Monitoring sheet length

Apparatus which carries out a method for monitoring the length of sheets (such as banknotes) passing a predetermined position includes two pairs of rollers 1, 2 defining respective nips 5, 6 through which a sheet passes. The passage of a sheet causes radial movement of portions 11 of the rollers 2 which is detected by detectors 12 mounted in a shaft 4. The passage of a sheet causes respective counters 15, 16 to be incremented at a relatively fast sheet. The length of the sheet is determined by monitoring the counter, counters 15 and 16, during the plurality of successive intervals. The leading edge of the sheet appears during the first of the successive intervals and the trailing edge of the sheet appears during the last of the successive intervals. The total length of the sheet is determined by counting the total number of intervals between the first and last interval and counting a portion of the first and last intervals, respectively, determined by the ratio of the counter in the first and last intervals and immediately following and preceding intervals, respectively.

FIELD OF THE INVENTION 
The invention relates to methods and apparatus for monitoring the length of 
sheets passing a predetermined position. 
DESCRIPTION OF THE PRIOR ART 
There is a requirement, particularly in the case of document counting and 
sorting, for example banknote counting and sorting to determine accurately 
the length of sheets passing a predetermined position. This determination 
can be used for a variety of purposes such as the detection of 
unacceptable sheets or to distinguish between sheets having different 
lengths so that they may be sorted in appropriate directions. 
In the past, sheets have been fed by transport means past a predetermined 
position and the presence of a sheet at the predetermined postion has been 
sensed at regularly spaced intervals. A rough indication of the length of 
a sheet is then derived by counting the number of sensing intervals at 
which a sheet was sensed. One problem with this arrangement is that, in 
general, leading and trailing edges of a sheet will not exactly coincide 
with a sensing instant. This means that prior art systems will determine 
different lengths for substantially identical sheets. In some cases, this 
may be acceptable but particularly in the case of detecting counterfeit 
documents and for distinguishing between certain genuine, but different 
denomination banknotes, a more accurate determination of length is 
required. 
It should be understood that in this specification the term "length" means 
the dimension of a sheet in the direction of movement of the sheet. In 
practice, this dimension may not be the longest dimension of the sheet. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a method of 
monitoring the length of sheets passing a predetermined position comprises 
(1) monitoring a relatively fast rate count at least in two pairs of 
intervals wherein 
(a) during one interval of each pair leading and trailing edges of the 
sheet are sensed respectively, the count being monitored for a period 
during the interval related to the time at which the leading or trailing 
edge passes the predetermined position and during the other interval of 
each pair the count is monitored for the entire interval, 
(b) during each interval the count is incremented at a constant rate, and 
(c) the duration of the intervals is long compared with the time between 
successive counts; 
(2) determining respective total counts from the monitored counts; 
(3) determining first and second values related to the lengths of the 
portions of the sheet passing the predetermined position during the 
intervals in which the leading and trailing edges are sensed by comparing 
the total counts monitored in the intervals of each pair; 
(4) determining a third value related to the length of the portion of the 
sheet passing the predetermined position between the intervals in which 
the leading and trailing edges are sensed; and 
(5) deriving a value related to the total length of the sheet passing the 
predetermined position from the first, second, and third values. 
The invention improves upon the prior art systems by determining values 
related to the length of the portions of the sheet passing the 
predetermined position during the intervals in which the leading and 
trailing edges are sensed. This enables a very accurate determination of 
the length of the sheet to be achieved. 
It should be noted that the invention has several advantages. Firstly, the 
overall measurement is substantially independent of the speed of the 
sheet. Secondly, the measurement accuracy is independent of the speed of 
the count while the count rate itself defines the leading and trailing 
edge resolutions. Thus, the faster the count rate, the greater the 
resolution obtained. Thirdly, the invention obviates the need for the 
expensive and bulky shaft encoding devices which would otherwise be 
necessary to achieve similar resolutions. 
Although it must be assumed that the speed of the sheet during any 
particular interval is substantially constant, the speeds in successive 
intervals do not necessarily have to be the same. Preferably, however, at 
least during each pair of intervals the sheet moves at the same 
substantially constant rate. The first and second values may then be 
determined simply by directly comparing the number of increments of the 
count in the successive intervals of each pair. This considerably 
simplifies the later processing steps since otherwise some additional 
compensation would be required. 
Preferably, the intervals of each pair are successive so as to maximize 
accuracy by minimizing the chance of significant changes in the count 
rate. 
Conveniently, the two pairs of intervals are separated by ten to twenty 
different intervals. However, in some methods each pair of intervals may 
share a common interval. This common interval, for which typically a total 
count will be determined, preferably occurs while a sheet is passing the 
predetermined position but it could occur outside this time. 
