envelope. As a result of the preferred sensor arrangement described above, controller  14  will not be able to tell whether the first envelope has been subject to unwanted variation.  
         [0082]    In the preferred embodiment, controller  14  is programmed to always treat a “first envelope” as if it has arrived late by the maximum expected time variation. As a result of this assumption, the first envelope will always be given a forward correction displacement by PCM  1 . If this assumption was not made, and the envelope was in fact late, then the second envelope might be too close behind to be properly corrected. Because there is no envelope in front of the first envelope, there is no danger that unnecessarily advancing the first envelope will cause it to come too close to an envelope in front of it.  
         [0083]    In an alternative embodiment, instead of assuming that the first envelope is late, the first envelope of a series of envelopes could be tracked as it travels through the inserter output subsystem. The system can be programmed to sense when the first envelope enters the inserter output subsystem, and to record a position or time stamp. Nominal arrival times (or displacements) can be established for the arrival of the first envelope at various downstream locations. Sensors detect the arrival of the envelope at the various locations and it is can be determined whether, in fact, the first envelope is traveling more slowly than nominally desired. If the first envelope is not late to PCM  1 , then no advancing displacement acceleration need be applied. This method has the advantage of potentially decreasing motor heating of PCM motor  11  by not requiring it to accelerate unnecessarily. A potential disadvantage to this method is that different style envelopes are not likely to all have the same nominal travel times.  
         [0084]    The present invention may also be utilized to correct variations larger than can be handled by a single PCM. If pitch corrections to be performed are too large for a single PCM  1  to correct, then additional PCM modules can be serially arranged to provide cascading pitch correcting profiles.  
         [0085]    In another alternative embodiment, rollers  10  at location A can be a soft nipped. Under that arrangement, hard-nipped rollers at location B could take control of an envelope before it was completely out of the control of rollers at location A. As a result, the size of PCM  1  will not be limited in the manner described above, and PCM  1  can effectively be made up of one set of rollers  10 , and be very short in length. However, soft nipped rollers at location A introduce additional variation into the system which can make correction less reliable.  
         [0086]    Although the invention has been described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the spirit and scope of this invention.