Abstract:
A device for stacking banknotes, comprising a cashbox and a stacker arranged to stack banknotes of predetermined dimensions in said cashbox, said cashbox having a surface including an aperture therein, said aperture having a dimension in a first direction of W, said device being arranged to receive a banknote at a position overlying said aperture, said banknote having a dimension in said first direction of L, said stacking means being arranged to push said banknote through said aperture and into a stacked position in said cashbox, wherein said banknote is pushed to a predetermined maximum depth D in said cashbox relative to said aperture such that D&lt;(L−W)/2.

Description:
TECHNICAL FIELD 
     This invention relates to an apparatus for forming a stack of sheet-like objects, in particular but not exclusively a stack of banknotes formed in a cashbox. 
     BACKGROUND ART 
     Various devices are known for forming stacks of banknotes. One such device is described in published European patent application No. 0684929. This discloses an apparatus which incorporates a pusher plate with which a banknote may be pushed from the plane along which the banknote is transported to the stacking mechanism (transport plane), into a cashbox situated adjacent to the banknote plane. The pusher plate is connected by a pivoted lever arrangement via a cam, to a drive motor. The pivoted lever arrangement operates with a “scissors action” to cause the pusher plate to push the banknote into the cashbox against the action of a spring mounted stack surface. The banknotes are retained in a stack in the cashbox, when the pusher plate is withdrawn, by flanges which abut the ends of the uppermost surface of the banknote stack. 
     Although this type of arrangement provides an efficient method of stacking banknotes, the required depth of stroke of the pusher plate is linked to the size of the aperture through which the banknote is pushed. Thus, a short depth of stroke is only possible if the aperture is relatively large. However, cashboxes with relatively large apertures suffer from the disadvantage of being difficult to make secure (i.e. self closing) on detachment from the stacking device. The cashbox aperture may be made smaller by increasing the depth of stroke of the pusher plate. However, an increased depth of stroke results in an increased cashbox depth for any given size of banknote stack. As space is often at a premium in such circumstances, for example in combined banknote validator and stacker devices, this too is an undesirable consequence. 
     Furthermore, if banknotes of differing lengths are to be stacked in a cashbox incorporating stack retaining flanges, the aperture must be significantly shorter than the length of the shortest banknote to be stacked. This is in order that the flanges at the ends of the aperture may retain even the shortest banknotes. This results in a minimum length of pusher plate stroke being further increased in order to successfully stack the longest banknotes through the same aperture size and hence a corresponding increase in the depth of the cashbox. 
     In order that the flanges should retain the stack of banknotes, it may be important that the banknotes are presented for stacking in a predetermined orientation. For example, if a banknote of maximum length is skewed on being stacked, its greater diagonal length may prevent it from being successfully stacked. Additionally, it may also be important that the banknotes are accurately positioned lengthwise with respect to the cashbox aperture, in order to be reliably stacked. A sufficient lengthwise offset will result either in an end of the banknote not being stacked, or alternatively an end of the banknote not being retained by a flange, or both. 
     As cashboxes used with such devices often incorporate a spring mounted stacking surface against which a pusher plate or piston must work, a further problem may arise in such devices. Namely, despite successfully pushing the banknote into the cashbox, the banknote may not completely flatten against the stack. As the stack surface is again biased against the retaining flanges by the spring mounted stacking surface banknotes may become crumpled, causing an irregular banknote stack. 
     U.S. Pat. No. 4,809,967 and U.S. Pat. No. 5,014,857 disclose a stacking device of the piston type which aims to address the problem of ensuring that banknotes flatten correctly on the stack surface during the stacking process. These disclosures teach to incorporate pivotally mounted “unfolding” plates in the piston assembly. These are arranged to displace horizontally as the piston stroke increases in the vertical direction; thus assisting in flattening a banknote against the stack. 
     However despite assisting with flattening banknotes in the stacking procedure the device of U.S. Pat. No. 4,809,967 and U.S. Pat. No. 5,014,857 suffers from the same drawback as that of EP 0684929A, in that a short depth if stroke is only possible of the cashbox aperture is relatively large; or, conversely a small aperture is only achievable if the stroke length is relatively long. 
     A further stacking device is disclosed in U.S. Pat. No. 4,834,230 and U.S. Pat. No. 4,807,736 which employs a pair of rotors in place of a piston in order to stack banknotes in a cashbox. However, like the device of U.S. Pat. No. 4,809,967 and U.S. Pat. No. 5,014,857, this device suffers from the disadvantage that a short depth of stroke is only possible if the cashbox aperture is relatively large. Additionally, such a device may suffer from the disadvantage of a banknote being incorrectly stacked (for example, one end of the banknote not being retained in the cashbox by a retaining flange) if the banknote is erroneously presented for stacking in a non-central manner. 
     A further such device is described in granted European patent 0470329. This discloses an apparatus which transports banknotes between opposing belts entrained around rollers of a carriage, which is arranged to traverse an open surface of a cashbox. As the carriage moves over the stack of banknotes, the entrained banknote is deposited on the stack. The stack of banknotes is retained in the cashbox by one of the transporting belts which lie across the uppermost surface of the banknote stack. 
     Such a device does not require vertical movement of the piston or pusher, and hence the cashbox depth can be smaller for a given capacity. However, this arrangement also requires the cashbox construction to be substantially open and consequently difficult to make secure on detachment from the stacking device. Indeed in such a design the aperture of the cashbox must be at least as large as the banknotes which are to pass through it. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a device for stacking banknotes, comprising a cashbox and a stacker arranged to stack banknotes of predetermined dimensions in said cashbox, said cashbox having a surface including an aperture therein, said aperture having a dimension in a first direction of W, said device being arranged to receive a banknote at a position overlying said aperture, said banknote having a dimension in said first direction of L, said stacking means being arranged to push said banknote through said aperture and into a stacked position in said cashbox, wherein said banknote is pushed to a predetermined maximum depth D in said cashbox relative to said aperture such that D&lt;(L−W)/2. 
