Apparatus and method for stacking sheet-like articles
FIELD OF INVENTION 
     The present invention relates generally to an apparatus and method for stacking sheet-like articles, and more particularly to a device for controlling the pressure in a stack when stacking sheet-like articles such as envelopes that are continuously being fed into the stack. 
     BACKGROUND OF THE INVENTION 
     Envelope processing systems, such as mail piece processing, sorting and bar code application systems, typically include an envelope stacking apparatus at the end of the system to secure the sorted mail pieces in a stacked position to facilitate orderly removal of the processed mail pieces from the system. The stacked mail pieces are manually or automatically removed from the stack and/or bound by an operator. 
     One such stacking apparatus is disclosed in U.S. Pat. No. 4,955,596, commonly assigned. An envelope to be stacked, or any suitable sheet-like article, is forcibly fed on edge into a discharge magazine where it is stacked in a somewhat compressed array with other, previously fed envelopes. The envelope enters the discharge magazine via a dual stacker belt transport configuration, wherein a pair vertically juxtaposed stacker belts rotate about rollers disposed in a triangular array. The rollers each are rotatably mounted on shafts having a fixed axis. 
     The discharge magazine includes multiple transport belts that may have smooth surfaces, or may have track-like protrusions extending above the discharge magazine floor to engage the bottom edges of the stacked envelopes and advance the envelopes away from the stacker belts to permit the free entry of additional envelopes into the stacker region. The transport belts are activated by a stack sensor mechanism that includes a spring biased, pivotally mounted lever arm which extends through a gap between the pair of stacker belts. The tip of the lever arm contacts the last envelope to enter the stack. As the stack gets larger and the laterally applied normal force of the stacked envelopes overcomes the bias of the lever arm spring of the sensor mechanism, the lever arm trips a switch that in turn activates a drive motor connected to the transport belts to move the envelopes away from the stacker belts. This reduces the normal force or pressure exerted by the stack of envelopes on the stacker belts, and provides space for the entry of subsequent envelopes into the stack. 
     Although the stack sensor lever arm in the prior art apparatus contacts the last envelope in the stack, the lever arm contacts the last stacked envelope over a small plane, or sometimes a point, and is therefore highly susceptible to planar and height variations associated with the last stacked envelope. In high speed mail processing systems, a problem arises when the last envelope in the stack tilts such that the bottom edge and the top edge of the envelope no longer form a substantially vertical plane. As the stack becomes increasingly tight, accurate pressure sensing is critical to avoid jamming. Jamming occurs as a consequence of erroneous stack pressure sensing, when a tilted edge of the last stacked envelope obstructs the entrance to the stack of the next envelope to be stacked. Such problems are compounded when the stacker is used for simultaneously stacking a plurality of different sized (varying in height and thickness) envelopes. 
     Erroneous stack pressure sensing typically occurs where the lever arm contacts the tilted envelope at a surface or point that is tilted furthest away from the stacker belts. This surface exerts less force on the lever arm than the surface closest to the stacker belt. The lever arm&#39;s small plane or point of contact may erroneously indicate that the stack can receive more envelopes when the stack is actually too tight to properly receive another envelope without first activating the transport belts. 
     In view of the foregoing, an object of the present invention is to provide a high speed stacking apparatus and method that accurately senses the stack pressure of tilted articles to substantially reduce the occurrence of article jamming. 
     Another object of the present invention is to provide an apparatus and method that automatically and accurately senses the pressure applied by a stacked group of processed envelopes on a stacker conveying mechanism regardless of the degree of tilt of the last envelope to enter the stack, and in response thereto generates a signal to actuate an envelope transport system for advancing the documents in the stack away from the stacker conveyor means, thereby relieving the pressure on the conveying mechanism and creating space for the facile entry of additional processed envelopes into the stack. 
     A further object of the present invention is the provision of a sensor apparatus for a conveying belt mechanism for an envelope stacker device whereby one of the roller elements supporting the belt mechanism is on a laterally displaceable axis, whereby the axial displacement of the roller element is responsive to the force applied to the sensor apparatus by the stack of envelopes regardless of the angle of vertical orientation of the envelopes in the stack as the envelopes engage the sensor apparatus. 
     Yet another object of the present invention is the provision of an automatically actuated kicker mechanism that senses the trailing edge of an envelope entering the stack, and kicks the trailing edge away from the conveyor belts and onto an auger which drives the envelope&#39;s tracking edge outward to cooperate with the stack pressure sensor and control device to ensure that space is provided in the stack to permit subsequent envelopes to be fed into the stack without jamming. 
