GB2209963A - Method and apparatus for shredding sheets - Google Patents

Abstract

A method and apparatus for shredding sheets (5) comprises a sheet feed system (4) for feeding sheets to a shredding station (6). A detector (24) senses each sheet fed to the shredding station (6) and causes a predetermined single sheet value to be arithmetically summed with a running count. The noise generated at the shredding station is monitored by one or two microphones (15, 16) and when the noise level exceeds a threshold indicating that the shredding of sheets is taking place, the running count is decremented at a predetermined rate. An error signal is generated if the running count exceeds a predetermined threshold. This condition will occur if the sheet feed rate to the shredder exceeds a rate at which the shredder can efficiently operate. <IMAGE>

Description

METHOD AND APPARATUS FOR SHREDDING SHEETS The invention relates to methods and apparatus for shredding sheets such as banknotes.

When a number of banknotes or other security documents have to be disposed of they are usually sorted into various groups and/or counted before being destroyed. When destruction takes place by means of an on-line shredding facility provided as part of the banknote processing machine, it is desirable to know that the banknotes are actually being destroyed efficiently.

This is important from the security point of view since the destroying authority must be sure that notes counted as destroyed are not unknowingly returned to circulation.

It is also important for proper machine operation so that any jamming of banknotes can be detected and undue stoppages can be avoided by the operator taking corrective action. If the rate of destroying notes is sufficiently low then shredding of single notes can take place and a direct method of correlation can he made. On the other hand, if the rate of destruction is sufficiently high then resort has to be made to overlapping of the notes as they are fed to the shredder and an indirect method of correlation then has to be used. Various methods of correlation have been proposed in the past.

Direct methods of monitoring the shredding operation include, for example an optical sensing and transmitting system across the exit throat of the shredder for observing the shreds; or the weighing of the shreds in a suitable receptacle. However, optical sensor and transmitter performance can deteriorate very quickly in the inherently dust atmosphere of the shredder and the weighing method is not sufficiently sensitive to monitor small numbers of notes. Moreover, the machine would need to be stopped to allow accurate weighing to take place so that a dynamic disposal system, such as vacuum extraction, would be difficult to implement.

Indirect methods of confirming that notes are being shredded efficiently by monitoring the shredder drive motor speed and current have not been successful because it has been found that these parameters do not vary sufficiently when single banknotes are passed through the shredder.

In accordance with one aspect of the present invention a method of shredding sheets comprises feeding sheets to a shredding station at which the sheets are shredded characterised by sensing each sheet fed to the shredding station and arithmetically summing in one sense a predetermined single sheet value with a running count; monitoring the noise generated at the shredding station and causing the running count to change at a predetermined rate in the opposite sense when the monitored noise indicates the shredding of sheets is taking place; and generating an error signal if the running count passes a predetermined threshold indicating an unacceptable operating condition.

In accordance with a second aspect of the present invention, apparatus for shredding sheets comprises a sheet shredder positioned at a shredding station; a sheet feed system for feeding sheets to the shredder; a sheet sensor for sensing each sheet fed by the sheet feeding system; counting means for storing a running count and for arithmetically summing in one sense a predetermined single sheet value with the running count when the sheet sensor detects the feeding of a sheet by the sheet feed system; noise monitoring means for monitoring the noise generated at the shredding station and for causing the counting means to change the running count in the opposite sense at a predetermined rate when the monitored noise indicates that the shredding of sheets is taking place, the counting means generating an error signal if the running count passes a predetermined threshold indicating an unacceptable operating condition.

The present invention provides a method and apparatus for monitoring the shredding operation when either single or overlapped sheets are fed to the shredding station and is based on the fact that sheets being shredded generate a characteristic acoustic signal.

We have found that mint banknotes produced a high frequency "crackle" as they are being distorted by the shredder blades and a low frequency noise due to structural vibration of the equipment. Old banknotes do not produce the high frequency noise to the same extent but still produce the low frequency noise at a detectable level.

In some examples, the noise monitoring means comprises a low frequency acoustic detector such as a piezo-electric transducer while in other examples the noise monitoring means comprises a microphone. The microphone may be adapted to detect higher frequencies associated with note crackle. In the preferred arrangement, the noise monitoring means is adapted to sense noise at two different frequencies and o generate a composite output signal.

We have found that high frequency crackle is directional in nature and careful positioning of the noise monitoring means is necessary.

Conveniently, the noise monitoring means is positioned at the entrance to the shredding station when high frequencies are to be detected. When low frequencies are to be detected, the noise monitoring means is preferably positioned at the exit of the shredding station.

Preferably, the step of monitoring the noise generated at the shredding station comprises generating an analogue signal related to the noise, comparing the analogue signal with a predetermined noise threshold, and generating a shredding signal indicating that the shredding of sheets is taking place when the analogue signal exceeds the noise threshold.

Typicall, in the comparison step, the amplitude of the analogue signal is compared with the noise threshold.