Similarly, it is preferable if the count is monitored only when a sheet is 
sensed at the predetermined position. This again reduces the risk of 
obtaining erroneous counts which could occur when monitoring part of the 
sheet transport system due to differences between the feed rate when a 
note is present and when a note is not present. 
During the interval when the leading edge of the sheet is sensed the count 
is preferably monitored from the detection of the leading edge to the end 
of the interval; similarly during the interval when the trailing edge is 
sensed, the count is preferably monitored from the beginning of the 
interval to the detection of the trailing edge. However, the count could 
be monitored from the time at which the trailing edge is sensed to the end 
of the interval and from the beginning of an interval to the time at which 
a leading edge is sensed. In this case, these counts could be subtracted 
from the count for a whole interval to produce the counts required. 
Typically step 5 comprises summing the first, second and third values to 
generate the fourth value. 
Step 4 may be carried out in any conventional way, but conveniently this 
step comprises determining the number of intervals during which the sheet 
moves through substantially the same distance past the predetermined 
position or positions. This number can then be multiplied by a constant 
relating to the distance of movement to provide the third value. 
In order to determine the time of commencement and termination of each 
interval, step 1 may include the step of monitoring the rotation of a 
shaft of transport means controlling movement of the sheets. This may most 
conveniently be achieved by using a conventional slotted timing disc. 
The counts which are monitored could be determined by monitoring a 
continuously incrementing count at appropriate times. Conveniently, 
however, the count is initiated at least when a leading edge of the sheet 
is sensed and is stopped when a trailing edge is sensed. 
Furthermore, the count could be incremented at different constant rates in 
each interval, but this would lead to more complex processing. 
Preferably, the method comprises carrying out steps 1-5 at two 
predetermined postions laterally offset from one another relative to the 
direction of movement of the sheets. By carrying out the method at two 
laterally offset positions, it is possible to compensate for skew fed 
sheets. 
Preferably, there are about 30 fast rate count pulses in each interval. 
Other numbers of fast rate count pulses are acceptable ranging from, for 
example, 10 to 50 per interval. The number depends on the distance moved 
by a sheet in one interval and the accuracy required. 
In accordance with a second aspect of the present invention, a method of 
detecting the acceptability of sheets comprises 
(a) monitoring the length of a sheet at two laterally spaced positions; 
(b) determining the difference between the monitored lengths; 
(c) comparing the difference with a predetermined threshold to determine 
whether the difference is large or small; 
(d) 
(i) if the difference is large, comparing each monitored length with 
predetermined reference values to determine the acceptability of the 
sheet; or 
(ii) if the difference is small, determining the average of the two 
lengths, and comparing the average with one or more predetermined 
reference values to determine the acceptability of the sheet. 
This method enables not only skew fed sheets to be accepted if they are 
genuine, but also sheets which have cuts, tears, holes and the like. 
Step a preferably comprises a method according to the first aspect ot the 
invention although the method is applicable to other known methods of 
monitoring sheet length. 
Step c may comprise determining the difference to be large if it exceeds 
the predetermined threshold and otherwise determining the difference to be 
small. 
It will be appreciated that the methods according to the first and second 
aspects of the invention are particularly applicable to banknote 
monitoring in, for example, banknote counting or sorting apparatus. 
In accordance with a third aspect of the present invention, apparatus for 
monitoring the length of sheets passing a predetermined position comprises 
transport means for transporting the sheets past the predetermined 
position; sensing means for sensing the presence of a sheet at the 
predetermined postion; a counter which may be incremented at a relatively 
fast rate; processing means for carrying out steps 1 to 5 of a method 
according to the first aspect of the invention; and comparison means for 
comparing the determined length with a reference and for providing a 
corresponding output signal. 
The sensing means may be provided by any conventional system such as an 
opacity detector. Preferably, however, the sensing means comprises a 
detector for detecting the passage of a sheet through a nip between a pair 
of rollers. An example of a suitable arrangement is illustrated in our 
copending European patent application No. 0130824.

DETAILED DESCRIPTION OF AN EMBODIMENT 
The sensing system shown in FIG. 1 comprises two pairs of rollers 1, 1' and 
2, 2', respectively. The rollers 1' are non-rotatably mounted on a shaft 
3, while the rollers 2, 2' are rotatably mounted on a shaft 4. The rollers 
1, 1' and 2, 2' form part of a transport system (not shown) for 
transporting single sheets from a hopper to a stacking position in order 
to count the number of sheets in the hopper. An example of such a counting 
system is described in more detail in the copending European patent 
application mentioned above and is incorporated in the De La Rue 2300 
banknote counting machine. Each pair of rollers 1, 2, 1', 2', 
respectively, defines a respective nip 5, 6. A slotted timing wheel 7 of 
conventional form is non-rotatably mounted to an extension of the shaft 3. 
