Sheet feeders are used in a variety of fields both for feeding relatively rigid sheets such as cards, as described in GB-A-1366151, and also flexible sheets such as paper sheets including security documents such as banknotes.

A typical sheet feeding system comprises a withdrawal system for withdrawing a sheet from one end of a stack; a separation system to which the withdrawn sheet is fed for separating the sheet from any other sheet withdrawn at the same time; and a transport system for transporting the separated sheet to downstream processing such as denomination detectors, authentication detectors and the like following which the sheet is fed to a storage location or to a dispense outlet. In WO-A-98/23513, EP-A-0260015, US-A-4867431 and US-A-4813658 we describe such sheet feeding systems.

A problem which can arise when handling certain types of sheet is that they are not easily separated from the rest of the stack. This can arise particularly with mint stacks of banknotes, known as mint"bricks", in which the sheets tend to adhere together.

In accordance with the present invention, a method of feeding flexible sheets from a stack comprises withdrawing a sheet from one end of the stack; separating the withdrawn sheet from any other sheets withdrawn at the same time; and transporting the sheet away from the stack, and is characterised in that the method further comprises, prior to the withdrawing step, feeding the said sheet in an opposite direction ("reverse feed") to the withdrawal direction so as partially to release it from the remainder of the stack.

We have found that by applying a reverse feed to the leading most sheet in the stack, prior to withdrawing the sheet, partially releases the sheet from the remainder of

the stack. This is possible in the case of flexible sheets, particularly paper sheets such as banknotes, which can be caused to buckle away from the rest of the stack providing the partial release required. The result of this is that a much more efficient feeding method is achieved with far fewer double feeds being received by the separation system and a lower rate of stream feed conditions arising.

Preferably, the reverse feed is performed while the separation and/or transport step is being carried out on a previous sheet or sheets. This optimises the operation of the system which does not need to wait until one sheet has been fully transported away before attempting to withdraw the next.

As mentioned above, the reverse feed can be used not only to release a sheet from a stack but to relieve a stream feed condition.

Typically, where the sheets comprise banknotes, the banknotes are transported thereafter to one or more of a denomination detector, an authenticity detector, and a soil detector.

The method may be implemented on a variety of different types of apparatus but conveniently is implemented on apparatus as described in WO-A-98/23513 incorporated herein by reference.

Some examples of sheet feed apparatus implementing a method according to the invention will now be described with reference to the accompanying drawings, in which:- Figures la and lb are schematic right and left side elevations respectively of a first example; Figure 2 is a plan of the apparatus shown in Figures la and lb ; Figure 3 is a schematic diagram illustrating the reverse feed operation; and, Figure 4 is a view similar to Figure la but of a second example.

The apparatus shown in Figures la, lb and 2 comprises a sheet store 1 containing a stack of sheets (usually of a common size) such as banknotes 2. The banknotes 2 are supported by a pressure plate 3 which is mounted to cause the sheet in the stack 2 furthest from the pressure plate to be urged against a pair of friction feed rollers 4 non- rotatably mounted on a shaft 5 journalled between a pair of side plates 6,7. The shaft 5 carries a toothed pulley 8 non-rotatably mounted laterally outwardly of the side plate 7, the toothed pulley 8 being connected to a stepper drive motor 9 via toothed drive belt 10 and a further toothed pulley 11.

The feed rollers 4 have a continuous rubber surface around their circumference.

The topmost sheet from the stack 2 is fed upon rotation of the rollers 4 into a separator system 12 comprising a pair of separator rollers 13 mounted via one- way clutches 13A (Figure 3) on a shaft 14 journalled between the side plates 6,7. A toothed pulley 15 is non- rotatably mounted to the shaft 14 laterally outwardly of the side plate 6 and is connected by a drive belt 16 to a stepper motor 17 via a further toothed pulley 18. The rollers 13 also have a continuous rubber element around their circumference.

