De La Rue Systems Ltd.

David R Worsley

Trevor J Conner

Martin Lane

Michel R Croset

Roy E Winchester

A method monitors the length of sheets, such as banknotes, passing a predetermined position. The length of a sheet is monitored at two laterally spaced locations, and the difference between the monitored lengths is compared. If the difference is small, this indicates that both monitored lengths are an accurate indication of the length of the sheet. An average of the two monitored lengths is determined and compared with at least one predetermined reference value to determine the acceptability of the sheet. If the average falls within a predetermined range, the sheet is acceptable.

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.

A method and apparatus for monitoring the passage of sheets past at least one sensing assembly (48,49). The sensing assembly (48,49) generates an output signal whose level varies in accordance with a characteristic of the sheets, for example the thickness of the sheets. The method comprises for the or each sensing assembly, presetting a first threshold (62) spaced by a predetermined amount from a datum output signal level (57) when no sheet is sensed, the predetermined amount being such that the first threshold (62) is not passed as a result of random noise variations in the output signal but is passed during the passage past the sensing means of all sheets which it is desired to monitor. The size of the output signal from the sensing means (48, 49) relative to the datum level is monitored when the output signal first passes the first threshold (62) corresponding to the passage of a first sheet. Single, double, and triple sheet thresholds (63, 64, 67) are generated from a monitored size and thereafter the output signal is monitored to sense when it passes one or more of the thresholds.

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Hampshire, United Kingdom

David R Worsley