The slots 8 of the wheel 7 are equally, circumferentially spaced around 
the wheel 7 and the light emitting diode and transistor of a detector 9 
are positioned on either side of the wheel 7. Output signals (C) from the 
detector 9 are fed to a microprocessor 10 such as an INTEL 8040. 
When a banknote enters the nips 5, 6 this will cause radial movement of 
rotatable portions 11 of the rollers 2, 2'. This movement will be detected 
by detectors 12, 12' mounted in the shaft 4 each of which provides a 
corresponding output signal which is fed to amplifiers 13, 14 
respectively. The output signals (A,B) from the amplifiers 13, 14 are fed 
to enabling inputs of respective counters 15, 16. An oscillator 17 
generates a substantially constant high pulse rate output signal (e.g. 21 
MHz) which is fed to each of the counters 15, 16. When the counters 15, 16 
are enabled by the respective signals A, B, they are incremented at the 
rate of the pulse signal from the oscillator 17. 
When no sheet is present in the nips 5, 6, first signals are output from 
the detectors 12, 12' and fed to the amplifiers 13, 14. The amplifier 13, 
14 then generates output signals which disable counters 15, 16. When a 
banknote enters the nips 5, 6, the detectors, 12 issue a second signal 
(which may in certain cases have zero amplitude) which causes amplifiers 
13, 14 to enable the respective counters 15, 16. It should be understood 
that the amplifiers 13, 14 are chosen to be suitable for causing the 
respective counters 15, 16 to be enabled when a sheet is detected. 
FIG. 2 illustrates the case where a banknote is fed slightly skew to the 
feed direction so that a leading edge of the banknote reaches the nip 6 
before the nip 5 is reached. For the sake of example, the situation 
relative to the nip 5 will be described in more detail. As soon as a 
banknote is detected in the nip 5, an appropriate signal is fed via the 
amplifier 14 to the counter 16 to enable the counter. This is indicated by 
the vertical line 18 in FIG. 2. The counter 16, which has previously been 
reset to zero, then starts to count pulses received from the oscillator 
17. 
At the same time, as the rollers 2, 2' rotate, periodic signals (C) are 
output from the detector 9 to the microprocessor 10 at a rate for example 
of 700 per second. These signals relate to the time intervals during which 
the rollers 2, 2' have rotated through a fixed angle corresponding to 
movement of the note through a distance of typically 4.7 mm. Since the 
slots 8 of the timing wheel 7 are equally spaced, this will correspond to 
equal angles of rotation. (Unequal spacing could also be used with more 
complex processing). It should be noted that the signals from the detector 
9 may not be equally spaced in time if the rollers 2, 2' do not rotate at 
a constant rate. However substantially equal units of length will be 
transported through the nips between each pair of slots. For convenience, 
successive signals from the detector 9 over the period concerned are 
labelled M-V respectively in FIG. 2. In practice, a much larger number of 
intervals will occur between the passage of leading and trailing edges, 
typically in the order 10-20. It should be noted that the pulse rate 
delivered by the oscillator 17 is considerably higher than the rate of 
pulses from the detector 9. 
As has previously been explained, as soon as the counter 16 is enabled it 
starts to count pulses from the oscillator 17. Thus, at the time P the 
microprocessor 10 will derive a count value from the counter 16 (and also 
the counter 15). At this time, the microprocessor 10 also causes the 
counters 15, 16 to be reset. At the next signal from the detector 9 (Q), 
the counters 15, 16 are again read (and reset) to determine second count 
values. It is assumed that the feed rate of the banknote in these two time 
intervals ending at P and Q is the same so that by simply taking the ratio 
of the counts read from the counter 16, it is possible to determine what 
proportion of the interval O-P corresponded to the presence of a note in 
the nip 5. 
At successive signals from the detector 9, the microprocessor 10 reads the 
counts from the counters 15, 16 and then resets the counters for the next 
interval. After the signal at time T, the trailing edge of the banknote 
passes through the nip 5 and the signal B from the detector 12 changes, as 
indicated by line 19 in FIG. 2, to disable the counter 16. At the time U, 
the microprocessor 10 reads the count in the counter 16 which will be much 
smaller than for a fully counted interval such as Q-R. Again, by 
determining the proportion of the count determined at the time U with the 
count determined at the time T a value related to the length of the final 
portion of the note can be determined. 
Since the occurrence of signals Q, R, S and T corresponds to the passage of 
a certain length of sheet, it is a simple matter to determine the full 
length of the sheet. For example if there are X intervals during which a 
sheet remains in the appropriate nip for the entire interval then the 
total length of the sheet may be represented by a value given by the 
formula: 
EQU (P/Q+X+U/T) 
where in this particular example X=4 and P, Q, U, T represent the counts 
determined at the end of these intervals. This value can be used as it 
stands or converted to an actual length if the rotation distance of the 
circumference 11 of the roller 2 between successive signals from the 
detector 9 is known. 