     It will be appreciated that where a standard reciprocating piston action is used to push a banknote through an aperture of a cashbox which is narrower than the width of the banknote, a relationship between the minimum required depth of stroke to push a given banknote completely through the aperture and the width of the aperture may be derived. 
     This minimum stroke depth occurs when the banknote is pushed through the aperture symmetrically across its width. In this case the banknote will be pushed entirely within the cashbox when the piston stroke, relative to the aperture, is equal to half the difference between the banknote width and the aperture width. 
     However in mechanisms according to the present invention the relationship between the aperture width and the stroke depth is not fixed in this manner for a given banknote size. Thus a reduced cashbox aperture size may be achieved without necessitating a long stroke length. Therefore improved cashbox security and a reduced cashbox size may advantageously be achieved. 
     In a further aspect of the invention there is provided a device for stacking documents comprising a stacker and a stack surface, the stacker being arranged to push a document partially through an aperture defined by at least one surface such that the document at least partially contacts the stack, the stacker being further arranged to move along the stack and under the surface, entraining the document through said aperture into a stacked position, wherein the stacker comprises an extensible membrane positioned between the stacker and the document, arranged to contact the document during the stacking procedure. 
     By incorporating a flexible membrane in the stacking device, between the stacker and the document (for example a banknote), the degree of control over the document may be increased. Thus the possibility of the document being incorrectly stacked, due to slippage between the stacker and the document or the document being damaged in the stacking process, is significantly reduced. 
     Other aspects and embodiments of the invention, with corresponding objects and advantages, will be apparent from the following description and claims. The invention will now be illustrated, by way of example only, with reference to the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a 0.62:1 scale diagram illustrating the structure and function of the banknote stacking mechanism according to a first embodiment of the invention; 
     FIG. 2 a  is a perspective view of a rotor which may be used in first, second and fourth embodiments of the invention; 
     FIG. 2 b  is a perspective view of an alternative rotor design which may be used in first, second and fourth embodiments of the invention; 
     FIGS. 3 a-d  are a series of diagrams shown in 1:1 scale illustrating the structure and function of the banknote stacking mechanism according to a second embodiment of the invention; 
     FIG. 4 a  illustrates a rotor according to the third embodiment of the invention; 
     FIG. 4 b  is a 1:1 scale drawing illustrating the structure and arrangement of the rotors according to the third embodiment of the invention, shown from above in the resting state; 
     FIG. 4 c  is a 1:1 scale drawing illustrating a side view of the arrangement of the rotors according to the third embodiment of the invention, in operation; 
     FIG. 5 is a plan view of the membrane used in the fourth embodiment of the invention; 
     FIGS. 6 a-d  are a series of diagrams shown in 1:1 scale illustrating the working of the fourth embodiment of the invention with the cashbox partially removed; 
     FIGS. 7 a-d  are a series of diagrams shown in 1:1 scale illustrating the working of fourth embodiment of the invention with the cashbox in place; 
     FIG. 8 is a perspective view of a banknote stacking mechanism according to the firth embodiment of the invention; 
     FIG. 9 is a cross sectional view of the banknote stacker of FIG. 8 illustrating its mode of operation; 
     FIGS. 10 a  and  10   b  illustrate a banknote handling machine including a cashbox with which a stacking mechanism according to the present invention may be used. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 1, a banknote stacking system according to the first embodiment of the invention is shown. The system comprises a banknote transport system, a stacking mechanism and a cashbox  5 . The stacking mechanism and the transportation mechanism are housed in a banknote handling apparatus, such as a validator (shown in FIG.  10 ), to which a cashbox  5  is removably attached. 
     Banknote Transport System 
     A banknote  1  is transported to the stacking mechanism in a direction perpendicular to the plane of the diagram by the transportation mechanism, which comprises opposing pairs of rollers  2   a ,  2   b  and  3   a ,  3   b . The banknote  1  is engaged by transportation rollers  2   a ,  2   b ,  3   a ,  3   b  parallel to its lengthwise edges. That is to say it is transported in the direction of its longitudinal axis. The spacing between the pairs of rollers  2   a ,  2   b  and  3   a ,  3   b  is arranged such that even the minimum size of banknote for which the mechanism is designed may be securely held and transported. 
     The rollers  2   a ,  2   b ,  3   a ,  3   b  position the banknote  1  above an aperture  4  of the cashbox  5 . In this embodiment, the aperture  4  is approximately half of the width of the banknote; i.e. approximately 31 mm across. The position of the leading edge of the banknote  1  is sensed using photosensors (not shown), or other suitable position sensing devices, which are occluded by the banknote  1  when it is in the correct position. The output from the photosensors is then used to inhibit further transport of the banknote  1 . 
     The rollers  2   a ,  2   b ,  3   a ,  3   b  are located on either side of the aperture  4 , such that the banknote  1  is gripped with a positive force and held flat and parallel to the aperture  4  prior to being stacked. This is achieved by mounting the lower rollers  2   a ,  3   a  on fixed axles  6  and mounting the opposing rollers  2   b ,  3   b  on shafts  7 , which are free to move to a limited extent in the vertical direction. The shafts  7  are biased downwards towards the lower rollers  2   a ,  3   a  by compression springs  8  contained within the shafts  7 . 
