Patent Publication Number: US-6217020-B1

Title: Method and apparatus for detecting proper mailpiece position for feeding

Description:
FIELD OF THE INVENTION 
     The invention disclosed herein relates generally to an apparatus for feeding and separation of mixed mailpieces and, more particularly, to an apparatus and method for detecting and providing proper position of a stack of mail pieces. 
     BACKGROUND 
     The processing and handling of mailpieces consumes an enormous amount of human and financial resources, particularly if the processing of the mailpieces is done manually. The processing and handling of mailpieces not only takes place at the Postal Service, but also occurs at each and every business or other site where communication via the mail delivery system is utilized. That is, various pieces of mail generated by a plurality of departments and individuals within a company need to be collected, sorted, addressed, and franked as part of the outgoing mail process. Additionally, incoming mail needs to be collected and sorted efficiently to ensure that it gets to the addressee in a minimal amount of time. Since much of the documentation and information being conveyed through the mail system are critical in nature relative to the success of a business, it is imperative that the processing and handling of both the incoming and outgoing mailpieces be done efficiently and reliably so as not to negatively impact the functioning of the business. 
     In view of the above, various automated mail handling machines have been developed for processing mail (removing individual pieces of mail from a stack and performing subsequent actions on each individual piece of mail). However, in order for these automatic mail handling machines to be effective, they must process and handle “mixed mail.” The term “mixed mail” is used herein to mean sets of intermixed mailpieces of varying size, thickness, and weight. In addition, the term “mixed mail” also includes stepped mail (i.e. an envelope containing therein an insert which is smaller than the envelope to create a step in the envelope), tabbed and untabbed mail products, and mailpieces made from different substrates. Thus, the range of types and sizes of mailpieces which must be processed is extremely broad and often requires tradeoffs to be made in the design of mixed mail feeding devices in order to permit effective and reliable processing of a wide variety of mixed mailpieces. 
     In known mixed mail handling machines which separate and transport individual pieces of mail away from a stack of mixed mail, the stack of “mixed mail” is first loaded onto some type of conveying system for subsequent sorting into individual pieces. The stack of mixed mail is moved as a stack by an external force to, for example, a shingling device. The shingling device applies a force to the lead mailpiece in the stack to initiate the separation of the lead mailpiece from the rest of the stack by shingling it slightly relative to the stack. The shingled mailpieces are then transported downstream to, for example, a separating device which completes the separation of the lead mailpiece from the stack so that individual pieces of mail are transported further downstream for subsequent processing. In the mailing machine described immediately above, the various forces acting on the mailpieces in moving the stack (shingling the mailpieces, separating the mailpieces and moving the individual mailpieces downstream) often act in a counterproductive manner relative to each other. For example, inter-document stack forces exist between each of the mailpieces that are in contact with each other in the stack. The inter-document stack forces are created primarily by the weight of the stack and additionally by the stack advance mechanism, the frictional forces between the documents, and potentially electrostatic forces that may exist between the documents. The inter-document forces tend to oppose the force required to shear the lead mailpiece from the stack. Additionally, the interaction of the force used to drive the shingled stack toward the separator and the forces at the separator can potentially cause a thin mailpiece to be damaged by being buckled as it enters the separator. Furthermore, in a conventional separator, there are retard belts and feeder belts that are used to separate the mailpiece from the shingled stack. Both the forces applied by the retard belts and the feeder belts must be sufficient to overcome the inter-document forces previously discussed. However, the force of the retard belts cannot be greater than the force of the feeder belts, or the mailpieces will not be effectively separated and fed downstream to another mail processing device. Moreover, if the feeding force being applied to the mailpieces for presenting them to the separator is too great, another potential problem which may occur is that a plurality of mailpieces will be forced through the separator without the successful separation of the mailpieces. 
     Another condition that affects the feeding of mailpieces is vertical orientation of the stack of mixed mail. The preferred orientation in which the most successful feeding occurs is when mail is leaning slightly against a paddle of the stack feeding device. When the mail is in this orientation, stack forces created by the weight of the mail are very low, and the mail is more easily separated and aligned for feeding downstream into, for example, a separating device. The high stack forces are created by improperly loaded mail stacks or mail stacks that have shifted creating improper lean. The shifting can be caused by the inertia of the stack as it a incrementally advances to the shingling device. The high forces of the stack may also cause damage to mailpieces as they are fed out of the stack and can cause greater wear on the nudger or feed rollers. The high stack forces can also cause multi-feeds. Additionally, improper alignment of leaning mail along the feed path to the separating device can cause the mail to stub as it enters the separating device and may also cause the mail to skew. 