In the preferred example, the counting means includes a down counter which is enabled by a signal from the noise monitoring means generated when the shredding of sheets is taking place.

It will be understood that the predetermined rate, predetermined single sheet value, and the predetermined running count threshold are chosen such that the error signal is generated when the rate of feeding sheets to the shredder exceeds the sheet shredding rate of the shredder by a predetermined amount. In this way, the sheet feed rate can be kept at a sufficientl low value to enable efficient shredding to take place. In addition, jamming of the shredder can be detected.

An example of banknote shredding apparatus for carrying out a method according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic end view of the apparatus; Figure 2 is a schematic side view of part of the apparatus shown in Figure 1; Figure 3 is a block circuit diagram of the shredding apparatus control system; Figure 4 is a partial perspective view of the shredding apparatus; and, Figure 5 is a block diagram illustrating the signal conditioner of Figure 3 in more detail.

Figure 1 illustrates a banknote sorting machine 1 which sorts banknotes in a known manner into notes suitable for reuse and notes which should be destroyed The notes which are to be destroyed are fed along a path 2 (Figure 2) to a rotating stacking wheel 3 where they are stacked onto a conveyor system 4. The conveyor system 4 comprises three, laterally spaced, endless belts (Figure 4) which carry the banknotes 5, possibly in an overlapped manner as shown in Figure 1, at a slower rate than that at which they are fed to the stacking wheel 3, towards a shredding station 6. s they approach the shredding station 6, the banknotes are gripped between the conveying system 4 and an auxiliarv conveyor belt 7 which feeds the banknotes into a shredder 8.

The shredder 8 comprises a pair of rollers 9, 10 carrying a large number of circumferentially spaced, axially extending cutting blades (not shown), the blades on each roller cooperating in a conventional manner to cut sheets passing between the rollers. The rollers 9, 10 are rotated by a motor 11 coupled to the rollers by a drive belt 12.

The shredded banknotes fall under gravity into a shredder funnel 13 and are vacuum extracted into the secure container. The shredder station 6 is positioned within a secure container 14 which prevents unauthorised access to the shredding system.

A first microphone 15 is positioned at the entry end of the shredder 8 while a second microphone 16 is positioned at the exit end of the shredder 8. The microphone 15 is adapted to sense high frequency noise such as banknote crackle while the microphone 16 is adapted to sense low frequency noise such as vibration of the shredder mechanism.

The microphones 15, 16 are coupled to a signal conditioner circuit 17 shown in more detail in Figure 5.

The circuit 17 comprises a pair of preamplifiers 30, 31 connected to microphones 15, 16 respectively. The preamplifiers 30, 31 are coupled to respective high and low pass filters 32, 33 whose outputs are rectified by full wave rectifiers 34, 35. The signals from the rectifiers 34, 35 are algebraically summed together by a summing circuit 36 and passed to a low pass filter 3? which smooths the signal in a conventional manner. The smoothed signal from the conditioner circuit 17 is fed to a digitiser circuit 18 in which the amplitude of this smoothed signal is compared with an empirically predetermined threshold. If the threshold is exceeded, this is taken to be indicating of the shredding of at least one sheet by the shredder 8 and a binary signal with a value "1" is output by the digitiser 18.

Otherwise, a binary "0" is output.

The output signal from the digitiser circuit 18 is fed to the enable input of a digital down counter 19 which also receives a clock signal 20. The clock signal fed to the down counter is generated at a rate which is empirically determined as will be explained in more detail below.

The current value of the running count is fed to a comparator circuit t 21 where it is compared with a maximum allowable value for the running count, the comparator circuit 21 generating an error signal if the maximum allowable count is exceeded. The current value of the running count is also fed to a summing circuit 22. The summing circuit 22 is adapted to sum with the running count a single sheet count representing the entry of a banknote into the shredding system, the single sheet count, corresponding to a single note, being stored in a memory 23. This addition is enabled by a sensor 24 positioned downstream of the stacking wheel 3.The sensor 24 may comprise an optical sensor positioned opposite a light source 25, the sensor 24 generating a signal indicating the passage of a single sheet when the amplitude of the optical signal impinging on the sensor 24 falls below a threshold value. The resultant count value produced by the circuit 22 is then set in the counter 19.

It will be seen from Figure 3 that each time a note is fed into the shredding system by the stacking wheel 3, the running count will be incremented by a constant multiple count step value; while as soon as the microphones 15, 16 detect the shredding of notes, the down counter will be enabled so that the running count will be decremented in single steps.

If for some reason the shredding system jammed then the noise monitored would drop and thus the down counter would be disenabled and the supply of further sheets to the shredding system would cause the running count rapidly to exceed the allowable maximum value causing the generation of the error signal.

In addition, if the supply of sheets to the shredding system is at too fast a rate at which the shredding system cannot efficiently shred the sheets then the down counter will not be decremented at a fast enough rate by the clock signal and again the running count will generally increase until the maximum allowable value is exceeded.