     Although rollers are used in the present embodiment for the transportation of the banknotes, a belt driven transportation system could alternatively be used. 
     Stacking Mechanism 
     The stacking mechanism comprises a pusher plate  9 , a rotor  10  and a stack support surface  13  located inside the cashbox  5 . 
     Pusher Plate 
     The pusher plate  9  comprises a flat plate made from a plastics material or metal. It is connected by the centre of its upper surface to a solenoid (not shown) using any suitable fastening. The solenoid is arranged to cause the pusher plate  9  to reciprocate in a vertical direction. The solenoid may however be replaced by other suitable means. For example, a pivoted lever arrangement driven by an electric motor via a cam, as discussed with reference to published European patent application No. 0684929. 
     Rotor 
     A detailed view of the rotor  10  is shown in FIG. 2 a . The rotor  10  comprises two rotor arms  20  mounted on an axle  11 . In this embodiment the rotor arms  20  have a straight sided profile. However, various other profiles may be used, for example a circular profile extending through 93°. as shown in FIG. 2 b . At one end of the axle  11  is situated a crank arm  21  through which rotational movement is applied to the rotor  10  by an electric motor and gear train (not shown). A support bar  22  connects the two rotor arms  20  and provides added rigidity to the rotor assembly. Adjacent the support bar  22 , situated between the extremities of the rotor arms  20 , is a rotating axle  23 , which forms a banknote engaging surface. Since it is free to rotate relative to the banknote  1  during the stacking process the levels of friction acting on the banknote  1  are reduced. This may be beneficial as the banknote  1  may otherwise be prone to tearing during the stacking process, especially if the mechanism is operating at high speed. The rotating axle  23  may alternatively be replaced by a non-rotating banknote contacting surface made from a low friction material such as PTFE. 
     The separation between the two rotor arms  20  in the direction of the axle  11 , is chosen such that the overall width of rotor  10  is slightly less than the corresponding dimension of the aperture  4 , through which it must pass. This ensures that a high degree of control over the banknote  1  is achievable during the stacking process. 
     The entire rotor assembly may be manufactured by any suitable means such as a one piece plastics injection moulding, with the exception of rotating axle  23  which may be joined to the main rotor assembly by means of a snap fit. Alternatively, it may be manufactured through individually machined or moulded plastics or metal components, or a combination thereof. 
     Stacking Process 
     Prior to the actuation of the stacking mechanism, the positive gripping force exerted by the roller  3   b  is removed from the banknote  1 . This achieved by raising the associated shaft  7  using a solenoid (not shown), against the spring force of the spring  8  to give a clearance between the rollers  3   a  and  3   b . Alternatively, this may equally be achieved by lowering the roller  3   a  relative to roller  3   b.    
     The benefit of giving a clearance between the opposing rollers  3   a  and  3   b  is to ensure that banknote  1  will not be subject to undue stress which might cause it to tear on being stacked. It should be noted that at this stage the rollers  2   a ,  2   b  continue to engage the right-hand end of the banknote  1  as shown in FIG.  1 . 
     The pusher plate  9  is initially situated in its resting position parallel to and slightly above the transport plane of the banknote  1 , as shown in FIG.  1 . On actuation, the pusher plate  9  descends through the transportation plane of the banknote  1 , through the aperture  4  of the cashbox  5  to the required depth. The required depth must be sufficient for the left-hand end of the banknote  1  to be entrained through the aperture  4  and fall beneath the left-hand abutment surface  15  as shown in FIG.  1 . The pusher plate  9  descends no further than the minimum distance required in order to ensure reliable stacking of the banknote  1 , in order to allow the depth of the cashbox  5  to be minimised for a given capacity. 
     This action causes the free left-hand end of the banknote  1  to be pushed through the aperture  4  of the cashbox  5  and on to a stack surface, which may be either a support plate  13 , or the surface of a stack of banknotes  12  already stacked on support plate  13 . 
     Since the right-hand end of the banknote  1  is held between the rollers  2   a ,  2   b , the surface of the banknote  1  will move laterally in relation to the pusher plate  9  as it descends into the cashbox  5 . This situation is illustrated by the dashed representations of the pusher plate and the banknote referenced  9 ′ and  1 ′ respectively. 
     The support plate  13  is supported upon a compression spring  14 . The compression spring  14  compresses to take up any excess travel in the length of stroke of the pusher plate  9 , beyond that required to bring the left hand end of banknote  9  into contact with stack surface  12 ;  13 , as shown in FIG.  1 . The position of the support plate  13  and the compression spring  14  when the pusher plate is fully lowered are shown by dashed representations of the support plate  13 ′ and the compression spring  14 ′. The degree to which the compression spring  14  is compressed depends upon the height of any existing banknote stack on the support plate  13 . 
     At this stage, the right-hand roller pair  2   a ,  2   b  is disengaged, thus freeing the right-hand end of the banknote  1 , as shown in FIG.  1 . However, as the left-hand end of the banknote  1  is securely maintained on the stack surface  12 ;  13  by the pusher plate  9 , the position of the banknote  1  is positively controlled throughout. 
     The rotor mechanism  10  is then actuated, driven by a reversible DC motor and drive train (not shown). The rotor  10  is rotated approximately 90° anti-clockwise, with reference to FIG. 1, from its resting position (shown in solid line) where the rotating axle  23  of the rotor  10  is positioned above the resting position of the pusher plate  9 , to its extended position (shown in dotted line referenced by numeral  10 ′). This causes the right-hand end of banknote  1  to be withdrawn from the clearance between rollers  2   a  and  2   b , entrained downwards through the aperture  4  and unrolled sideways along the stack surface  12 ;  13 , such that it falls beneath the right-hand hand abutment surface  16 , as shown in FIG.  1 . 
