Method and apparatus for sensing sheets

Apparatus for sensing the passage of sheets through a nip, said apparatus comprising:

• a pair of guide members whose surfaces define said nip, said guide members being moveable in a cyclic manner to advance said sheets through said nip and being displaceable with respect to one another as a function of the thickness of one or more sheets passing through said nip;
• sensing means for sensing a deflection of said guide members relative to one another and for generating a train of signals whose frequency varies as a function of the amount of relative deflection of said guide members;
• counter means for generating n present count values for each cycle of movement of said guide members, n being a integer greater than 1, each present count value being representative of the number of signals generated by said sensing means during a respective sampling interval corresponding to a respective portion of said surface of said guide members;
• storing means for storing n no-sheet count values for a single cycle of movement of said guide members, each no-sheet count value representing the relative deflection of said guide members with respect to one another when no sheet is present between said nip for a respective one of said portions of said surface of said guide members so that each of said no-sheet count values corresponds to a respective one of said present count values;
• comparing means for comparing, for each cycle of movement of said guide members, each present count value with its corresponding no-sheet count value and for generating a respective difference signal for each said comparison;
• and
• determining means for obtaining an average of said difference signals for each said cycle of movement of said guide means and for comparing said average with a predetermined threshold value to determine the presence or absence of a sheet in said nip.