The length measurement described above can vary by a small amount in 
accordance with the thickness of the note being fed. FIG. 3 illustrates 
two examples of the output signal 20, 21 due to the passage of a 
relatively thin note and a relatively thick note respectively. The 
presence of a note in the nip is determined by comparing these output 
signals 20, 21 with a threshold 22 so that a note is only detected when 
this minimum threshold 22 is exceeded. However, it will be seen that the 
signal 20 due to a thin note takes longer to exceed the threshold 22 than 
the signal 21 corresponding to a thick note. this means that the counter 
enabling signals Y.sub.1, Y.sub.2 (equivalent to signals A, B in FIGS. 1 
and 2) will be generated at different times, as can be seen in FIG. 3, 
depending upon the thickness of a note. This is, in certain circumstances, 
undesirable since it will lead to an error in the determination of the 
length of the note. 
To compensate for thickness, it is therefore preferred that the 
microprocessor 10 reduce the calculated length of a banknote having a 
thickness greater than some minimum thickness, say due to a minimum 
thickness banknote, in accordance with the following formula: 
EQU Compensated length=Measured length-T.sub.n /K 
where T.sub.n is representative of the thickness of the sheet as determined 
by the signals from the detector and will lie between certain minimum and 
maximum thickness values. K is chosen to ensure that the units of T.sub.n 
/K correspond to the units of "measured length". 
In a typical example, the minimum thickness of a sheet which is allowable 
may be 20 while the maximum allowable thickness is 60, T.sub.n lying 
between these values and being typically 25. In this example, K is chosen 
to be 10. 
The values of T.sub.n and K depend on the processing equipment used and K 
is chosen empirically. 
In the case shown in FIG. 2, the banknote is skew fed and this can be 
compensated for by using a formula of the form: 
True length= 
EQU Calculated length.times.Cosine (Arctangent (Ls/Ld)) 
OR 
True length= 
##EQU1## 
where Ld=distance between detectors 12, 12' being typically 60 mm. 
and 
Ls=length of skew measurement between left and right detectors 12, 12'. 
The micriprocessor 10 is used to calculate the true length. Note that the 
true length equals the calculated length when the skew measurement is 
zero. 
An alternative method for determining the acceptability of banknotes will 
now be described. In this method, the apparatus shown in FIG. 1 is again 
used but the microprocessor 10 determines the length of the banknote as 
separately detected by each detector 12. These lengths L.sub.1, L.sub.2 
are then processed in the following manner. 
FIG. 4 is a flow diagram illustrating part of the operation of the 
microprocessor. 
After the two note lengths L.sub.1, L.sub.2 have been determined 23, the 
difference between the two lengths is calculated 24. This difference is 
then compared with a preset threshold D in a step 25. Typically, in the 
case of banknotes, D may be set to about 10 mm. 
If the difference between the measured note length is greater than D (step 
26) this is taken to indicate a banknote, part of which is torn, cut, or 
folded and the like. However, such a banknote may still be a genuine 
banknote and the method set out in FIG. 4 provides a way in which the 
banknote may be accepted despite this apparent difference in note length. 
In step 27, the measured length L.sub.1 is compared with an allowable range 
of lengths which may be defined in terms of a nominal length and a 
tolerance or, preferably, by upper and lower values. These values may have 
been preset or previously determined from the first sheet of a batch which 
is fed through the note counter. 
An example of suitable maximum and minimum length limits is 68 mm and 62 mm 
respectively. 
If L.sub.1 is found to lie within the allowable range, the microprocessor 
10 generates an acceptance signal (step 28) which may be used to increment 
a running total of a counter. 
If L.sub.1 is found not to lie within the allowable range, L.sub.2 is 
compared with the allowable range (step 29), and a similar acceptance 
signal is generated if L.sub.2 is found to lie within the range. 
If, however, both L.sub.1 and L.sub.2 lie outside the allowable range, then 
a reject signal is generated (30) by the microprocessor 10. This may be 
used, for example, to stop the machine to enable the unacceptable note to 
be extracted. 
If, in step 26, the difference between the measured length is found to be 
less than or equal to the threshold D, the average of the two lengths is 
determined, 31. This is because the difference may be due to a genuine 
sheet being fed slightly askew. 
The average measured length is then compared with the same allowable range 
used in steps 27 to 29, in a step 32. If the average is found to lie 
within the range, then the general acceptance signal 28 is generated while 
if it is outside the range, the reject signal is generated. 
Of course, it should be understood that additional tests may be carried out 
on the banknotes as they are fed so that although the length may be 
determined to be acceptable, the banknote may still be judged not to be 
genuine if it fails, for example, an opacity test.