     It will be noted from FIG. 1 that the maximum depth of penetration of the rotor  10  into the cashbox  5  is no more than that of the pusher plate  9 . This ensures that the movement of the rotor  10  is not obstructed by the stack surface  12 ;  13 . 
     It will also be noted that the maximum dimensions of the pusher plate  9  are limited by the corresponding dimensions of the aperture  4 . Within this constraint it is desirable that the banknote contacting area of the pusher plate  9  is large to increase the control over the positioning of the banknote  1 . Unlike known stacking systems, the size of the pusher plate  9  of the present embodiment is not directly related to the depth of stroke of pusher plate. 
     When the banknote  1  is fully contacting stack surface  12 ;  13 , the rotor  10  rotates clockwise, as shown in FIG. 1, back to its resting position and subsequently the pusher plate  9  is also returned to its resting position above the banknote transport plane. As the pusher plate  9  is returned to this position, the compression spring  14  returns the stack surface  12 ;  13  to its uppermost limit, against the movement of the pusher plate  9 . This movement of the stack surface is limited by the abutment surfaces  15 ,  16  located on the interior surface of the cashbox  5 . 
     Thus, stack surface  12 ;  13  is continually under a compressive load between compression spring  14  and pusher plate  9  or abutment surfaces  15 ,  16 . Because the banknote is flattened on the stack surface by the stacking mechanism, the scope for a banknote to become incorrectly positioned prior to being forced against the abutment surfaces  15 , 16  is greatly reduced. 
     Subsequently, rollers  2   a ,  2   b ,  3   a ,  3   b  are re-engaged in order to receive a further banknote  1  to be stacked, at which time the stacking cycle is ready to restart. 
     In this embodiment, despite the fact that the pusher plate  9  and the initial position of banknote  1  are centrally located with respect to the rollers  2   a ,  2   b ,  3   a ,  3   b , the final stacked position of the banknote  1  is offset with respect to this position. This offset is a function of the distance between the banknote transport plane and the length of stroke of pusher plate  9 . 
     It will be apparent to the skilled reader that the present embodiment of the invention is tolerant of misalignment of the banknote  1  as it is presented for stacking at the stacking mechanism, since no datum edge is relied upon in order to effect the stacking operation. Furthermore, because each banknote  1  is effectively stacked by positioning part of the banknote  1  on the stack  12  and subsequently flattening the remainder against the stack  12 , this embodiment is also able to cope with a wide range of banknote sizes. 
     Second Embodiment 
     Referring to FIG. 3, a stacking mechanism according to the second embodiment of the invention is shown. Features in the second embodiment which are similar to features already discussed with reference to the first embodiment, are referenced using the same numerals and are not discussed further in detail. Unlike the first embodiment, the second embodiment does not utilise a pusher plate or piston in the stacking process but incorporates two rotors with the circular profile shown in FIG. 2 b  and as described with reference to the first embodiment. 
     Banknote Transport System 
     In this embodiment, the banknote  1  is transported to the stacking mechanism by a banknote transport system similar to that described with reference to the first embodiment. 
     However, in this embodiment the banknote  1  is transported in the region of the stacking mechanism by drive rollers  30  situated above the banknote transport plane and at either side of the cashbox aperture  4 . Each drive roller  30  is opposed by a trapped bearing  32  situated beneath the banknote transportation plane. 
     The drive rollers  30  are supported rigidly on axles  31  and the trapped bearings  32  are mounted along opposing edges  26  of the cashbox aperture  4 , such that they have two rotational degrees of freedom. 
     The trapped bearings  32  may be manufactured from metal or plastics material and are mounted proud of the profile of the upper surface of the cashbox  5 . The drive rollers  30  are manufactured from plastics or any other suitable material and have a rubberised tyre or circumferential surface to positively grip the banknote  1 . 
     The spacing between the drive rollers  30  and the trapped bearings  32  on either side of the aperture  4  is such that even the minimum width of banknote for which the mechanism is designed may be securely held and transported. 
     In this embodiment (illustrated in FIGS. 3 a-d  in 1:1 scale) the maximum banknote width is approximately 95 mm. The minimum banknote width is approximately 70 mm. In this instance this is limited by the spacing of abutment surfaces  15  and  16 . In practice this spacing could be reduced to a slightly greater width than the aperture width if required. In this embodiment the aperture width is approximately 24 mm. 
     As in the first embodiment, transportation belts may be used in the place of rollers. 
     Stacking Mechanism 
     The stacking mechanism in this embodiment comprises two rotors  10 , each as described with reference to the first embodiment. Each rotor  10  is mounted and driven in a similar manner to that described with reference to the first embodiment. Referring to FIG. 3 a , the rotors  10  are shown to be mounted opposing each other, with sufficient clearance between them in order that they do not interfere with each other when they are rotated about their axes  11 . 
     Stacking Process 
     Referring to FIG. 3 a , a banknote  1  is shown having been transported between the drive rollers  30  and the trapped bearings  32  to a position above the cashbox aperture  4 . The banknote  1  is shown as being transported to the stacking mechanism in a direction perpendicular to the plane of the diagram by the transportation mechanism. 
     As with the first embodiment, prior to the actuation of the stacking mechanism, the positive gripping force exerted by the rollers  30  is removed from the banknote  1 . This is achieved by raising the associated mounting axles  31  to give a clearance between the rollers  30  and the trapped bearings  32 . However, unlike the first embodiment in which the rollers on one side of the banknote only are released, this occurs on both sides of the banknote  1  in the present embodiment. 