     Thus, one of the problems of the prior art is that there can be failure to feed the mailpieces. Another problem of the prior art is that there can be poor separation of mail. Another problem of the prior art is that mail can be damaged by stubbing. Still another problem of the prior art is that multifeeds can occur when feeding mail. Yet another problem of the prior art is that stack forces can cause increased wear on feed rollers. 
     SUMMARY OF THE INVENTION 
     This invention overcomes the disadvantages of the prior art by providing a method and apparatus for detecting and providing proper mailpiece position when feeding mixed mailpieces. This in turn affords better mailpiece processing. The present invention is directed, in a general aspect, to a nudger for a mixed mail feeder and, in particular, to an apparatus and method of providing and detecting proper position in a stack of mixed mail. The apparatus generally comprises a nudger arm for detecting proper positioning of the mailpiece and a lean detection arm for detecting proper lean of the mailpiece with respect to the nudger. The apparatus helps to correct mailpiece lean which can cause the mailpiece not to feed. The method comprises, generally, a determination that when both the lean detection arm and the nudger arm are in a position indicating that the lead mailpiece is in the proper position, the stack of mixed mail is decelerated and fed to, for example, a separator, for further processing. The deceleration is performed at a slow rate and provides for a predetermined amount of over travel by the stack of mixed mail. This ensures proper contact of the lead mailpiece with the nudger rollers for feeding the mailpieces for further processing. The nudger rollers continue to feed the lead mailpieces until one or both of the lean detection arm and the nudger arm move out of the position(s) for proper mailpiece feeding, or a leading edge of the mailpiece blocks a downstream sensor. 
     Thus, an advantage of the present invention is that there is less failure to feed the mailpieces. Another advantage of the present invention is that consistent proper positioning of mailpieces for feeding is provided. Another advantage of the present invention is that less mailpiece damage occurs. Another advantage of the present invention is that less multi-feeds occur. Another advantage of the present invention is that there is less wear on feed rollers. Other advantages of the invention will in part be obvious and will in part be apparent from the specification. The aforementioned advantages are illustrative of the advantages of the various embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a presently preferred embodiment of the invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. 
     FIG. 1 is a perspective view of the inventive mail handling machine. 
     FIG. 2 is an enlarged to plan view of FIG.  1 . 
     FIG. 3 is an enlarged detailed view of the nudger wall of FIG.  1 . 
     FIG. 4 is an enlarged top plan view partially in section along line V—V of FIG. 3 showing details of the nudger roller drive system. 
     FIG. 5 is a perspective view of a portion of the mail handling machine illustrating an embodiment with a lean detection arm, tapered nudger rollers, a continuous belt and a leaning lead mailpiece (with dashed lines). 
     FIG. 6 is a perspective view of a portion of the mail handling machine of FIG. 5 illustrating the nudger arm, the lean detection arm and the nudger rollers. 
     FIG. 7 is a simplified top view of the portion of the mail handling machine of FIG.  5 . 
     FIG. 8 is an alternate embodiment of the simplified top view of FIG. 8 illustrating an alternate configuration of the lean detection arm. 
     FIG. 9 a  is a top view of the lean detection arm. 
     FIG. 9 b  is a right side view of the lean detection arm. 
     FIG. 10 a  is a simplified front view of an embodiment of the mail handling machine illustrating mailpiece lean (inward against the nudger wall) against the lean detection arm of FIG.  5 . 
     FIG. 10 b  is a simplified front view of an embodiment of the mail handling machine illustrating mailpiece lean against the lean (outward away from the nudger wall) detection arm of FIG.  5 . 
     FIG. 11 is a flowchart illustrating the method of accelerating and decelerating the stack of mailpieces. 