     SUMMARY OF THE INVENTION 
     The above objects and advantages are provided by the apparatus and method for stacking sheet-like articles disclosed herein. The invention includes an envelope conveying apparatus for stacking a series of sequentially fed sheet-like envelopes. The apparatus includes an elastic stacker belt assembly, one element of which comprises a movable roller element forming part of the conveying means, such as a spring biased roller, for sensing a force exerted by the stack of articles on the envelope conveying apparatus. 
     One embodiment of the invention comprises an envelope conveying apparatus having a dual belt system which extends around a portion of the moveable sensing roller and feeds the envelopes directly and sequentially into the stack. The moveable sensing roller also varies the tension of the conveying belts as a function of the force exerted by the stack of articles on the stacker belt system. 
     This embodiment of the invention includes a moveable dual belt system supported by a plurality of rollers, which belt system comprises the means for conveying envelopes into a stack of previously fed envelopes. One of the rollers is located adjacent the stack of envelopes, whereby the portion of the conveyor belt system passing over that particular roller is in contact with the stack of envelopes, and specifically in direct contact with the most recent envelope added to the stack. This one roller is rotatably supported on an axially moveable shaft disposed, in one embodiment, at one end of a lever arm, which lever arm is pivotally mounted to the base of the stacking mechanism. The other end of the lever arm, which is beyond the pivotal mounting point, includes means for biasing the lever arm such that the roller on the opposite end of the lever arm is urged toward the stack of envelopes. The belt portion extending around the axially moveable roller is biased to pivot into contact with the stack of articles with a force that counteracts the ever increasing force applied by the stack against the envelope conveying belt system. The force of the stack moves the axially moveable roller, which then acts as a sensing means to detect when the stack force or pressure reaches a predetermined maximum value. When this value is reached, the movement of the roller activates a motor operated drive mechanism which causes a horizontally disposed belt transport system upon which the stack of envelopes is supported to move the stack of envelopes away from, and relieve the pressure upon, the belt system of the envelope conveying means. 
     Another embodiment includes an envelope conveying system having a roller moveable in a substantially linear direction and a biasing element, such as a spring, operably coupled to the roller support structure for counteracting the force exerted by the stack of articles on the belt system of the envelope conveying means. 
     Another aspect of the present invention provides a kicker arm assembly which senses the trailing edge of an envelope entering the stacker region adjacent the envelope conveyor belt system, and applies a force to kick the trailing edge of each envelope away from the belt system and onto an auger to provide additional force to move the trailing edge of each document entering the stack out of the path of the leading edge of each subsequently fed envelope. 
     The method of the present invention for stacking sheet-like articles on edge of the present invention includes: conveying the individual documents into a stack of documents; sensing the force exerted by the stack of documents on a stacker infeed belt regardless of the angular disposition of the last documents added to the stack, and reducing the force exerted by the stack of documents on the stacker belts in response to the sensing of the force by moving the stack of documents in a direction away from the stacker belt system. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     FIG. 1 is a top view schematically depicting a stacking apparatus constructed in accordance with the present invention, with the envelope kicker mechanism not shown; 
     FIG. 2 is a front elevation view of the stacker belts and kicker mechanism forming the document drive of the present invention, with a portion of the stacker belts and rollers cut away to illustrate the kicker mechanism; 
     FIG. 3 is a top view schematically illustrating the kicker arm assembly of the present invention for kicking the trailing edge of each envelope away from the envelope conveying belt system and onto an auger element; and 
     FIG. 4 is an additional embodiment of a moveable sensor mechanism constructed in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 generally illustrates the preferred embodiment of the stacking apparatus 10 of the present invention having an introductory conveying path 12 for a document such as an envelope 14, a controllable discharge or document transport magazine 16, a stacking belt assembly 18, and a movable roller sensing mechanism 20. The introductory document conveying path 12 includes a feed belt 22 rotatable about a plurality of rollers 24 which drive belt 22 in the direction shown by arrow A. The path 12 may accept envelopes from a prior envelope feeding stage or other envelope processing stage. The feed belt 22 drives the envelope into contact with the stacking belt assembly 18 by virtue of the spatial proximity of the two belt assemblies. Alternately, feed belts 22 may comprise a pair of vertically separated 0-rings extended over pulleys used in place of rollers 24. The 0-rings are adapted to provide the same function as belts 22, which is to advance an envelope into contact with stacking belt assembly 18, as will be explained. 