     FIG. 3 a  illustrates the start of the stacking process. The rotors  10  are caused to rotate in synchronism about their respective axles  11  in the directions indicated by the arrows in the Figure. As was described with reference to the first embodiment, the movement of the rotors  10  is entrained using an electric motor and a gear train (not shown). As the angle of rotation of each of the rotors  10  increases, the rotating axles  23  of the rotors  10  are brought into contact with the upper surface of the banknote  1 , in a roughly central position with respect to the banknote  1 . The synchronous operation of the rotors  10  ensures that the force exerted on banknote  1  is even. The possibility of the banknote  1  being skewed upon being stacked is therefore diminished. 
     Continued rotation of rotors  10  causes the banknote  1  to be entrained around the rotating axle  23  of each rotor  10  and onto stack surface  12 ;  13 , as is shown in FIG. 3 b.    
     As the trapped bearings  32  are free to rotate both in the direction of transportation of the banknote  1  and in the perpendicular direction, the banknote  1  is freely moveable both in the transportation stage, and subsequently downwards in the direction of the cashbox  5  during the stacking process. 
     Alternatively, this objective may be achieved by arranging the trapped bearings  32  to be moveable with respect to the fixed drive rollers  30 . Prior to the stacking process they may be lowered in order to allow the banknote  1  to be stacked freely. 
     As the rotors  10  continue to rotate, their rotating axles  23 , diverge from one another along the upper surface of the banknote  1 . As previously described, the rotation of the rotating axles  23  ensures that no undue frictional forces are exerted on banknote  1 , thus reducing the chance of banknote  1  being damaged during the stacking process. 
     As the rotors  10  rotate further, their depth in the cashbox  5  increases. This is allowed for by the compression spring  14  which allows the support surface  13  to be depressed. As is shown in FIGS. 3 c  and  3   d , the further rotation of the rotors  10  causes the rotating axles  23  of the respective rotors  10  to diverge. This has the effect of causing the banknote  1  to be further entrained about the trapped bearings  32  as the banknote  1  progressively enters the cashbox  5 , until it has entirely entered the cashbox  5  and is flattened against stack surface  12 ;  13 , as is shown in FIG. 3 d . This occurs at the maximum degree of rotation of the rotors  10 ; approximately 90°. It is desirable that the actual degree of rotation of the rotors  10  is sufficient to make the banknote contacting portions  23  of the rotors  10  reach or just pass the point of maximum depth of penetration into the cashbox  5 . This facilitates the unrolling of the banknote and reduces the risk of the banknote being incorrectly stacked. 
     At this point, as the rotors  10  are circular in profile the ends of each rotor are positioned directly beneath the axis about which they rotate. 
     As the rotors  10  rotate in the reverse direction, out of the cashbox  5 , the banknote stack is biased under the influence of the spring  14  towards the aperture  4 , against the retreating rotors  10 . As the rotors  10  withdraw from cashbox  5  entirely, the stack surface  12 ;  13  is urged by the compression spring  14  against the abutment surfaces  15 ,  16  situated on the inside of the upper surface of the cashbox  5 . The abutment surfaces  15 ,  16  ensure that positive control over the stack surface  12 ;  13  is always maintained. 
     This embodiment of the invention yields the same advantages as the first embodiment. In addition, however, the aperture  4  of the cashbox  5  may be smaller in this embodiment due to the absence of the pusher plate, which may increase the degree of security which may be imparted to a cashbox for use with this embodiment. In this embodiment of the invention the minimum width of the aperture  4  must be at least twice the thickness of rotor arm  20 , approximately 14 mm. Therefore a minimum aperture width of approximately 15 mm may be achieved in this embodiment. 
     Furthermore the speed with which a banknote may be stacked may be increased as in this embodiment both rotors  10  act simultaneously, as opposed to the arrangement in the first embodiment where the rotor and the pusher plate are actuated at different times. 
     Third Embodiment 
     The third embodiment of the invention operates in a similar manner to that described with reference to the second embodiment and similar features will not be described further in detail. 
     In this embodiment, the rotors  40  are of a slightly different design compared to those previously described. 
     Referring to FIG. 4 a , a rotor according to the present embodiment is illustrated. Unlike the rotor  10  previously described, rotor  40  has no to support bar  22  or rotating axle  23 . Rotor  40  has three rotor arms  41  (although this number could be higher or lower). At the end of each rotor arm  41  is a wheel  42 . Each wheel  42  forms a banknote engaging surface, which fulfils the same function as the rotating axle  23  of rotor  10 . Alternatively, the rotating wheels  42  may be replaced by non-rotating banknote contacting surface made from a low friction such as PTFE. 
     The arms  41  of opposing rotors  40  are thus arranged to interdigitate. This is illustrated in FIGS. 4 b  and  4   c  which respectively show the rotor structure and arrangement from above in the resting state and from the side in operation. 
     This provides the added advantage that aperture  4  of cashbox  5  may be made narrower, yet still allow the entry of the rotors in order to stack the banknotes; thus, cashbox  5  may be more easily made secure when it is removed from the validator. Specifically, the minimum width of the cashbox aperture  4  (approximately 10 mm in this embodiment) is limited by the thickness of one rotor arm  41 , which in this case is 7 mm. 
     Fourth Embodiment 
     In the fourth embodiment the stacking mechanism operates in a similar manner to that described with reference to the second and third embodiments and similar features will not be described further. However, in the fourth embodiment the positional control exerted over the banknote  1  during the stacking process is improved through the use of a banknote contacting membrane  50  interposed between the rotors  10 ;  40  and the banknote  1 . 