     FIG. 12 is a time v. velocity graph illustrating the movement of the advancing stack of mailpieces. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a mixed mail feeder  1  is shown. Mixed mail feeder  1 , as will be discussed in more detail below, separates individual mailpieces  3  from a stack of mixed mail generally designated at  5  and transports the individual mailpieces  3  to a subsequent mail processing station  7 . Mail processing station  7  can be any one of a plurality of devices such as a meter for printing postage on the mailpiece  3 , an OCR reader for reading addresses off the mailpiece  3 , a sorting device for sorting the individual mailpieces  3  to designated bins or areas, or even a scale that weighs the mailpiece. The key point is that the mixed mail feeder  1  functions to separate individual mailpieces  3  from a stack of mixed mail  5  and deliver the individual mailpieces  3  sequentially to the mail processing station  7 . 
     Mixed mail feeder  1  includes a table  9  upon which all of the components of the mixed mail feeder  1  are mounted. At an input end of the mixed mail feeder  1 , generally designated by the arrow  11 , the stack of mixed mail  5  is placed on edge by an operator in front of a guide wall  13 . Guide wall  13  acts as a support against which the stack of mixed mail  5  rests. Moreover, guide wall  13  includes a cylindrical portion  13   a  which is mounted to slide on a guide rod  15  fixedly attached to platform  10  which is mounted to table  9 . 
     Platform  10  has first and second slots  17 ,  19 , in a horizontal surface  21  thereof. The slots  17 ,  19  each permit a top portion of a respective individual continuous belt  23 ,  25  to project therethrough. Belts  23 ,  25  each have a plurality of individual track portions  27  over the full extent of the belts  23 ,  25 . The bottom of guide wall  13  removably fits in adjacent track portions  27  of each of belts  23  and  25  so that guide wall  13  moves with belts  23 ,  25  in the direction of arrow A (alternatively, a single belt can be used). Moreover, as guide wall  13  moves in the direction of arrow A with the belts  23 ,  25 , the cylindrical portion  13   a  slides along guide rod  15  to keep the standing orientation of guide wall  13  in the position shown in FIG.  1 . 
     Continuous belts  23 ,  25  are mounted in a conventional manner around a pulley at each end (not shown). One pulley is an idler pulley, while the other is driven by a motor  29 . The motor  29  drives a common shaft (not shown) connected to the drive pulleys of each of the belts  23 ,  25  such that the belts  23 ,  25  will be driven at the same velocity to move around their respective idler and driven pulleys. Thus, as the belts  23 ,  25  move around the pulleys in the direction of arrow A, the guide wall  13  moves therewith so that the entire stack of mixed mail  5  is moved toward a nudger wall  31 . As will be discussed in more detail below, the stack of mixed mail  5  will have individual mailpieces  3  moved from the stack of mixed mail  5  downstream so that the stack of mixed mailpieces  5  is continuously reduced in size. When the guide wall  13  has been moved to a point where it is desirable to add additional pieces of mixed mail to the stack  5 , the guide wall  13  can be lifted out of the individual tracks  27  of the belts  23 ,  25  by pulling the guide wall  13  up to rotate, via the cylindrical portion  13   a , about the guide rod  15 . Once the bottom of the guide wall  13  is clear of the individual tracks  27  of the belts  23 ,  25 , it can be slid backward in the opposite direction from that of arrow A and placed in a desired position to receive additional mixed mail. In an alternate embodiment, a single belt fitted with cogs may be used. In the alternate embodiment the mailpieces  3  in the stack of mixed mail  5  engages with the cogs on the belt and be driven toward the nudger wall  31 . 
     Referring to FIGS. 1,  2 , and  3 , nudger wall  31  includes a plurality of rollers  33  mounted therein in a conventional manner to be freely rotatable. Furthermore, nudger wall  31  has a cutout  35  in a lower corner thereof through which driven nudger rollers  37  project. Moreover, a plurality of roller bars  38  are rotatably mounted in a conventional manner in a slot  40  of platform  10 . Thus, as guide wall  13  pushes the stack of mixed mail  5  toward nudger wall  31 , individual pieces of mail  3  fall off the end of belts  23 ,  25  on top of the rollers  38  and into contact with the nudger rollers  37 . While in the preferred embodiment the roller bars  38  are not driven, they could be driven to provide additional forward feed force to the mailpiece  3 . In one embodiment, a continuous belt  36  (shown in FIG. 5) is driven around the roller bars  38 . Use of the continuous belt  36  provides a greater coefficient of friction as compared to the roller bars and thus  15  improves the feed force and provides for a simple drive structure. Additionally, the driven continuous belt  36  is helpful when mailpieces are being manually placed on the belt  36  since the drive helps to pull the mailpiece into the mixed mail feeder  1 . 