In one example, if the maximum capacity of the shredder system is ten notes and the down counter 19 is able to count down from 60,000, each note entering the shredder system could represent a count of 5000 and the maximum allowed count would therefore be 50,000. If the nominal shredding rate was 0.5 sec/note, the decrement rate would need to be a minimum of 10 Hz to cause the running count to fall to zero by the time the note had been shredded. In fact since notes may overlap during the shredding process and appear shorter than the actually are, the decrementing rate would need to be more than lOKHz, for example 15 KHz. It will be seen therefore that the clock rate should be chosen to correspond to the fastest acceptable shredding rate.

In this case, a single note shredded would cause the counter 19 to decrement to zero in 0.33 seconds, thus three notes entering but overlapping and appearing as two would still cause the counter to decrement to zero by the time thev passed, as in the same way would six notes entering and being shredded as three overlapped plus 3 single notes. Thus the system can cater for short term overloads but if 12 notes entered and by the time the twelfth was counted in, less than two had been counted out, the maximum count of 50,000 would be exceeded, and the error signal would be generated.

It should be understood that although the invention has been described as making use of a down counter and the incrementing of a running count by a fixed value when sheets enter the system, it could equally well be performed by making use of an upcounter and decrementing the running count when sheets enter the system. In this case, the comparator 21 would issue the error signal if the running count was less than a minimum allowable value.

The construction of the shredding system and its speed of operation will determine the frequencies that will dominate when sheets are being shredded. The filters associated with each microphone 15, 16 should then be chosen to accentuate the dominant frequencies which occur during shredding.

In the example just described, frequency analysis of the signal produced by banknotes being shredded show that the predominant frequencies are in the 6 KHz and 63 Hz third octive band range. Mint banknotes exhibit a very high increase over background noise (10 dR) at 6 KHz (crackle) whereas very worn/limp notes give a neglible output of 6 KHz. Both mint and worn give a significant increase over background noise (6 to 10 dB) at 63 Hz.

All other frequency bands either fluctuate too greatly to have the change monitored or the signal change is less then 3 dB.

The monitoring system described could be used with other forms of sheet destruction systems such as cross-cut shredding devices which are likely to produce more high frequency noise than say a strip shredder because of the increased paper deformation.

The error signal may be used to control a display indicating to the operator that an error condition has occurred. In addition, or alternatively, the error signal can be used to stop the drive to the convevor system 4 so as to prevent any further sheets from being fed.

Claims (11)

1. A method of shredding sheets comprises feeding sheets to a shredding station at which the sheets are shredded characterised by sensing each sheet fed to the shredding station and arithmetically summing in one sense a predetermined single sheet value with a running count; monitoring the noise generated at the shredding station and causing the running count to change at a predetermined rate in the opposite sense when the monitored noise indicates the shredding of sheets is taking place; and generating an error signal if the running count passes a predetermined threshold indicating an unacceptable operating condition.

2. A method according to claim 1, wherein the monitoring step comprises generating an analogue signal related to the monitored noise, comparing the analogue signal with a predetermined noise threshold, and generating a shredding signal indicating that the shredding of sheets is taking place when the analogue signal exceeds the noise threshold.

3. A method according to claims 1 or 2, wherein in the one sense the running count is incremented, the error signal being generated if the count exceeds the threshold.

4. A method according to any of the preceding claims, wherein the noise monitoring step is performed at one or both of the entry and exit of the shredding station.

5. A method of shredding sheets substantially as hereinbefore described with reference to the accompanying drawings.

6. Apparatus for shredding sheets, the apparatus comprising a sheet shredder positioned at a shredding station; a sheet feed system for feeding sheets to the shredder; a sheet sensor for sensing each sheet fed by the sheet feeding system; counting means for storing a running count and for arithmetically summing in one sense a predetermined single sheet value with the running count when the sheet sensor detects the feeding of a sheet by the sheet feed system; noise monitoring means for monitoring the noise generated at the shredding station and for causing the counting means to change the running count in the opposite sense at a predetermined rate when the monitored noise indicates that the shredding of sheets is taking place, the counting means generating an error signal if the running count passes a predetermined threshold indicating an unacceptable operating condition.

7. Apparatus according to claim 6, wherein the counting means comprises a counter which is enabled by a signal from the noise monitoring means generated when the shredding of sheets is taking place to decrement the running count.

8. Apparatus according to claim 7, wherein the sensor causes the running count to be incremented by the predetermined single sheet value when a sheet is sensed.

9. Apparatus according to any of claims 6 to 8, wherein the noise monitoring means comprises a microphone connected to a high pass filter, the microphone being positioned at the entrance to the shredding station.

10. Apparatus according to any of claims 6 to 9, wherein the noise monitoring means comprises a microphone connected to a low pass filter, the microphone being positioned at the exit to the shredding station.

11. Apparatus for shredding sheets substantially as hereinbefore described with reference to the accompanying drawings.