A set of contra-rotating rollers 19 are included within the separator system 12 and cooperate with the rollers 13 to prevent more than one sheet being fed through the separator system. The contra rollers 19 are mounted on a shaft 20 journalled between the side plates 6,7 and are adjustable towards and away from the shaft 14 so as to adjust the gap between the rollers 13 and 19. The contra rollers 19 are step rotated in an anti-clockwise direction by a linkage system (not shown) in order to even out the wear that occurs on their circumferential surface. A scale 21 is provided to enable the position of the contra- rotating rollers 19 and hence the spacing between the rollers 13,19 to be indicated.

Sheets are fed by the separator system 12 to the main sheet transport system whose entry pinch point is defined by a pair or pairs of rollers 22,23 mounted on shafts 24,25 respectively journalled between the side plates 6,7. Each shaft 24,25 carries non-rotatably a respective toothed pulley 26,27 connected via drive belts (not shown) to a main drive motor (also not shown).

A sensor system 28 is provided for sensing when a sheet has been received by the sheet transport system, the sensor system 28 being connected to a controller 29.

The operation of the system will now be described with reference also to Figure 3. Figure 3 shows a mint brick of banknotes 2 arranged horizontally instead of vertically as shown in Figure la. The method of operation is, however, the same in both cases. Initially, the controller 29 causes the motor 9 to rotate the feed rollers 4 in a reverse direction as shown by an arrow 30 in Figure 3.

This causes the end most banknote 31 contacting the feed rollers 4 to buckle as shown in Figure 3 and thus to separate it from the adjacent document in the stack. This movement is in response to a pulse drive to the motor 9.

The motor 9 is then caused to rotate the feed rollers in their feed direction (clockwise in Figure 3) to push the end most banknote 31 between the rollers 13,19. At this stage, the rollers 13 take over the feed control of the banknote 31 and feed the banknote into the nip defined between the rollers 22,23 of the main transport system.

Once the arrival of the banknote at the transport system is detected by the sensor 28, the controller 29 stops the motor 9 so as to prevent further rotation of the feed rollers 4. The main transport rollers 23,24 feed the banknote forward, a one-way clutch connection 13A between the separation rollers 13 and the shaft 14 allowing the separation rollers 13 to free wheel on the shaft in the feed direction. The banknotes are then fed past conventional detectors indicated schematically at 50, such

as visible, ir or uv pattern detectors, authenticity and/or soil detectors.

If a number of banknotes have to be fed in one operation from the stack 2, as soon as one banknote has been fed from the stack and is being fed forward by the separation system 12, and is no longer contacting the feed rollers 4, the feed rollers 4 can be reverse driven as described above to begin the process of separating the next banknote from the remainder of the stack so it is ready for feeding to the separation system 12 at the earliest opportunity.

Optionally, the stream feed detector 35 (Figure 3) may be positioned to detect stream fed banknotes exiting the separation system 12. If such a stream feed is detected, the feed rollers 4 can be operated in reverse to prevent further stream feed.

Although the system is described using friction feed rollers, other feed systems such as friction belts or vacuum rollers/belts could also be used.

Although the mechanism described above has the advantage that the withdrawal or feed system is operated independently of the separator system, it requires the two systems to be independently driven. In the preferred embodiment, Figure 4, shaft 5 carries a gear 30 non rotatably mounted laterally outwardly of the side plate 7, gear 30 meshing with a drive gear 31 rotatably mounted on a stub shaft 32 attached to the outer face of the side plate 7. Attached to the drive gear 31 is toothed pulley 33 connected via toothed belt 10 and toothed pulley 11 to the shaft of motor 9.

Also engaging with the drive gear 31 is the separator roller system drive gear 35, non rotatably mounted to the outer component of a one way clutch 36, the inner component of which is mounted non rotatably laterally outwardly of the side plate 7 on to the end of shaft 14. In this example the separator rollers 13 are non rotatably mounted on shaft 14.