     As seen in FIGS. 1 and 2, the stacker belt assembly 18 comprises a pair of elastic stacker belts 26a and 26b rotatable about axially fixed dual idler rollers 28a and 28b, axially fixed drive rollers 30a and 30b, and axially movable dual rollers 32a and 32b. Moveable roller 32 is rotatably mounted on a shaft 34 which shaft is mounted on a pivotally mounted lever arm 36. Lever arm 36 is rotatably mounted on a shaft 38, which in turn is fixed to a base plate 40 and a top plate 42 (FIG. 2) which form part of the static support assembly for the stacking apparatus 10. 
     The stacker belts 26a and 26b extend about a triangular course of travel formed by the rollers 28a, 28b, 30a, 30b, 32a and 32b. A drive shaft 44 is connected to a prime mover and to rollers 30a and 30b, and drives the axially fixed drive rollers 30a and 30b in a clockwise direction as viewed in FIG. 1. As will be explained, shaft 34 supporting rollers 32a and 32b is axially biased toward the stack of envelopes 46, and forces a portion 64 of belts 26a and 26b outwardly and into contact with the last stacked envelope 50 of the stack 46. 
     The movable roller sensing mechanism 20 includes the axially moveable dual rollers 32a and 32b, lever arm 36, and a biasing mechanism generally denoted 52. Lever arm 36 has a first end 54a and a second end 54b. Moveable rollers 32a and 32b are attached to the first end 54a, and are rotatable about shaft 34 and axially pivotal about shaft or post 38, as previously described. The second end 54b of arm 36 engages one end of biasing mechanism 52 via an adjustable screw 56. The distal end of the biasing mechanism 52 is secured to a non-movable post 58. The biasing mechanism 52 extends through the space between dual stacker belts 26a and 26b (FIG. 2). Adjustable screw 56 extends toward the wand 60 of a switch mechanism comprising microswitch 62, whereby the movement of screw 56 actuates the microswitch. As viewed in FIG. 1, biasing mechanism 52 biases lever arm 36 in a counterclockwise direction around shaft 38, forcing rollers 32a and 32b and belt portion 64 outward towards envelope stack 46. 
     Stop member 66 provides a limit to the counterclockwise rotation of the lever arm 36 and moveable rollers 32a and 32b about shaft 38. The biasing mechanism 52 serves to urge the lever arm 36 away from the switch mechanism 62. Switch mechanism 62 is electrically coupled to a motor 69 or other prime mover that controls the movement of magazine conveyor belts 68a and 68b. 
     Discharge magazine 16 includes conveyor belts 68a and 68b, an adjustable compression plate 70 slidable along guide rod 72, and a document stop element 74. Each conveyor belt 68a, 68b, extends around a pair of pulleys (not shown). One of the pulleys associated with each belt 68a, 68b is driven by a suitable motor 69, which motor 69 is operatively connected to and actuated by switch 62. The conveyor belts 68a and 68b transport the envelopes in stack 46 in a direction shown by arrow 76, and are activated by the motor when the adjustable screw 56 contacts the switch mechanism 62. The stack of envelopes 46 is vertically disposed on top of and supported by belts 68a and 68b. 
     After the last to be stacked envelope 50, as will be explained, reaches discharge magazine 16 and the leading edge of envelope 50 abuts stop element 74, portion 64 of stacker belt 26 holds envelope 50 at an angle relative to the longitudinal direction of magazine 16. In addition, portion 78 of belt 26 extends away from discharge magazine 16 on the upstream side of rollers 32a and 32b relative to belt portion 64. As a result, a variable entrance angle 80 is formed between envelope 50 and belt portion 78. This is the entrance angle thru which envelopes 14 are transported to stack 46. 
     The present invention also includes a kicker arm assembly 82 (FIGS. 2, 3) for kicking the trailing edge of each envelope 14 away from belt portion 78 and onto auger 84. Auger 84 (FIGS. 1, 3) comprises a helix 85 rising slightly above the upper surface of base plate 40 which engages the bottom edge of each envelope 14 as the envelope is kicked away from belt portion 78 by kicker arm assembly 82. The auger 84 moves the trailing edge of each envelope 14 through entrance angle 80, thereby creating a space for the advancement of the next envelope into the stack 46 without causing interference with the trailing edge of the preceding envelope. 
     Kicker arm assembly includes a vertically disposed mounting bracket 86 fixed to base plate 40, as best seen in FIG. 2. A mounting plate 88 is fixed to and extends from the top of bracket 86, and an aperture 90 extends through an outer portion of mounting plate 88. Shaft 92 is rotatably mounted through aperture 90, and extends downward through an aperture in base plate 40. The lower end of shaft 92 is attached to the operating shaft 94 of a rotary operating solenoid 96. Shaft 94 is adapted to be rotated through a limited circular angle when solenoid 96 is actuated. 
