Apparatus for detecting the thickness of sheets

Apparatus for detecting the passage of single and multiple sheets (5) along a flow path, comprising:

• a surface (6) in the flow path for supporting the sheet or sheets;
• a follower (1 or 52) biased such that it normally abuts against the supporting surface and follows the profile of the supporting surface and the overlying sheet or sheets;
• a transducer (4 or 53) responsive to the position of the follower to provide a thickness signal (TP) indicative of the linear displacement of the follower relative to the supporting surface due to the passage of the sheet or sheets;
• a control circuit (FIGS. 3A, 3B) which compares the thickness signal (TP) with at least one stored threshold level signal to provide an output signal (31, 32, 33) indicative of whether the detected thickness corresponds to that of a single sheet or multiple sheets;
• the control circuit including:
• means (9) responsive to the transducer output signal in the absence of a sheet for repeatedly updating a stored no-sheet signal level;
• adjustable threshold level generating means (10) including adjustable means for generating first and second signals (TP1, TP2) which correspond to changes in the output of the transducer in response to the passage of a single sheet and two superimposed sheets, respectively;
• and means connecting the first and second signal generating means to the output of the updating means to add the updated no-sheet signal level to the first and second signals to obtain updated first and second threshold levels which will be exceeded by the output of the transducer in response to the passage of a single sheet and two superimposed sheets, respectively.