Method and apparatus for detecting abnormal thickness conditions such as

Abstract

Sheets moving in a feed direction and arranged at spaced intervals are fed through nips arranged to rollingly engage opposing sides of the sheet, each nip being formed by a roller rotating about a fixed axis and a cooperating swingably mounted roller. Resilient adjustable biasing means urges an assembly upon which the swingably mounted roller is pivotally mounted, as well as an arm forming part of the pivotally mounted roller assembly, in a first direction. A sensor is positioned adjacent the free end of said arm. Sheets moving through said nip cause displacement of said pivotal assembly against the biasing force in accordance with the thickness of the sheet passing through the nip. The biasing means comprises a thick, and accurate resilient element designed to permit the roller assembly to follow even abrupt thickness changes rapidly and accurately without experiencing overshoot or undershoot. The sensor senses the position of a field generating element mounted upon the free end of said arm to provide a signal representative of the size and direction of the displacement. The output signals developed during the passage of a sheet through said nips are compared against a threshold. The sensors are periodically examined to provide an offset signal for automatically compensating for changes in output signal strength due to changes in temperature, the new zero offset being generated during the time that no sheet, is passing between the aforesaid nips. The offset signal is combined with the sensor signal to provide a compensated sensor signal which is then compared with a reference level representing a predetermined thickness.