     A pair of extendable arms 98a, 98b are fixed to shaft 92, and a pair of kicker arms 100a, 100b are attached to extendable arms 98a, 98b respectively. Kicker arm 100a is vertically located on shaft 92 such that when shaft 92 is rotated by solenoid 96 in the counterclockwise direction 16 as viewed in FIG. 3, kicker arm 100a extends between belts 26a and 26b. In similar fashion, kicker arm 100b is vertically located on shaft 92 such that when shaft 92 is rotated counterclockwise (FIG. 3) by solenoid 96, kicker arm 100b extends in the space between belt 26b and the upper surface of base plate 40. When solenoid 96 is actuated, shaft 92 rotates in a clockwise direction (FIG. 3), moving kicker arms 100a and 100b to the retracted position seen in FIG. 3. When solenoid 96 is de-activated, a spring mechanism (not shown) biases the outer ends of kicker arms 100a and 100b to extend outward beyond the vertical plane of belt portions 78 to engage the trailing edge of a moving envelope 14 as the envelope is driven towards the stack 46, thereby driving the trailing edge of the envelope through angle 80 and onto auger 84 and helix 85. The helix drives the trailing edge of the envelope in a direction away from belt portion 78, providing space for the transport of the next envelope into the stack without jamming. 
     A photocell sensor element 102 (FIG. 3) is mounted on base plate 40 adjacent the path traveled by each envelope 14 and just ahead of mounting bracket 86. Sensor element 102 is electrically connected through line 103 to solenoid 96. As each envelope 14 advances, sensor 102 detects the leading edge of the envelope, and sends a signal through line 103 to actuate the solenoid, rotating shaft 92 clockwise (FIG. 3), thus retracting kicker arms 100a, 100b out of the path of the advancing envelope. As the envelope 14 moves forward, sensor 102 eventually detects the trailing edge of the envelope, and sends another signal through line 103 which de-actuates solenoid 96, whereby the spring mechanism rotates shaft 92 counterclockwise, extending kicker arms 100a, 100b outward beyond the vertical plane of belts 26a, 26b. As stated previously, kicker arms 100a, 100b force the trailing edge of the envelope 14 outward and onto auger 84. The vertical location of kicker arms 100a, 100b is preferably fixed such that the arms will contact regular sized envelopes as well as flat or larger sized envelopes. 
     In the operation of the embodiment disclosed in FIGS. 1 and 2, an envelope 14 is conveyed by the stacking belt assembly 18 along linear introductory path 12 until the leading edge of the envelope 14 contacts the most recently stacked envelope 50 after passing through the acute entrance angle 80. As the stacker belts 26a and 26b move, the leading edge of each envelope 14 is bent around bend point 104 and interposed between the most recently stacked envelope 50 and portion 64 of stacker belts 26a and 26b. The trailing edge of the envelope is displaced through the entrance angle 80 with the aid of auger 84 so that the trailing edge &#34;fishtails&#34; through the entrance angle. The bottom margin of the trailing edge of the envelope 14 is engaged to ride in the helical threads 85 of the rotating auger element 84 to propel the trailing edge of each envelope 14 into the stack and away from stacking belt assembly 18 to provide space for subsequently fed envelopes. 
     The present invention includes a unique sensing mechanism that provides a broad plane of contact with the last stacked envelope and also varies the stacker belt tension about the rollers 28a and 28b, 30a and 30b, and 32a and 32b. The moveable roller sensing mechanism 20 forms part of the stacking belt assembly 18. As more envelopes are sequentially stacked in the discharge magazine 16, a normal compressive force or pressure is developed in the stack 46 in opposition to the bias element 52 exerts on the moveable rollers 32a and 32b. This normal force causes the lever arm 36 to rotate in a clockwise direction, thereby slightly decreasing the entrance angle 80 and reducing the tension on the stacker belts 26a and 26b. When the force applied by the stack 46 to the movable rollers 32a and 32b exceeds the force applied to the moveable rollers by the biasing mechanism 52, the adjustable screw 56 engages microswitch 62 and activates the motor driving magazine conveyor belts 68a and 68b. 
     The conveyor belts 68a and 68b then convey the envelopes away from the stacker belts 26a and 26b in the direction of arrow 76, thereby relieving the pressure force previously exerted on the movable rollers 32a and 32b and allowing the lever arm 36 to rotate in the counterclockwise direction under the bias of mechanism 52. This causes the adjustable screw 56 to disengage from switch mechanism 62 which de-activates the motor connected to magazine conveyor belts 68a and 68b. The envelopes in the front part of stack 46 fan out as pressure is relieved, allowing additional envelopes to be sequentially fed into the stack without interference from the trailing edge of previously stacked envelopes. 
