Sheet length detector with skew compensation

DETAILED DESCRIPTION One object of my invention is to provide a sheet length detector which compensates for skew.
Another object of my invention is to provide a sheet length detector which operates even where there is initial sheet slippage.
Other and further objects will be apparent from the following description.
In general, my invention contemplates a sheet length detector in which sheets are successively advanced past a pair of sheet sensors which are disposed at transversely spaced locations on the feed path on either side of the nip formed by a pair of opposing feed members.
A timing interval is begun upon the actuation of both sheet sensors and is ended upon the subsequent deactuation of both sheet sensors following their initial actuation.
The duration of the timing interval, which is preferably measured using pulses generated synchronously with the movement of the feed member, provides a measure of the sheet length.
This measure is then corrected for skew, which is determined from the time lapse, if any, between the deactuation of one sensor and the deactuation of both sensors.
The corrected measure may be compared with a fixed reference or, if desired, with a previously obtained length measurement or composite of such measurements.
This method of length detection is especially effective in minimizing inaccuracies due to slippage, since the friction feed members of the systems described above are generally most effective in the region of the sensors.
By starting the timing interval only when both sensors have been actuated, I ensure that the sheet has become fully enmeshed in the drive train.