     The nudger rollers  37  are mounted to be driven into rotation within a nudger arm  39 . The four nudger rollers  37  are driven together by a motor  41 , mounted on nudger arm  39 , via a drive train  43  as shown schematically in FIG.  2  and in detail in FIG.  4 . As shown in FIGS. 2 and 4, all of the nudger rollers  37  are driven into rotation in a clockwise direction. Accordingly, as the stack of mixed mail  5  is moved toward nudger wall  31 , the lead mailpiece  3   a  is forced into contact with the nudger rollers  37 . The force of the driven nudger rollers  37  acts against the lead mailpiece  3   a  to move the mailpiece  3   a  in the direction of a conventional separator device  45 , thereby shingling the lead mailpiece  3   a  from the stack of mixed mail  5  as shown in FIGS. 1 and 2. The shingled mailpiece is then transported to the nip of separator  45  which operates in a conventional manner to separate the lead mailpiece  3   a  from the shingled stack and deliver it to take-away rollers  65  which transport the individual lead mailpiece  3   a  further downstream to mail processing station  7 . As is readily apparent to one skilled in the art, the microprocessor  61  controls all of the motors typically associated with the stack advance, shingling device, separator, and take away rollers and includes known clock structure for determining the predetermined time periods discussed above. The nudger rollers  37  continue to drive until a lead edge of the lead mail piece  3   a  is substantially through the separator device  45  where it is sensed by a sensor (not shown). Upon being sensed, the microprocessor  61  is signaled to stop the driving of the nudger roller  37 . The nudger rollers  37  have an over running clutch (not shown). Mailpieces pulled by the separator device  45  freely rotate the nudger rollers  37 . Disengaging the nudger rollers  37  reduces the amount of pullout force needed to pull the lead mailpiece  3   a  from the stack and produces less failures to feed. 
     Referring to FIGS. 3 and 4, the details of the drive system  43  are shown. Motor  41  has a shaft  41   a  connected to a pulley  42 . A continuous belt  44  is disposed around pulley  42  and a second pulley  46 . Pulley  46  is fixedly mounted to a rotatable shaft  48  mounted in nudger arm  39 . Also, fixedly mounted to shaft  48  is a third pulley  50 . Additional shafts  52 ,  54  are also rotatably mounted in nudger arm  39  and respectively have fourth and fifth pulleys  56 ,  58  fixedly mounted thereto. Nudger rollers  37  are mounted on a corresponding one of shafts  52 ,  54 . Accordingly, as motor  41  rotates pulley  42  in the clockwise direction of FIG. 4, pulley  46  and hub  48  are driven in the clockwise direction as well. Since a continuous belt  60  passes around pulleys  48 ,  56 , and  58 , shafts  52 ,  54  are forced to rotate in the clockwise direction causing a corresponding rotational movement in all of nudger rollers  37 . 
     In order for the nudger rollers  37  to effectively feed the stack of mixed mail  5  into the separator  45 , accurate control of the normal force applied to the stack of mixed mail  5  by the interaction of the guide wall  13  and the nudger rollers  37  needs to be achieved. The normal force is created by a spring  49  that is fixedly mounted at one end to the nudger wall  31  and at its other end to a mounting platform  50  of nudger arm  39 . The nudger arm  39  is pivotally mounted about a conventional pivot structure  51  so that the spring  49  biases the nudger rollers  37  through the cutout  35  and into contact with the lead mailpiece  3   a . Thus, as the guide wall  13  is advanced in the direction of the nudger wall  31 , the nudger arm  39  is forced to rotate in the clockwise direction of FIG. 2 around pivot structure  51  in opposition to the biasing force of the spring  49 . As the spring  49  is extended due to the rotation of nudger arm  39  about the pivot structure  51 , the force exerted by the spring  49  is continually increased by a known amount. The normal force is discussed in U.S. Pat. No. 5,971,391, assigned to the assignee of the present invention, and herein incorporated by reference. 