     Undesirable variations in pressure sensing by the movable roller sensing mechanism 20 due to slack in the stacker belts 26a and 26b is further reduced by the direction of movement of the stacker belts 26a and 26b. The drive roller 30 rotates clockwise as viewed in FIG. 1, and pulls the stacker belts 26 tightly over movable rollers 32a and 32b while &#34;pushing&#34; the stacker belts 26a and 26b toward idler rollers 28a and 28b. Therefore, any slack in the stacker belts 26a and 26b is developed in the top run of the triangular path between drive rollers 30a and 30b and idler rollers 28a and 28b. Belts 26a and 26b remain taught as they travel from idler rollers 28a and 28b to axially moveable rollers 32a and 32b, and from moveable idler rollers 32a and 32b to drive rollers 30a and 30b.  This taughtness of belts 26a and 26b adjacent rollers 32a and 32b enhances the accuracy of roller sensing mechanism 20. 
     As illustrated in FIG. 2, the lever arm 36 is vertically situated adjacent the gap between stacker belts 26a and 26b. Shaft 38 and non-moveable stop member or post 66 are fixedly secured to base plate 40 via attachment bolts 110, 112. 
     The plane of contact between the movable roller sensing mechanism 20 and envelopes 14 comprises the broad surfaces of the dual stacker belts 26a and 26b, rather than a separate rod or arm type sensing lever with a small plane of contact, as found in the prior art. The stacker belts 26a and 26b of the present invention are used both to transport envelopes directly into the stack 46 and also to form a broad pressure sensing surface which senses stack pressure accurately regardless of the tilt of the forward envelopes. 
     Kicker arm assembly 82 cooperates with the stacking belt assembly 18 and the roller sensing mechanism 20 to move the trailing edge of each envelope away from the path of subsequently fed envelopes as each prior envelope reaches the stack 46. As described previously, kicker arm assembly operates to kick the trailing edge of each envelope as it reaches the stack in a direction toward the stack and onto auger 84, and out of the primary path of envelope travel, as defined by introductory conveying path 12 and belt portion 78 of stacking belt assembly 18. The trailing edge of each envelope is therefore removed from possible interference with the leading edge of an incoming envelope. As the number of unimpeded envelopes entering the stack increases, the normal force applied by the stack of envelopes against the roller sensing mechanism 20 increases to the point where the compressed stack of envelopes presents another impediment to rapid introduction of envelopes into the stack. As explained, when the normal force reaches a predetermined limit, the magazine conveyor belts 68a and 68b are driven to relieve the stack pressure adjacent the stacking belt assembly 18. 
     As appreciated by those having ordinary skill in the art, a single belt 26 configuration may also be suitable, provided the width of the belt 26 that forms the contact surface is proportionally wide enough to contract a substantial portion of each envelope. 
     FIG. 4 illustrates an alternative embodiment of the roller sensing mechanism of the present invention, comprising a substantially linearly moving sensor element. A spring loaded sensing roller 20&#39; replaces the roller sensing mechanism 20 shown in FIGS. 1, 2 and 3. As illustrated in FIG. 4, linear displacement occurs in the horizontal direction, as indicated by arrow 120, as opposed to the rotational displacement of the moveable sensing mechanism 20 of FIG. 1. A hollow cylindrical member 122 is fixedly mounted to a vertically extending sleeve 124 through which rotatable shaft 34 extends. Moveable roller 32 is rotatably mounted on shaft 34. The cylindrical member 122 houses biasing element 126, which is substantially restricted to linear movement by guide plates 128 and 130. A lever 132 is attached to an end of sleeve 124, and is adapted to contact and move wand 134 of microswitch 136. 
     The operation of the alternate embodiment of FIG. 4 is similar to the operation of the embodiment of FIG. 1. As stack pressure increases, moveable roller 32&#39; moves horizontally in the direction of the application of stack pressure, driving shaft 34&#39;, sleeve 124, hollow cylindrical member 122 and lever 132 in the same direction. When the stack pressure has reached a predetermined maximum limit, lever 132 comes into contact with wand 134, activating microswitch 136 and moving magazine conveyor belts 68a and 68b in a direction away from stacking belt assembly 18. Although a spring is shown as a representation of biasing element 126, other suitable biasing elements can be substituted therefor. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those having ordinary skill in the art that numerous variations in form and detail may be made without departing from the spirit and scope of the invention, as set forth in the following claims.