Monitoring sheet length

Claim 1

A method of detecting the acceptability of sheets comprising 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.

Claim 4

A method according to any of claims 1 to 3, wherein the reference values comprise a pair of maximum and minimum length values.

Claim 7

A method according to any of claims 1 to 6, wherein step a comprises a method of monitoring the length of sheets passing a predetermined position (5,6), the method comprising 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 (5,6), 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 (5,6) 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 (5,6) 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 (5,6) from the first, second, and third values; wherein steps 1-5 are carried out at two predetermined positions (5,6) laterally offset from one another relatively to the direction of movement of the sheets.