     A mechanism may be used to provide additional force in the situation where stalled mail is detected. That is, once the microprocessor  61  determines that a stall has occurred, utilization of a solenoid  71  (as shown in FIG. 2) provides additional normal force in an attempt to overcome the stalled situation. The solenoid  71  is fixedly mounted to the platform  9  and has one end fixedly mounted to a moveable plunger  75  of solenoid  71 . When the nudger arm  39  is positioned in the normal force operating range, the plunger of the solenoid is not extended, thereby providing no additional normal force. However, when stalled mail is detected, the microprocessor  61  energizes the solenoid  71  to withdraw the plunger  75  such that the plunger  75  is extended to provide an additional normal force to the mixed mail stack  5  via the nudger rollers  37 . The force applied by the solenoid  71  can be consistently applied for a predetermined period of time or can be pulsed to help the stalled mail break away. It should be noted that if the creation of additional normal force by the solenoid does not clear the stalled mailpieces, the noise created by the solenoid operation is a signal to the operator that a stall situation has occurred that needs to be manually resolved. The solenoid provides for efficient operation of the mail handling device, because it does not require shutting down the device each time a stall occurs but rather attempts automated resolution of the stall. 
     As shown in the perspective view of FIG. 5, the nudger arm  39  further comprises a lean detection arm  120  for detecting the position of the lead mailpiece  3   a . FIG. 6 a  illustrates a simplified perspective view of the nudger arm  39 , nudger roller  37  and lean detection arm  120  configuration. The lean detection arm  120  is positioned between the first and second rows,  37 ′ and  37 ″ respectively, of nudger rollers  37 . The lean detection arm  120  is spring biased (not shown) in a counter clockwise direction and is pivotally mounted about a conventional pivot structure  121  such that the arm is movable being movable between an extended position and a compressed position of the spring. FIG. 7 illustrates a finger  124  projecting from the lean detection arm  120 . The finger  124  aligns with a through-beam sensor  126  (lean detection sensor) mounted on the nudger arm  39  when the lean detection arm  120  is in the position where it has been rotated clockwise, and the biasing spring has been compressed. In an alternate embodiment, shown in FIG. 8, the lean detection arm  120  and through-beam sensor  126  may be fixedly mounted, for example, on table  9  or other suitable adjacent stationary portion of the mail handling device. When the through beam sensor is blocked by the finger  124 , a signal is sent to the controller indicating that the lead mailpiece  3   a  is in a preferred position, that is the lead mailpiece  3   a  has the proper lean for feeding. In this embodiment, the maximum allowable lean angle that a mailpiece can have with respect to the nudger wall  31  is dependent upon the position of the nudger rollers  37 . Whereas in the embodiment where the lean detection arm  120  pivot structure is mounted on the nudger arm  39  (as shown in FIG.  7 ), the maximum allowable lean angle is independent of the position of the nudger rollers  37 . Additionally, mounting the lean detection arm  120  on the nudger arm  39 , as illustrated in FIG. 7, allows for easier access to the lead detection arm, because the nudger arm  39  can swing outward and away from the nudger wall  31 . 
     The geometry of the lean detection arm  120  illustrated in FIG. 9 a , assists in the detection and proper feeding of mailpieces of various sizes. The lean detection arm  120  comprises a first end  120 ′ and a second end  120 ″. The first end  120 ′ is configured for mounting with the conventional pivot structure  121 . The second end comprises a trigger point  119 , a flag  124  and a ridge adjacent to the flag  124 . The geometry provides for more accurate detection of short mailpieces such as postcards and allows the mailpieces to hold the lean detection arm  120  in the sensed position as they are being fed into the separator or other downstream processing device. As they are being fed downstream, the mailpieces  3  travel between the trigger point  119  and the ridge  125  causing the normal force to be maintained against the lean detection arm  120 . Additionally, the geometry of the lean detection arm  120  (further illustrated in the right side view of FIG. 9 b ) allows for manual feeding of mailpieces  3  which may be slid into the nudger area from behind the first end of the lean detection arm  120  or may be dropped into the nudger area from above the lean detection arm  120 . In either case, the angling of the lean detection arm  120  allows the manual feeding without providing harsh edges on the lean detection arm  120  which may catch and/or damage the mailpieces  3  or the lean detection arm  120 . 
     For proper feeding, the nudger arm  39  is preferred to be in a particular position that allows the mailpieces  3  to be fed down stream without stubbing on downstream devices such as the separator device  45  or on a guide plate  6  (shown in FIG.  2 ). The position of the nudger arm  39  is sensed using a through-beam sensor  128  (stack advance sensor) which is preferably fixedly mounted on the table  9  or other suitable adjacent stationary portion of the mail handling device. When the nudger arm  39  rotates in a clockwise direction as the mailpiece is advanced in the direction of the nudger wall  31 , the nudger arm  39  blocks the through beam sensor  128 , and a signal is sent to the microprocessor  61  indicating that the lead mailpiece  3   a  is in a preferred position for feeding. 