     Membrane 
     A membrane  50  according to the present embodiment is illustrated in plan view in FIG.  5 . The membrane  50  may be made of various wear resistant materials which may be produced in thin flexible sheets and suitable for rolling on rollers; such as polyester, mylar (TM), kevlar (TM) and Gore-tex (TM). 
     The membrane  50  is symmetrical about the dotted centre line and has a single connection point  51  situated at each end. The connection points  51  provide a means of attaching the membrane  50  to rollers  53 ,  54  upon which the membrane  50  is wound. It is advantageous to have a single point of attachment to each roller as this reduces the possibility of the membrane  50  becoming skewed when it is wound on or off the rollers  53 ,  54 . 
     The membrane  50  also comprises a central friction strip  52 , situated on its banknote contacting side. This is beneficial in terms of increasing control over the banknote  1  during the stacking process by increasing the level of friction between the membrane  50  and the banknote  1 . In the present embodiment the friction strip  52  is made from vulcanised rubber which is bonded to the membrane  50 . However, it may be made from any other suitable high friction material and attached to the membrane by any other suitable method, such as by stitching. 
     The membrane  50  is mounted upon rollers  53 ,  54 , as shown in FIG. 6, which are spring loaded and mounted in the chassis of the stacker mechanism. This is achieved using springs (not shown) internal to the rollers  53 ,  54 . The effect of the springs is to bias the rollers  53 ,  54  in the directions indicated by the arrows in FIG. 6 a . Therefore, in its resting state the membrane  50  is held taught between the rollers  53 ,  54 , entrained over two guide rollers  55 ,  56 , which are also mounted in the chassis of the stacker mechanism, as shown in FIG.  6 . 
     Stacking Operation 
     Referring to FIG. 7, a stacking mechanism according to the fourth embodiment of the invention is shown. 
     FIG. 7 a  illustrates the start of the stacking cycle, which is as described with reference to the second and third embodiments, with the exception of the addition of membrane  50 , and so common features will not be discussed further in detail. 
     As the rotors  10 ;  40  are caused to rotate about their respective axes  11  they contact the membrane  50 , which is positioned between the banknote  1  and the rotors  10 ;  40 . Further rotation of the rotors  10  causes the membrane  21  to be pushed downwards and entrained first around the guide rollers  55 ,  56 , as shown in FIG. 7 a  and then around trapped bearings  32 , which are located at either side of the aperture  4 . The purpose of the guide rollers  55 ,  56  is to prevent the membrane  50  from snagging on the rollers  30 . 
     The rollers  53 ,  54  are caused to rotate in the directions indicated by the arrows in FIG. 7 a , against their respective spring force bias, as the membrane  50  unrolls from them under the action of the rotors  10 ;  40 . As the rotors  10 ;  40  move the membrane  50  downwards through the banknote transportation plane, as shown in FIG. 7 b , the banknote  1  is contacted by the friction strip  52 . As the friction strip  52  displaces only in a vertical sense, and hence remains centred in the mechanism throughout the stacking process, it serves to reduce any skewing of the banknote which might otherwise occur. 
     As the rotors  10 ;  40  rotate further, as shown in the sequence illustrated in FIGS. 7 b  to  7   d , the banknote  1  is pushed through the cashbox aperture  4  and brought into contact with the stack surface  12 ; 13  as shown in FIG. 7 b . The banknote  1  is then unrolled in a sideways direction with respect to the stack surface  12 ; 13  as shown in FIGS. 7 c  and  7   d.    
     Subsequently, as the rotors  10 ;  40  rotate in reversed directions on exiting the cashbox  5 , membrane  50  is tensioned by the springs in axles  53 ,  54 , which ensure that there is no slack in the membrane  50  during the removal of rotors  10 ;  40 , from cashbox  5 . Since there is no relative movement between the membrane  50  and the stacked banknote  1  in the plane of the surface of the stack  12 ; 13 , the banknote  1  is not disturbed by the withdrawal of the rotors  10 ;  40  and the membrane  50 . 
     Fifth Embodiment 
     Referring to FIGS. 8 and 9, a stacking mechanism according to the fifth embodiment of the invention is shown. In general terms, the mechanism of this embodiment fulfils the same functions as those described in the first embodiment. Features in this embodiment which are similar to features already discussed are referenced using the same reference numerals and will not be discussed further in detail. 
     Whereas the mechanism of the first embodiment incorporates a stacking mechanism and a transportation mechanism which are housed in a banknote handling apparatus, to which a cashbox is removably attached, the mechanism of the current embodiment incorporates part of the transportation mechanism and the entire stacking mechanism in the cashbox itself. This feature greatly enhances the level of security which may be provided for a detachable cashbox. As a result of this feature, the aperture  4  through which banknotes are stacked is internal to the outer casing of the cashbox. Therefore, on being detached from the banknote handling device, for example a validator, there is no external aperture large enough to allow a person to tamper with the contents of the cashbox. 
     Transportation Mechanism 
     Referring to FIG. 8, it will be noted that the cashbox according to the present embodiment consists of an inner and an outer envelope, referenced by numerals  60  and  61  respectively. A banknote  1  is introduced into the cashbox  5  in the direction of arrow “A”, by the transportation mechanism of a banknote handling apparatus to which the cashbox  5  is attached. The aperture (not shown) through which a banknote  1  may be introduced into the cashbox need only be slightly larger than the width-wise cross sectional dimensions of the largest banknote  1  with which the apparatus is designed to work, further increasing the level of security of the cashbox  5 . On entering the cashbox  5 , the banknote  1  is engaged by opposing pairs of belts  62 ,  62   a  and  63 ,  63   a  which are arranged to grip the banknote  1  along each of its longitudinal edges. The belts  62 ,  62   a  and  63 ,  63   a  are driven by rollers  64 , which in turn are driven by a connection (not shown) from the banknote handling apparatus drive mechanism through an aperture (not shown) in the wall of cashbox  5 . The upper belts  62 ,  63  of the drive arrangement are biased using springs  65  in order to keep the banknote  1  firmly in contact with opposing belts  62   a ,  63   a.    
     Stacking Mechanism 
     Referring to FIG. 9, it can be seen that as with previous embodiments, in this embodiment banknotes are stacked onto a plate  13  which is supported by a spring  14 . This allows the banknote stack  12  to be displaced by the stacking mechanism as a new banknote  1  is stacked and to return as the stacking mechanism retreats in order that the uppermost banknote  1  in the stack  12  abuts the abutment surfaces  15 ,  16  of the upper wall  66  of the inner envelope  60  of the cashbox  5 . Thus, the banknote stack  12  is always maintained under positive control as discussed in previous embodiments. 
     Referring again to FIG. 8, the stacking mechanism comprises an actuation lever  70  which is moveable in the direction of the arrow shown in FIG. 8 by an external drive mechanism (not shown). This may take the form of a simple gear, for example, connected via an aperture in the cashbox wall to an electric motor housed in the banknote handling apparatus. The rotation of actuation lever  70  causes the rigidly connected assembly of rod  71 , connecting arm  72  and roller axle  73  to rotate about the longitudinal axis of rod  71 , such that the roller axle  73  enters the cashbox aperture  4  (best seen in FIG. 9) in a radial channel  90  in the end wall of the inner cashbox envelope  60 . 
     The actuation lever  70 , rod  71 , connecting arm  72  and roller axle  73  may be manufactured from any suitable rigid material such as steel and interconnected using standard manufacturing techniques. 
     The roller axle  73  has mounted at either end a roller  74 ,  75 . Each roller  74 ,  75  is provided with a rubber tyre for engaging a piston  80 ,  81 ,  84  which will be described in more detail below. The roller axle  73  is secured at the end of roller  74  only, to connecting arm  72 ; thus avoiding the need for providing further channels in the internal envelope  60 , which would be required for securing the second end of roller axle  73 . The roller axle  73  is free to rotate against the spring bias of an internally mounted spring (not shown) housed in connecting arm  72 , the biasing of which acts in the direction of the arrow shown in FIG.  9 . The rollers  74  and  75  are mounted on the roller axle  73  such that they are free to rotate independently of the roller axle  73 . 
     The banknote stacking mechanism further comprises a piston assembly, as mentioned above. The piston assembly comprises a banknote engaging plate  80 . The plate  80  is dimensioned such that it just fits through the aperture  4  of the upper surface of the inner envelope  60  of cashbox  5 , as viewed in FIGS. 8 and 9. The aperture  4  is in turn dimensioned such that its length (in the direction of banknote transportation) exceeds the length of the longest banknote with which the apparatus is designed to function. 
     The piston assembly is mounted in a slot  86  in the end wall of the inner envelope  60  which receives a reduced width portion of a guide piece  81  of the piston body, such that the guide piece  81  is free to move linearly in the slot  86 . The guide piece  81  is held in a planar relationship with the end wall of the inner envelope  60  by the end wall of the outer envelope, with which it is a sliding fit. The guide piece  81  is acted on by a spring  83  which biases the piston body towards the upper surface  66  of the inner envelope  60  of cashbox  5  as viewed in FIGS. 8 and 9, such that in its resting condition, as is shown in FIG. 9, the plate  80  of the piston body is situated above the plane of a banknote  1  which is held between each side of the transport mechanism. 
     The piston body also comprises an arm  84  which extends perpendicularly to the guide piece  81  and which is co-planar with the plate  80 . The entire piston body assembly may be made from any suitable rigid material, such as steel or a plastics material and may be made as a one piece moulding or may be assembled, using standard manufacturing techniques from components parts. 
     Entrained about the roller axle  73  is a membrane  91 , similar to that described in the fourth embodiment. One edge of the membrane  91  is secured to the roller axle  73 . The membrane  91  extends from near the roller  75 , along approximately the entire length of the plate  80 . 
     The other edge of the membrane  91  is secured to a longitudinal edge of plate  80 , for example by adhesion, as is shown in FIGS. 8 and 9. 
     Mode of Operation 
     As has been described with reference to the previous embodiments, the banknote  1  is transported by the transportation mechanism and held stationary above the aperture  4  prior to the initiation of the stacking procedure. Subsequently, the belt transport system  62  is raised relative to its opposing belt  62   a  in order to create a clearance between the belts  62  and  62   a  such that an edge of the banknote  1  may be withdrawn during the stacking operation. This is initiated by the rotation of actuation lever  70  in the direction indicated by the arrow on FIG.  9  and as previously described this results in the rotation of roller axle  73  into the inner envelope  60  of cashbox  5  along the radial slot  90  in the end wall of the inner cashbox  60 . In so doing, roller  74  acts on the arm  84  of the piston body, forcing the piston body to slide vertically down into the inner envelope  60  of cashbox  5 , along slot  86 . This in turn causes the underside of the plate  80  to come into contact with the upper surface of the banknote  1 , which is entrained by the plate  80  through the aperture  4  and onto the upper surface of the stack of banknotes  12  in the cashbox, or, onto the support plate  13  if the cashbox is empty. 
     Once the piston plate  80  has secured one edge of the banknote  1  against the banknote stake  12 , the second banknote edge is release by the raising of the belt transport system  63  relative to its opposing belt transport system  63   a.    
     As the actuation lever  70  continues to rotate in the direction of the arrow shown in FIG. 8, the action of roller  74  continues to force the piston body downwards against the action of spring  14  shown in FIG.  9 . Thus, as the roller axle  73  moves across the upper surface of the plate  80 , the membrane  91  is wound onto the roller axle  73  by virtue of the biasing spring (not shown) in connecting arm  72  which acts upon the roller axle  73 . This continues until the point at which the roller axle  73  passes off the right hand edge of plate  80 , as viewed in FIG.  9 . 
     Continued rotation of the actuation lever  70  causes the membrane  91  to unwind, against the action of the spring (not shown) acting upon the roller axle  73  until the roller axle  73  reaches its maximum depth of penetration into the inner envelope  60  of the cashbox  5 . This state is shown in FIG. 9 by the dashed representation of connecting arm  72 ′, roller axle  73 ′, roller  75 ′, plate  80 ′, membrane  91 ′, banknote stack  12 ′ and support plate  13 ′. Thus, the action of roller axle  73 , together with that of the membrane  91  has at this point flattened the remainder of the banknote  1  against the stack  12 . 
     It should be noted that in this embodiment, as with the mechanism of the first embodiment, the final stacked position of the banknote is laterally offset with regard to the position of the banknotes during transportation. 
     It should also be noted that at this point, roller  74  continues to exert a downward force on the piston body, via the extreme end of arm  84 . This is despite the fact that the roller axle  73  is no longer situated above plate  80 . 
     The actuation mechanism then proceeds to drive actuation lever  73  in the reverse direction to rotate the roller axle  73  back out of the inner envelope  60  of cashbox  5  along the radial path defined by slot  90 . The biasing force of spring  83  causes the piston body to return to its normal position, shown in full line in FIG.  9 . 
     Similarly the biasing force of the spring (not shown) which acts on roller axle  73  causes the membrane  91  to be once again wound onto the roller axle  73  up until the point at which the roller axle  73  again reaches the upper surface of the plate  80 , leaving the banknote in its stacked position. And thereafter to unwind again as the position shown in FIG. 9 is approached. 
     The skilled reader will appreciate that the present embodiment has the advantages described earlier with respect to the first embodiment of being tolerant of misalignment of the banknote  1  as it is presented for stacking, since no datum edge is relied upon in order to effect the stacking operation. 
     Similarly, because each banknote  1  is effectively stacked by positioning part of the banknote  1  on the stack  12  and subsequently flattening the remainder against the stack  12 , this embodiment is also able to cope with a wide range of banknote sizes. However, in addition, the presence of the membrane  91  further increases the control which may be exerted upon the banknote  1  during the stacking operation. 
     Furthermore, the tensile stresses imparted to the banknote  1  are reduced by the presence of the membrane  50 . Therefore, the chances of the banknote  1  being torn by the stacking process are further reduced. Accordingly, the speed of the stacking cycle may be further increased. 
     The skilled reader will understand that a banknote stacking apparatus according to the present invention may be used in various applications, particularly where banknotes are automatically accepted and validated such as in automated vending machines and banknote changing machines. Referring to FIG. 10 a  a banknote validating machine  100  is shown in conjunction with a cashbox  5 . Referring now to FIG. 10 b , an idealised sectional view through the machine  100  is shown. This shows a banknote  1  on the point of being inserted into an aperture  101  from where it is transported along a banknote transportation system  102  by a drive unit  103  and validated by a validation apparatus  104 . The transportation system  102  then transports the banknote  1  to a stacking arrangement  105  so that the banknote  1  may be stacked in the cashbox  5  as has been described in previous embodiments, the stacking arrangement  105  may be located in the validator  100  as it is shown in FIG. 10 b  or alternatively in the cashbox  5  itself. 
     Furthermore, it will be appreciated by the skilled reader that the stacking arrangement  105  employed in a banknote accepting machine may conform to any one of the previously described embodiments. 
     It will be apparent from the forgoing that various modifications and variations may be employed in relation to the above-described embodiments without departing the spirit or scope of the present invention. In particular, features of the embodiments described may be employed individually or in individual combinations without departing from the scope of the invention. 
     For example the skilled reader will appreciate that the present invention as described in the second, third and fourth embodiments, could be used to insert documents such as banknotes, loosely through an aperture; thus obviating the need to any stack forming means. 
     Furthermore, the skilled reader will appreciate that by adjusting the clearance between the upper and the lower halves of the banknote transport mechanism, the present invention could be used to stack bundles of banknotes, which have been held, for example, in a temporary storage device such as an escrow. 
     The skilled reader will also appreciate that various modifications may be made to the mechanism with which the rotors and the pusher plate are driven. For example, both the rotors and the pusher plate may be driven by a single, non-reversible electric motor, their actuation timing being controlled through the use of cams, for example. Furthermore, the banknote transport mechanism may be arranged to deliver banknotes for stacking at predetermined intervals, allowing the continuous operation of the stacking mechanism. 
     The skilled reader will also realise that the inventive concept of the present invention may be realised using stacking members which would not normally be termed rotors. For example, the opposing rotors of the second embodiment may be replaced with parallel rods, each supported at either end in an “L” shaped channel. By moving the rods in the “L” shaped channels the required downward and sideways movement for stacking a sheet according to the present invention may be accomplished.