     FIGS. 10 a  and  10   b  illustrate the position of the lead mailpiece  3   a  and acceptable angle with respect to the wall  31 . In order for the mailpiece to be moved by the nudger rollers  37 , the mailpiece must cause the nudger arm  39  and the lean detection arm  120  to be in a compressed position and that position must be sensed by sensors  126  and  128 . This position may happen when the angle θ between the mailpiece and the nudger wall  31  is in a range of about 0 to 1.5 degrees when the mailpiece is leaning toward the wall as shown in FIG. 10 a , and in a range of about 0 to 8 degrees when the mailpiece is leaning away from the wall as shown in FIG. 10 b . The angles correspond to the angle of the guide plate  6  at the entrance of the separator (shown in FIG.  5 ). That is, the guide plate  6  is preferably at an angle of about 8 degrees to vertical to help prevent stubbing when mailpieces are fed in a “lean away” position. 
     FIG. 11 is a flowchart illustrating the steps of advancing the stack of mixed mail  5  towards the nudger arm  39  and lean detection arm  120 , feeding the mailpieces  3  and controlling the stack advance. At step  130 , the method begins. At step  132 , the stack of mixed mail  5  is advanced in the direction of the nudger and accelerated to a predetermined velocity. At step  134 , the stack of mixed mail  5  continues to advance at a constant velocity. At step  136  a query is made as to whether the stack advance sensor  128  is blocked or satisfied. If at step  136 , the stack advance sensor is not satisfied, then step  134  is repeated, and the stack of mixed mail  5  continues to advance at constant velocity. Next, at step  138 , a query is made as to whether the lean detection sensor is satisfied or blocked. If at step  138 , the lean detection sensor  126  is not satisfied, then at step  140 , the stack of mixed mail  5  continues to advance at a constant velocity. If at step  138 , the lean detection sensor  126  is satisfied, then at step  142 , the stack of mixed mail  5  is decelerated to a stop over a predetermined distance of stack travel. The predetermined distance causes some over-travel of the mailpiece and helps to ensure that the mailpiece is in contact with the nudger rollers  37 . Next, at step  144 , the nudger rollers  37  are driven, and the lead mailpiece  3   a  is fed down stream for further processing. At step  144 , a query is made as to whether the lean detection sensor  126  or the stack advance sensor  128  are still satisfied. If at step  146 , both sensors continue to be satisfied (an indication that there is a mailpiece in the preferred position for feeding), then step  144  is repeated, and the nudger rollers  37  feed the next mailpiece down stream for processing. If one or both of the lean detection sensor and the stack advance sensor is not satisfied, then the method returns to step  132 , and the stack advance accelerates. Steps  134 - 146  are repeated as explained above. Thus, feeding downstream is enabled in a range from the sensor trigger point to the over travel position. 
     FIG. 12 illustrates a preferred stack advance profile in a plot of time verses velocity. As can be seen from the graph, the stack of mixed mail  5  is accelerated very rapidly to a constant velocity and once the sensors  126 ,  128  become blocked the stack of mixed mail  5  is decelerated more gradually to a stop. The gradual deceleration of the stack of mixed mail  5  helps to prevent toppling of the stack of mixed mail  5  toward the nudger wall  31 . If the stack of mixed mail  5  topples toward the nudger wall  31 , the stack normal force will be great and can cause the mailpieces  3  to stall because the nudger rollers  37  may not be able to overcome the stack normal force and shingle the lead mailpiece  3   a  from the front of the stack of mixed mail  5 . The gradual deceleration is chosen to produce an over travel of the mailpieces  3  after the sensors  126 ,  128  are satisfied. This ensures good contact with the nudger rollers  37  for feeding downstream. If the over travel is too great, the interdocument forces in the stack of mixed mail  5  becomes too great and the mailpieces  3  may not be fed. If the over travel is too little, the lead mailpiece  3   a  may not have enough contact with the nudger rollers  37  for proper feeding, or the leaning stack may be creating too much force on the lead mailpiece which is also leaning. The stack advance profile may be determined by one of ordinary skill in the art. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims.